[RESEND][PATCH v18 0/8] Single RunQueue Proxy Execution (v18)

[RESEND][PATCH v18 0/8] Single RunQueue Proxy Execution (v18)

[John Stultz]

Hey All,

Just wanted to resend this v18 iteration of the Single
RunQueue Proxy-Execution series, as its been a bit quiet.
No changes from last time, though I did add K Prateek Nayak's
Tested-by tags.

So here is v18 of the Proxy Execution series, a generalized form
of priority inheritance.

As I’m trying to submit this work in smallish digestible pieces,
in this series, I’m only submitting for review the logic that
allows us to do the proxying if the lock owner is on the same
runqueue as the blocked waiter: Introducing the
CONFIG_SCHED_PROXY_EXEC option and boot-argument, reworking the
task_struct::blocked_on pointer and wrapper functions, the
initial sketch of the find_proxy_task() logic, some fixes for
using split contexts, and finally same-runqueue proxying. 

As I mentioned earlier, for the series I’m submitting here, it has
only barely changed from v17. With the main difference being
slightly different order of checks for cases where we don’t
actually do anything yet (more on why below), and use of
READ_ONCE for the on_rq reads to avoid the compiler fusing
loads, which I was bitten by with the full series.

However for the full proxy-exec series, there are a few
differences:
* Suleiman Souhlal noticed an inefficiency in that we evaluate
  if the lock owner’s task_cpu() is the current cpu, before we
  look to see if the lock owner is on_rq at all. With v17 this
  would result in us proxy-migrating a donor to a remote cpu,
  only to then realize the task wasn’t even on the runqueue,
  and doing the sleeping owner enqueuing. Suleiman suggested
  instead that we evaluate on_rq first, so we can immediately do
  sleeping owner enqueueing. Then only if the owner is on a
  runqueue do we proxy-migrate the donor (which requires the
  more costly lock juggling). While not a huge logical change,
  it did uncover other problems, which needed to be resolved.

* One issue found was there was a race where if
  do_activate_blocked_waiter() from the sleeping owner wakeup
  was delayed and the task had already been woken up elsewhere.
  It’s possible if that task was running and called into
  schedule() to be blocked, it would be dequeued from the
  runqueue, but before we switched to the new task,
  do_activate_blocked_waiter() might try to activate it on a
  different cpu. Clearly the do_activate_blocked_waiter() needed
  to check the task on_cpu value as well.

* I found that we still can hit wakeups that end up skipping the
  BO_WAKING -> BO_RUNNALBE transition (causing find_proxy_task()
  to end up spinning waiting for that transition), so I re-added
  the logic to handle doing return migrations from
  find_proxy_task() if we hit that case.

* Hupu suggested a tweak in ttwu_runnable() to evaluate
  proxy_needs_return() slightly earlier.

* Kuyo Chang reported and isolated a fix for a problem with
  __task_is_pushable() in the !sched_proxy_exec case, which was
  folded into the “sched: Fix rt/dl load balancing via chain
  level balance” patch

* Reworked some of the logic around releasing the rq->donor
  reference on migrations, using rq->idle directly.

* Sueliman also pointed out that some added task_struct elements
  were not being initialized in the init_task code path, so that
  was good to fix.

You can find the full series here:
  https://github.com/johnstultz-work/linux-dev/commits/proxy-exec-v18-6.16-rc3/
  https://github.com/johnstultz-work/linux-dev.git proxy-exec-v18-6.16-rc3

Issues still to address with the full series (not the patches
submitted here):
* Peter suggested an idea that instead of when tasks become
  unblocked, using (blocked_on_state == BO_WAKING) to protect
  against running proxy-migrated tasks on cpu’s they are not
  affined to, we could dequeue tasks first and then wake them.
  This does look to be cleaner in many ways, but the locking
  rework is significant and I’ve not worked out all the kinks
  with it yet. I am also a little worried that we may trip
  other wakeup paths that might not do the dequeue first.
  However, I have adopted this approach for the
  find_proxy_task() forced return migration, and it’s working
  well.

* Need to sort out what is needed for sched_ext to be ok with
  proxy-execution enabled.

* K Prateek Nayak did some testing about a bit over a year ago
  with an earlier version of the series and saw ~3-5%
  regressions in some cases. Need to re-evaluate this with the
  proxy-migration avoidance optimization Suleiman suggested now
  implemented.

* The chain migration functionality needs further iterations and
  better validation to ensure it truly maintains the RT/DL load
  balancing invariants (despite this being broken in vanilla
  upstream with RT_PUSH_IPI currently)

I’d really appreciate any feedback or review thoughts on the
full series as well. I’m trying to keep the chunks small,
reviewable and iteratively testable, but if you have any
suggestions on how to improve the series, I’m all ears.

Credit/Disclaimer:
—--------------------
As always, this Proxy Execution series has a long history with
lots of developers that deserve credit: 

First described in a paper[1] by Watkins, Straub, Niehaus, then
from patches from Peter Zijlstra, extended with lots of work by
Juri Lelli, Valentin Schneider, and Connor O'Brien. (and thank
you to Steven Rostedt for providing additional details here!)

So again, many thanks to those above, as all the credit for this
series really is due to them - while the mistakes are likely mine.

Thanks so much!
-john

[1] https://static.lwn.net/images/conf/rtlws11/papers/proc/p38.pdf

Cc: Joel Fernandes <joelagnelf@nvidia.com>
Cc: Qais Yousef <qyousef@layalina.io>  
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Zimuzo Ezeozue <zezeozue@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Metin Kaya <Metin.Kaya@arm.com>
Cc: Xuewen Yan <xuewen.yan94@gmail.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: kuyo chang <kuyo.chang@mediatek.com>
Cc: hupu <hupu.gm@gmail.com>
Cc: kernel-team@android.com

John Stultz (4):
  sched: Add CONFIG_SCHED_PROXY_EXEC & boot argument to enable/disable
  sched: Move update_curr_task logic into update_curr_se
  sched: Fix runtime accounting w/ split exec & sched contexts
  sched: Add an initial sketch of the find_proxy_task() function

Peter Zijlstra (2):
  locking/mutex: Rework task_struct::blocked_on
  sched: Start blocked_on chain processing in find_proxy_task()

Valentin Schneider (2):
  locking/mutex: Add p->blocked_on wrappers for correctness checks
  sched: Fix proxy/current (push,pull)ability

 .../admin-guide/kernel-parameters.txt         |   5 +
 include/linux/sched.h                         |  72 ++++-
 init/Kconfig                                  |  12 +
 kernel/fork.c                                 |   3 +-
 kernel/locking/mutex-debug.c                  |   9 +-
 kernel/locking/mutex.c                        |  18 ++
 kernel/locking/mutex.h                        |   3 +-
 kernel/locking/ww_mutex.h                     |  16 +-
 kernel/sched/core.c                           | 257 +++++++++++++++++-
 kernel/sched/deadline.c                       |   7 +
 kernel/sched/fair.c                           |  65 +++--
 kernel/sched/rt.c                             |   5 +
 kernel/sched/sched.h                          |  22 +-
 13 files changed, 448 insertions(+), 46 deletions(-)

-- 
2.50.0.727.gbf7dc18ff4-goog

[RESEND][PATCH v18 1/8] sched: Add CONFIG_SCHED_PROXY_EXEC & boot argument to enable/disable

[John Stultz]

Add a CONFIG_SCHED_PROXY_EXEC option, along with a boot argument
sched_proxy_exec= that can be used to disable the feature at boot
time if CONFIG_SCHED_PROXY_EXEC was enabled.

Also uses this option to allow the rq->donor to be different from
rq->curr.

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: John Stultz <jstultz@google.com>
---
v7:
* Switch to CONFIG_SCHED_PROXY_EXEC/sched_proxy_exec= as
  suggested by Metin Kaya.
* Switch boot arg from =disable/enable to use kstrtobool(),
  which supports =yes|no|1|0|true|false|on|off, as also
  suggested by Metin Kaya, and print a message when a boot
  argument is used.
v8:
* Move CONFIG_SCHED_PROXY_EXEC under Scheduler Features as
  Suggested by Metin
* Minor rework reordering with split sched contexts patch
v12:
* Rework for selected -> donor renaming
v14:
* Depend on !PREEMPT_RT to avoid build issues for now
v15:
* Depend on EXPERT while patch series upstreaming is
  in progress.
v16:
* Allow "sched_proxy_exec" without "=true" to enable
  proxy-execution at boot time, in addition to the
  "sched_proxy_exec=true" or "sched_proxy_exec=false" options
  as suggested by Steven
* Drop the "default n" in Kconfig as suggested by Steven
* Add !SCHED_CLASS_EXT dependency until I can investigate if
  sched_ext can understand split contexts, as suggested by
  Peter
v17:
* Expanded the commit message a bit to clarify that the option
  allows the split contexts (rq->donor and rq->curr) to be
  different tasks, from feedback from Juri Lelli

Cc: Joel Fernandes <joelagnelf@nvidia.com>
Cc: Qais Yousef <qyousef@layalina.io>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Zimuzo Ezeozue <zezeozue@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Metin Kaya <Metin.Kaya@arm.com>
Cc: Xuewen Yan <xuewen.yan94@gmail.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: kuyo chang <kuyo.chang@mediatek.com>
Cc: hupu <hupu.gm@gmail.com>
Cc: kernel-team@android.com
---
 .../admin-guide/kernel-parameters.txt         |  5 ++++
 include/linux/sched.h                         | 13 +++++++++
 init/Kconfig                                  | 12 ++++++++
 kernel/sched/core.c                           | 29 +++++++++++++++++++
 kernel/sched/sched.h                          | 12 ++++++++
 5 files changed, 71 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f1f2c0874da9d..5b89464ca570a 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6387,6 +6387,11 @@
 	sa1100ir	[NET]
 			See drivers/net/irda/sa1100_ir.c.
 
+	sched_proxy_exec= [KNL]
+			Enables or disables "proxy execution" style
+			solution to mutex-based priority inversion.
+			Format: <bool>
+
 	sched_verbose	[KNL,EARLY] Enables verbose scheduler debug messages.
 
 	schedstats=	[KNL,X86] Enable or disable scheduled statistics.
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4f78a64beb52c..ba2a02ead8c7e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1664,6 +1664,19 @@ struct task_struct {
 	randomized_struct_fields_end
 } __attribute__ ((aligned (64)));
 
+#ifdef CONFIG_SCHED_PROXY_EXEC
+DECLARE_STATIC_KEY_TRUE(__sched_proxy_exec);
+static inline bool sched_proxy_exec(void)
+{
+	return static_branch_likely(&__sched_proxy_exec);
+}
+#else
+static inline bool sched_proxy_exec(void)
+{
+	return false;
+}
+#endif
+
 #define TASK_REPORT_IDLE	(TASK_REPORT + 1)
 #define TASK_REPORT_MAX		(TASK_REPORT_IDLE << 1)
 
diff --git a/init/Kconfig b/init/Kconfig
index af4c2f0854554..6844b25303913 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -875,6 +875,18 @@ config UCLAMP_BUCKETS_COUNT
 
 	  If in doubt, use the default value.
 
+config SCHED_PROXY_EXEC
+	bool "Proxy Execution"
+	# Avoid some build failures w/ PREEMPT_RT until it can be fixed
+	depends on !PREEMPT_RT
+	# Need to investigate how to inform sched_ext of split contexts
+	depends on !SCHED_CLASS_EXT
+	# Not particularly useful until we get to multi-rq proxying
+	depends on EXPERT
+	help
+	  This option enables proxy execution, a mechanism for mutex-owning
+	  tasks to inherit the scheduling context of higher priority waiters.
+
 endmenu
 
 #
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 8988d38d46a38..853157b27f384 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -119,6 +119,35 @@ EXPORT_TRACEPOINT_SYMBOL_GPL(sched_compute_energy_tp);
 
 DEFINE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
 
+#ifdef CONFIG_SCHED_PROXY_EXEC
+DEFINE_STATIC_KEY_TRUE(__sched_proxy_exec);
+static int __init setup_proxy_exec(char *str)
+{
+	bool proxy_enable = true;
+
+	if (*str && kstrtobool(str + 1, &proxy_enable)) {
+		pr_warn("Unable to parse sched_proxy_exec=\n");
+		return 0;
+	}
+
+	if (proxy_enable) {
+		pr_info("sched_proxy_exec enabled via boot arg\n");
+		static_branch_enable(&__sched_proxy_exec);
+	} else {
+		pr_info("sched_proxy_exec disabled via boot arg\n");
+		static_branch_disable(&__sched_proxy_exec);
+	}
+	return 1;
+}
+#else
+static int __init setup_proxy_exec(char *str)
+{
+	pr_warn("CONFIG_SCHED_PROXY_EXEC=n, so it cannot be enabled or disabled at boot time\n");
+	return 0;
+}
+#endif
+__setup("sched_proxy_exec", setup_proxy_exec);
+
 /*
  * Debugging: various feature bits
  *
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 475bb5998295e..6b61e0c7f6e78 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1151,10 +1151,15 @@ struct rq {
 	 */
 	unsigned int		nr_uninterruptible;
 
+#ifdef CONFIG_SCHED_PROXY_EXEC
+	struct task_struct __rcu	*donor;  /* Scheduling context */
+	struct task_struct __rcu	*curr;   /* Execution context */
+#else
 	union {
 		struct task_struct __rcu *donor; /* Scheduler context */
 		struct task_struct __rcu *curr;  /* Execution context */
 	};
+#endif
 	struct sched_dl_entity	*dl_server;
 	struct task_struct	*idle;
 	struct task_struct	*stop;
@@ -1349,10 +1354,17 @@ DECLARE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
 #define cpu_curr(cpu)		(cpu_rq(cpu)->curr)
 #define raw_rq()		raw_cpu_ptr(&runqueues)
 
+#ifdef CONFIG_SCHED_PROXY_EXEC
+static inline void rq_set_donor(struct rq *rq, struct task_struct *t)
+{
+	rcu_assign_pointer(rq->donor, t);
+}
+#else
 static inline void rq_set_donor(struct rq *rq, struct task_struct *t)
 {
 	/* Do nothing */
 }
+#endif
 
 #ifdef CONFIG_SCHED_CORE
 static inline struct cpumask *sched_group_span(struct sched_group *sg);
-- 
2.50.0.727.gbf7dc18ff4-goog

[RESEND][PATCH v18 2/8] locking/mutex: Rework task_struct::blocked_on

[John Stultz]

From: Peter Zijlstra <peterz@infradead.org>

Track the blocked-on relation for mutexes, to allow following this
relation at schedule time.

   task
     | blocked-on
     v
   mutex
     | owner
     v
   task

This all will be used for tracking blocked-task/mutex chains
with the prox-execution patch in a similar fashion to how
priority inheritance is done with rt_mutexes.

For serialization, blocked-on is only set by the task itself
(current). And both when setting or clearing (potentially by
others), is done while holding the mutex::wait_lock.

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
[minor changes while rebasing]
Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Connor O'Brien <connoro@google.com>
[jstultz: Fix blocked_on tracking in __mutex_lock_common in error paths]
Signed-off-by: John Stultz <jstultz@google.com>
---
v2:
* Fixed blocked_on tracking in error paths that was causing crashes
v4:
* Ensure we clear blocked_on when waking ww_mutexes to die or wound.
  This is critical so we don't get circular blocked_on relationships
  that can't be resolved.
v5:
* Fix potential bug where the skip_wait path might clear blocked_on
  when that path never set it
* Slight tweaks to where we set blocked_on to make it consistent,
  along with extra WARN_ON correctness checking
* Minor comment changes
v7:
* Minor commit message change suggested by Metin Kaya
* Fix WARN_ON conditionals in unlock path (as blocked_on might already
  be cleared), found while looking at issue Metin Kaya raised.
* Minor tweaks to be consistent in what we do under the
  blocked_on lock, also tweaked variable name to avoid confusion
  with label, and comment typos, as suggested by Metin Kaya
* Minor tweak for CONFIG_SCHED_PROXY_EXEC name change
* Moved unused block of code to later in the series, as suggested
  by Metin Kaya
* Switch to a tri-state to be able to distinguish from waking and
  runnable so we can later safely do return migration from ttwu
* Folded together with related blocked_on changes
v8:
* Fix issue leaving task BO_BLOCKED when calling into optimistic
  spinning path.
* Include helper to better handle BO_BLOCKED->BO_WAKING transitions
v9:
* Typo fixup pointed out by Metin
* Cleanup BO_WAKING->BO_RUNNABLE transitions for the !proxy case
* Many cleanups and simplifications suggested by Metin
v11:
* Whitespace fixup pointed out by Metin
v13:
* Refactor set_blocked_on helpers clean things up a bit
v14:
* Small build fixup with PREEMPT_RT
v15:
* Improve consistency of names for functions that assume blocked_lock
  is held, as suggested by Peter
* Use guard instead of separate spinlock/unlock calls, also suggested
  by Peter
* Drop blocked_on_state tri-state for now, as its not needed until
  later in the series, when we get to proxy-migration and return-
  migration.
v16:
* Clear blocked on before optimistic spinning

Cc: Joel Fernandes <joelagnelf@nvidia.com>
Cc: Qais Yousef <qyousef@layalina.io>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Zimuzo Ezeozue <zezeozue@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Metin Kaya <Metin.Kaya@arm.com>
Cc: Xuewen Yan <xuewen.yan94@gmail.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: kuyo chang <kuyo.chang@mediatek.com>
Cc: hupu <hupu.gm@gmail.com>
Cc: kernel-team@android.com
---
 include/linux/sched.h        |  5 +----
 kernel/fork.c                |  3 +--
 kernel/locking/mutex-debug.c |  9 +++++----
 kernel/locking/mutex.c       | 22 ++++++++++++++++++++++
 kernel/locking/ww_mutex.h    | 18 ++++++++++++++++--
 5 files changed, 45 insertions(+), 12 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index ba2a02ead8c7e..aa7966141a090 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1238,10 +1238,7 @@ struct task_struct {
 	struct rt_mutex_waiter		*pi_blocked_on;
 #endif
 
-#ifdef CONFIG_DEBUG_MUTEXES
-	/* Mutex deadlock detection: */
-	struct mutex_waiter		*blocked_on;
-#endif
+	struct mutex			*blocked_on;	/* lock we're blocked on */
 
 #ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
 	/*
diff --git a/kernel/fork.c b/kernel/fork.c
index 1ee8eb11f38ba..5f87f05aff4a0 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2123,9 +2123,8 @@ __latent_entropy struct task_struct *copy_process(
 	lockdep_init_task(p);
 #endif
 
-#ifdef CONFIG_DEBUG_MUTEXES
 	p->blocked_on = NULL; /* not blocked yet */
-#endif
+
 #ifdef CONFIG_BCACHE
 	p->sequential_io	= 0;
 	p->sequential_io_avg	= 0;
diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
index 6e6f6071cfa27..758b7a6792b0c 100644
--- a/kernel/locking/mutex-debug.c
+++ b/kernel/locking/mutex-debug.c
@@ -53,17 +53,18 @@ void debug_mutex_add_waiter(struct mutex *lock, struct mutex_waiter *waiter,
 {
 	lockdep_assert_held(&lock->wait_lock);
 
-	/* Mark the current thread as blocked on the lock: */
-	task->blocked_on = waiter;
+	/* Current thread can't be already blocked (since it's executing!) */
+	DEBUG_LOCKS_WARN_ON(task->blocked_on);
 }
 
 void debug_mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter,
 			 struct task_struct *task)
 {
+	struct mutex *blocked_on = READ_ONCE(task->blocked_on);
+
 	DEBUG_LOCKS_WARN_ON(list_empty(&waiter->list));
 	DEBUG_LOCKS_WARN_ON(waiter->task != task);
-	DEBUG_LOCKS_WARN_ON(task->blocked_on != waiter);
-	task->blocked_on = NULL;
+	DEBUG_LOCKS_WARN_ON(blocked_on && blocked_on != lock);
 
 	INIT_LIST_HEAD(&waiter->list);
 	waiter->task = NULL;
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index a39ecccbd106a..e2f59863a866e 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -644,6 +644,8 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 			goto err_early_kill;
 	}
 
+	WARN_ON(current->blocked_on);
+	current->blocked_on = lock;
 	set_current_state(state);
 	trace_contention_begin(lock, LCB_F_MUTEX);
 	for (;;) {
@@ -680,6 +682,12 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 
 		first = __mutex_waiter_is_first(lock, &waiter);
 
+		/*
+		 * As we likely have been woken up by task
+		 * that has cleared our blocked_on state, re-set
+		 * it to the lock we are trying to aquire.
+		 */
+		current->blocked_on = lock;
 		set_current_state(state);
 		/*
 		 * Here we order against unlock; we must either see it change
@@ -691,8 +699,11 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 
 		if (first) {
 			trace_contention_begin(lock, LCB_F_MUTEX | LCB_F_SPIN);
+			/* clear blocked_on as mutex_optimistic_spin may schedule() */
+			current->blocked_on = NULL;
 			if (mutex_optimistic_spin(lock, ww_ctx, &waiter))
 				break;
+			current->blocked_on = lock;
 			trace_contention_begin(lock, LCB_F_MUTEX);
 		}
 
@@ -700,6 +711,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 	}
 	raw_spin_lock_irqsave(&lock->wait_lock, flags);
 acquired:
+	current->blocked_on = NULL;
 	__set_current_state(TASK_RUNNING);
 
 	if (ww_ctx) {
@@ -729,9 +741,11 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 	return 0;
 
 err:
+	current->blocked_on = NULL;
 	__set_current_state(TASK_RUNNING);
 	__mutex_remove_waiter(lock, &waiter);
 err_early_kill:
+	WARN_ON(current->blocked_on);
 	trace_contention_end(lock, ret);
 	raw_spin_unlock_irqrestore_wake(&lock->wait_lock, flags, &wake_q);
 	debug_mutex_free_waiter(&waiter);
@@ -942,6 +956,14 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
 		next = waiter->task;
 
 		debug_mutex_wake_waiter(lock, waiter);
+		/*
+		 * Unlock wakeups can be happening in parallel
+		 * (when optimistic spinners steal and release
+		 * the lock), so blocked_on may already be
+		 * cleared here.
+		 */
+		WARN_ON(next->blocked_on && next->blocked_on != lock);
+		next->blocked_on = NULL;
 		wake_q_add(&wake_q, next);
 	}
 
diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
index 37f025a096c9d..00db40946328e 100644
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -284,6 +284,14 @@ __ww_mutex_die(struct MUTEX *lock, struct MUTEX_WAITER *waiter,
 #ifndef WW_RT
 		debug_mutex_wake_waiter(lock, waiter);
 #endif
+		/*
+		 * When waking up the task to die, be sure to clear the
+		 * blocked_on pointer. Otherwise we can see circular
+		 * blocked_on relationships that can't resolve.
+		 */
+		WARN_ON(waiter->task->blocked_on &&
+			waiter->task->blocked_on != lock);
+		waiter->task->blocked_on = NULL;
 		wake_q_add(wake_q, waiter->task);
 	}
 
@@ -331,9 +339,15 @@ static bool __ww_mutex_wound(struct MUTEX *lock,
 		 * it's wounded in __ww_mutex_check_kill() or has a
 		 * wakeup pending to re-read the wounded state.
 		 */
-		if (owner != current)
+		if (owner != current) {
+			/*
+			 * When waking up the task to wound, be sure to clear the
+			 * blocked_on pointer. Otherwise we can see circular
+			 * blocked_on relationships that can't resolve.
+			 */
+			owner->blocked_on = NULL;
 			wake_q_add(wake_q, owner);
-
+		}
 		return true;
 	}
 
-- 
2.50.0.727.gbf7dc18ff4-goog

[RESEND][PATCH v18 3/8] locking/mutex: Add p->blocked_on wrappers for correctness checks

[John Stultz]

From: Valentin Schneider <valentin.schneider@arm.com>

This lets us assert mutex::wait_lock is held whenever we access
p->blocked_on, as well as warn us for unexpected state changes.

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
[fix conflicts, call in more places]
Signed-off-by: Connor O'Brien <connoro@google.com>
[jstultz: tweaked commit subject, reworked a good bit]
Signed-off-by: John Stultz <jstultz@google.com>
---
v2:
* Added get_task_blocked_on() accessor
v4:
* Address READ_ONCE usage that was dropped in v2
* Reordered to be a later add on to the main patch series as
  Peter was unhappy with similar wrappers in other patches.
v5:
* Added some extra correctness checking in wrappers
v7:
* Tweaks to reorder this change in the patch series
* Minor cleanup to set_task_blocked_on() suggested by Metin Kaya
v15:
* Split out into its own patch again.
* Further improve assumption checks in helpers.
v16:
* Fix optimistic spin case that can call schedule()
v17:
* Fix typos caught by Metin Kaya
* Add lockdep_assert_held_once and drop the READ_ONCE in
  __get_task_blocked_on(), as suggested by Juri Lelli

Cc: Joel Fernandes <joelagnelf@nvidia.com>
Cc: Qais Yousef <qyousef@layalina.io>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Zimuzo Ezeozue <zezeozue@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Metin Kaya <Metin.Kaya@arm.com>
Cc: Xuewen Yan <xuewen.yan94@gmail.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: kuyo chang <kuyo.chang@mediatek.com>
Cc: hupu <hupu.gm@gmail.com>
Cc: kernel-team@android.com
---
 include/linux/sched.h        | 54 ++++++++++++++++++++++++++++++++++--
 kernel/locking/mutex-debug.c |  4 +--
 kernel/locking/mutex.c       | 32 ++++++++++-----------
 kernel/locking/ww_mutex.h    |  6 ++--
 4 files changed, 70 insertions(+), 26 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index aa7966141a090..1d7f625adbb5e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -34,6 +34,7 @@
 #include <linux/sched/prio.h>
 #include <linux/sched/types.h>
 #include <linux/signal_types.h>
+#include <linux/spinlock.h>
 #include <linux/syscall_user_dispatch_types.h>
 #include <linux/mm_types_task.h>
 #include <linux/netdevice_xmit.h>
@@ -2177,6 +2178,57 @@ extern int __cond_resched_rwlock_write(rwlock_t *lock);
 	__cond_resched_rwlock_write(lock);					\
 })
 
+static inline void __set_task_blocked_on(struct task_struct *p, struct mutex *m)
+{
+	WARN_ON_ONCE(!m);
+	/* The task should only be setting itself as blocked */
+	WARN_ON_ONCE(p != current);
+	/* Currently we serialize blocked_on under the mutex::wait_lock */
+	lockdep_assert_held_once(&m->wait_lock);
+	/*
+	 * Check ensure we don't overwrite existing mutex value
+	 * with a different mutex. Note, setting it to the same
+	 * lock repeatedly is ok.
+	 */
+	WARN_ON_ONCE(p->blocked_on && p->blocked_on != m);
+	p->blocked_on = m;
+}
+
+static inline void set_task_blocked_on(struct task_struct *p, struct mutex *m)
+{
+	guard(raw_spinlock_irqsave)(&m->wait_lock);
+	__set_task_blocked_on(p, m);
+}
+
+static inline void __clear_task_blocked_on(struct task_struct *p, struct mutex *m)
+{
+	WARN_ON_ONCE(!m);
+	/* Currently we serialize blocked_on under the mutex::wait_lock */
+	lockdep_assert_held_once(&m->wait_lock);
+	/*
+	 * There may be cases where we re-clear already cleared
+	 * blocked_on relationships, but make sure we are not
+	 * clearing the relationship with a different lock.
+	 */
+	WARN_ON_ONCE(m && p->blocked_on && p->blocked_on != m);
+	p->blocked_on = NULL;
+}
+
+static inline void clear_task_blocked_on(struct task_struct *p, struct mutex *m)
+{
+	guard(raw_spinlock_irqsave)(&m->wait_lock);
+	__clear_task_blocked_on(p, m);
+}
+
+static inline struct mutex *__get_task_blocked_on(struct task_struct *p)
+{
+	struct mutex *m = p->blocked_on;
+
+	if (m)
+		lockdep_assert_held_once(&m->wait_lock);
+	return m;
+}
+
 static __always_inline bool need_resched(void)
 {
 	return unlikely(tif_need_resched());
@@ -2216,8 +2268,6 @@ extern bool sched_task_on_rq(struct task_struct *p);
 extern unsigned long get_wchan(struct task_struct *p);
 extern struct task_struct *cpu_curr_snapshot(int cpu);
 
-#include <linux/spinlock.h>
-
 /*
  * In order to reduce various lock holder preemption latencies provide an
  * interface to see if a vCPU is currently running or not.
diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
index 758b7a6792b0c..949103fd8e9b5 100644
--- a/kernel/locking/mutex-debug.c
+++ b/kernel/locking/mutex-debug.c
@@ -54,13 +54,13 @@ void debug_mutex_add_waiter(struct mutex *lock, struct mutex_waiter *waiter,
 	lockdep_assert_held(&lock->wait_lock);
 
 	/* Current thread can't be already blocked (since it's executing!) */
-	DEBUG_LOCKS_WARN_ON(task->blocked_on);
+	DEBUG_LOCKS_WARN_ON(__get_task_blocked_on(task));
 }
 
 void debug_mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter,
 			 struct task_struct *task)
 {
-	struct mutex *blocked_on = READ_ONCE(task->blocked_on);
+	struct mutex *blocked_on = __get_task_blocked_on(task);
 
 	DEBUG_LOCKS_WARN_ON(list_empty(&waiter->list));
 	DEBUG_LOCKS_WARN_ON(waiter->task != task);
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index e2f59863a866e..80d778fedd605 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -644,8 +644,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 			goto err_early_kill;
 	}
 
-	WARN_ON(current->blocked_on);
-	current->blocked_on = lock;
+	__set_task_blocked_on(current, lock);
 	set_current_state(state);
 	trace_contention_begin(lock, LCB_F_MUTEX);
 	for (;;) {
@@ -685,9 +684,9 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 		/*
 		 * As we likely have been woken up by task
 		 * that has cleared our blocked_on state, re-set
-		 * it to the lock we are trying to aquire.
+		 * it to the lock we are trying to acquire.
 		 */
-		current->blocked_on = lock;
+		set_task_blocked_on(current, lock);
 		set_current_state(state);
 		/*
 		 * Here we order against unlock; we must either see it change
@@ -699,11 +698,15 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 
 		if (first) {
 			trace_contention_begin(lock, LCB_F_MUTEX | LCB_F_SPIN);
-			/* clear blocked_on as mutex_optimistic_spin may schedule() */
-			current->blocked_on = NULL;
+			/*
+			 * mutex_optimistic_spin() can call schedule(), so
+			 * clear blocked on so we don't become unselectable
+			 * to run.
+			 */
+			clear_task_blocked_on(current, lock);
 			if (mutex_optimistic_spin(lock, ww_ctx, &waiter))
 				break;
-			current->blocked_on = lock;
+			set_task_blocked_on(current, lock);
 			trace_contention_begin(lock, LCB_F_MUTEX);
 		}
 
@@ -711,7 +714,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 	}
 	raw_spin_lock_irqsave(&lock->wait_lock, flags);
 acquired:
-	current->blocked_on = NULL;
+	__clear_task_blocked_on(current, lock);
 	__set_current_state(TASK_RUNNING);
 
 	if (ww_ctx) {
@@ -741,11 +744,11 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 	return 0;
 
 err:
-	current->blocked_on = NULL;
+	__clear_task_blocked_on(current, lock);
 	__set_current_state(TASK_RUNNING);
 	__mutex_remove_waiter(lock, &waiter);
 err_early_kill:
-	WARN_ON(current->blocked_on);
+	WARN_ON(__get_task_blocked_on(current));
 	trace_contention_end(lock, ret);
 	raw_spin_unlock_irqrestore_wake(&lock->wait_lock, flags, &wake_q);
 	debug_mutex_free_waiter(&waiter);
@@ -956,14 +959,7 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
 		next = waiter->task;
 
 		debug_mutex_wake_waiter(lock, waiter);
-		/*
-		 * Unlock wakeups can be happening in parallel
-		 * (when optimistic spinners steal and release
-		 * the lock), so blocked_on may already be
-		 * cleared here.
-		 */
-		WARN_ON(next->blocked_on && next->blocked_on != lock);
-		next->blocked_on = NULL;
+		__clear_task_blocked_on(next, lock);
 		wake_q_add(&wake_q, next);
 	}
 
diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
index 00db40946328e..086fd5487ca77 100644
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -289,9 +289,7 @@ __ww_mutex_die(struct MUTEX *lock, struct MUTEX_WAITER *waiter,
 		 * blocked_on pointer. Otherwise we can see circular
 		 * blocked_on relationships that can't resolve.
 		 */
-		WARN_ON(waiter->task->blocked_on &&
-			waiter->task->blocked_on != lock);
-		waiter->task->blocked_on = NULL;
+		__clear_task_blocked_on(waiter->task, lock);
 		wake_q_add(wake_q, waiter->task);
 	}
 
@@ -345,7 +343,7 @@ static bool __ww_mutex_wound(struct MUTEX *lock,
 			 * blocked_on pointer. Otherwise we can see circular
 			 * blocked_on relationships that can't resolve.
 			 */
-			owner->blocked_on = NULL;
+			__clear_task_blocked_on(owner, lock);
 			wake_q_add(wake_q, owner);
 		}
 		return true;
-- 
2.50.0.727.gbf7dc18ff4-goog

[RESEND][PATCH v18 4/8] sched: Move update_curr_task logic into update_curr_se

[John Stultz]

Absorb update_curr_task() into update_curr_se(), and
in the process simplify update_curr_common().

This will make the next step a bit easier.

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: John Stultz <jstultz@google.com>
---
Cc: Joel Fernandes <joelagnelf@nvidia.com>
Cc: Qais Yousef <qyousef@layalina.io>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Zimuzo Ezeozue <zezeozue@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Metin Kaya <Metin.Kaya@arm.com>
Cc: Xuewen Yan <xuewen.yan94@gmail.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: kuyo chang <kuyo.chang@mediatek.com>
Cc: hupu <hupu.gm@gmail.com>
Cc: kernel-team@android.com
---
 kernel/sched/fair.c | 26 +++++++++-----------------
 1 file changed, 9 insertions(+), 17 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7a14da5396fb2..edcc7d59ecc3b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1155,6 +1155,14 @@ static s64 update_curr_se(struct rq *rq, struct sched_entity *curr)
 	curr->exec_start = now;
 	curr->sum_exec_runtime += delta_exec;
 
+	if (entity_is_task(curr)) {
+		struct task_struct *p = task_of(curr);
+
+		trace_sched_stat_runtime(p, delta_exec);
+		account_group_exec_runtime(p, delta_exec);
+		cgroup_account_cputime(p, delta_exec);
+	}
+
 	if (schedstat_enabled()) {
 		struct sched_statistics *stats;
 
@@ -1166,13 +1174,6 @@ static s64 update_curr_se(struct rq *rq, struct sched_entity *curr)
 	return delta_exec;
 }
 
-static inline void update_curr_task(struct task_struct *p, s64 delta_exec)
-{
-	trace_sched_stat_runtime(p, delta_exec);
-	account_group_exec_runtime(p, delta_exec);
-	cgroup_account_cputime(p, delta_exec);
-}
-
 static inline bool did_preempt_short(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 {
 	if (!sched_feat(PREEMPT_SHORT))
@@ -1211,13 +1212,8 @@ static inline bool do_preempt_short(struct cfs_rq *cfs_rq,
 s64 update_curr_common(struct rq *rq)
 {
 	struct task_struct *donor = rq->donor;
-	s64 delta_exec;
 
-	delta_exec = update_curr_se(rq, &donor->se);
-	if (likely(delta_exec > 0))
-		update_curr_task(donor, delta_exec);
-
-	return delta_exec;
+	return update_curr_se(rq, &donor->se);
 }
 
 /*
@@ -1242,10 +1238,6 @@ static void update_curr(struct cfs_rq *cfs_rq)
 	update_min_vruntime(cfs_rq);
 
 	if (entity_is_task(curr)) {
-		struct task_struct *p = task_of(curr);
-
-		update_curr_task(p, delta_exec);
-
 		/*
 		 * If the fair_server is active, we need to account for the
 		 * fair_server time whether or not the task is running on
-- 
2.50.0.727.gbf7dc18ff4-goog

[RESEND][PATCH v18 5/8] sched: Fix runtime accounting w/ split exec & sched contexts

[John Stultz]

Without proxy-exec, we normally charge the "current" task for
both its vruntime as well as its sum_exec_runtime.

With proxy, however, we have two "current" contexts: the
scheduler context and the execution context. We want to charge
the execution context rq->curr (ie: proxy/lock holder) execution
time to its sum_exec_runtime (so it's clear to userland the
rq->curr task *is* running), as well as its thread group.

However the rest of the time accounting (such a vruntime and
cgroup accounting), we charge against the scheduler context
(rq->donor) task, because it is from that task that the time
is being "donated".

If the donor and curr tasks are the same, then it's the same as
without proxy.

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: John Stultz <jstultz@google.com>
---
v16:
* Renamed update_curr_se to update_se_times, as suggested by
  Steven Rostedt.
* Reworded the commit message as suggested by Steven Rostedt
v17:
* Renamed update_se_times to update_se, as suggested by Peter
* Added comment calrifying cfs_rq->curr being rq->donor.se as
  suggested by Peter
* Reworked to fix issue Peter pointed out with thread group
  accounting being done on the donor, rather then the running
  execution context.
* Slight rewording of the commit message to further clairifiy
  things.

Cc: Joel Fernandes <joelagnelf@nvidia.com>
Cc: Qais Yousef <qyousef@layalina.io>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Zimuzo Ezeozue <zezeozue@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Metin Kaya <Metin.Kaya@arm.com>
Cc: Xuewen Yan <xuewen.yan94@gmail.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: kuyo chang <kuyo.chang@mediatek.com>
Cc: hupu <hupu.gm@gmail.com>
Cc: kernel-team@android.com
---
 kernel/sched/fair.c | 40 ++++++++++++++++++++++++++++------------
 1 file changed, 28 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index edcc7d59ecc3b..c34e0891193a7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1143,30 +1143,40 @@ static void update_tg_load_avg(struct cfs_rq *cfs_rq)
 }
 #endif /* CONFIG_SMP */
 
-static s64 update_curr_se(struct rq *rq, struct sched_entity *curr)
+static s64 update_se(struct rq *rq, struct sched_entity *se)
 {
 	u64 now = rq_clock_task(rq);
 	s64 delta_exec;
 
-	delta_exec = now - curr->exec_start;
+	delta_exec = now - se->exec_start;
 	if (unlikely(delta_exec <= 0))
 		return delta_exec;
 
-	curr->exec_start = now;
-	curr->sum_exec_runtime += delta_exec;
+	se->exec_start = now;
+	if (entity_is_task(se)) {
+		struct task_struct *donor = task_of(se);
+		struct task_struct *running = rq->curr;
+		/*
+		 * If se is a task, we account the time against the running
+		 * task, as w/ proxy-exec they may not be the same.
+		 */
+		running->se.exec_start = now;
+		running->se.sum_exec_runtime += delta_exec;
 
-	if (entity_is_task(curr)) {
-		struct task_struct *p = task_of(curr);
+		trace_sched_stat_runtime(running, delta_exec);
+		account_group_exec_runtime(running, delta_exec);
 
-		trace_sched_stat_runtime(p, delta_exec);
-		account_group_exec_runtime(p, delta_exec);
-		cgroup_account_cputime(p, delta_exec);
+		/* cgroup time is always accounted against the donor */
+		cgroup_account_cputime(donor, delta_exec);
+	} else {
+		/* If not task, account the time against donor se  */
+		se->sum_exec_runtime += delta_exec;
 	}
 
 	if (schedstat_enabled()) {
 		struct sched_statistics *stats;
 
-		stats = __schedstats_from_se(curr);
+		stats = __schedstats_from_se(se);
 		__schedstat_set(stats->exec_max,
 				max(delta_exec, stats->exec_max));
 	}
@@ -1213,7 +1223,7 @@ s64 update_curr_common(struct rq *rq)
 {
 	struct task_struct *donor = rq->donor;
 
-	return update_curr_se(rq, &donor->se);
+	return update_se(rq, &donor->se);
 }
 
 /*
@@ -1221,6 +1231,12 @@ s64 update_curr_common(struct rq *rq)
  */
 static void update_curr(struct cfs_rq *cfs_rq)
 {
+	/*
+	 * Note: cfs_rq->curr corresponds to the task picked to
+	 * run (ie: rq->donor.se) which due to proxy-exec may
+	 * not necessarily be the actual task running
+	 * (rq->curr.se). This is easy to confuse!
+	 */
 	struct sched_entity *curr = cfs_rq->curr;
 	struct rq *rq = rq_of(cfs_rq);
 	s64 delta_exec;
@@ -1229,7 +1245,7 @@ static void update_curr(struct cfs_rq *cfs_rq)
 	if (unlikely(!curr))
 		return;
 
-	delta_exec = update_curr_se(rq, curr);
+	delta_exec = update_se(rq, curr);
 	if (unlikely(delta_exec <= 0))
 		return;
 
-- 
2.50.0.727.gbf7dc18ff4-goog

[RESEND][PATCH v18 6/8] sched: Add an initial sketch of the find_proxy_task() function

[John Stultz]

Add a find_proxy_task() function which doesn't do much.

When we select a blocked task to run, we will just deactivate it
and pick again. The exception being if it has become unblocked
after find_proxy_task() was called.

This allows us to validate keeping blocked tasks on the runqueue
and later deactivating them is working ok, stressing the failure
cases for when a proxy isn't found.

Greatly simplified from patch by:
  Peter Zijlstra (Intel) <peterz@infradead.org>
  Juri Lelli <juri.lelli@redhat.com>
  Valentin Schneider <valentin.schneider@arm.com>
  Connor O'Brien <connoro@google.com>

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
[jstultz: Split out from larger proxy patch and simplified
 for review and testing.]
Signed-off-by: John Stultz <jstultz@google.com>
---
v5:
* Split out from larger proxy patch
v7:
* Fixed unused function arguments, spelling nits, and tweaks for
  clarity, pointed out by Metin Kaya
* Fix build warning Reported-by: kernel test robot <lkp@intel.com>
  Closes: https://lore.kernel.org/oe-kbuild-all/202311081028.yDLmCWgr-lkp@intel.com/
v8:
* Fixed case where we might return a blocked task from find_proxy_task()
* Continued tweaks to handle avoiding returning blocked tasks
v9:
* Add zap_balance_callbacks helper to unwind balance_callbacks
  when we will re-call pick_next_task() again.
* Add extra comment suggested by Metin
* Typo fixes from Metin
* Moved adding proxy_resched_idle earlier in the series, as suggested
  by Metin
* Fix to call proxy_resched_idle() *prior* to deactivating next, to avoid
  crashes caused by stale references to next
* s/PROXY/SCHED_PROXY_EXEC/ as suggested by Metin
* Number of tweaks and cleanups suggested by Metin
* Simplify proxy_deactivate as suggested by Metin
v11:
* Tweaks for earlier simplification in try_to_deactivate_task
v13:
* Rename rename "next" to "donor" in find_proxy_task() for clarity
* Similarly use "donor" instead of next in proxy_deactivate
* Refactor/simplify proxy_resched_idle
* Moved up a needed fix from later in the series
v15:
* Tweaked some comments to better explain the initial sketch of
  find_proxy_task(), suggested by Qais
* Build fixes for !CONFIG_SMP
* Slight rework for blocked_on_state being added later in the
  series.
* Move the zap_balance_callbacks to later in the patch series
v16:
* Move the enqueue_task_rt() out to later in the series, as
  suggested by K Prateek Nayak
* Fixup whitespace error pointed out by K Prateek Nayak
* Use put_prev_set_next_task as suggested by K Prateek Nayak
* Try to rework find_proxy_task() locking to use guard and
  proxy_deactivate_task() in the way Peter suggested.
v17:
* Slightly simplified variable names per suggestion from
  Juri Lelli
* Minor comment and commit message tweaks suggested by Peter

Cc: Joel Fernandes <joelagnelf@nvidia.com>
Cc: Qais Yousef <qyousef@layalina.io>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Zimuzo Ezeozue <zezeozue@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Metin Kaya <Metin.Kaya@arm.com>
Cc: Xuewen Yan <xuewen.yan94@gmail.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: kuyo chang <kuyo.chang@mediatek.com>
Cc: hupu <hupu.gm@gmail.com>
Cc: kernel-team@android.com
---
 kernel/sched/core.c  | 100 +++++++++++++++++++++++++++++++++++++++++--
 kernel/sched/sched.h |  10 ++++-
 2 files changed, 106 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 853157b27f384..dc82d9b8bee2c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6614,7 +6614,8 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
  * Otherwise marks the task's __state as RUNNING
  */
 static bool try_to_block_task(struct rq *rq, struct task_struct *p,
-			      unsigned long *task_state_p)
+			      unsigned long *task_state_p,
+			      bool deactivate_cond)
 {
 	unsigned long task_state = *task_state_p;
 	int flags = DEQUEUE_NOCLOCK;
@@ -6625,6 +6626,9 @@ static bool try_to_block_task(struct rq *rq, struct task_struct *p,
 		return false;
 	}
 
+	if (!deactivate_cond)
+		return false;
+
 	p->sched_contributes_to_load =
 		(task_state & TASK_UNINTERRUPTIBLE) &&
 		!(task_state & TASK_NOLOAD) &&
@@ -6648,6 +6652,89 @@ static bool try_to_block_task(struct rq *rq, struct task_struct *p,
 	return true;
 }
 
+#ifdef CONFIG_SCHED_PROXY_EXEC
+static inline struct task_struct *proxy_resched_idle(struct rq *rq)
+{
+	put_prev_set_next_task(rq, rq->donor, rq->idle);
+	rq_set_donor(rq, rq->idle);
+	set_tsk_need_resched(rq->idle);
+	return rq->idle;
+}
+
+static bool __proxy_deactivate(struct rq *rq, struct task_struct *donor)
+{
+	unsigned long state = READ_ONCE(donor->__state);
+
+	/* Don't deactivate if the state has been changed to TASK_RUNNING */
+	if (state == TASK_RUNNING)
+		return false;
+	/*
+	 * Because we got donor from pick_next_task(), it is *crucial*
+	 * that we call proxy_resched_idle() before we deactivate it.
+	 * As once we deactivate donor, donor->on_rq is set to zero,
+	 * which allows ttwu() to immediately try to wake the task on
+	 * another rq. So we cannot use *any* references to donor
+	 * after that point. So things like cfs_rq->curr or rq->donor
+	 * need to be changed from next *before* we deactivate.
+	 */
+	proxy_resched_idle(rq);
+	return try_to_block_task(rq, donor, &state, true);
+}
+
+static struct task_struct *proxy_deactivate(struct rq *rq, struct task_struct *donor)
+{
+	if (!__proxy_deactivate(rq, donor)) {
+		/*
+		 * XXX: For now, if deactivation failed, set donor
+		 * as unblocked, as we aren't doing proxy-migrations
+		 * yet (more logic will be needed then).
+		 */
+		donor->blocked_on = NULL;
+	}
+	return NULL;
+}
+
+/*
+ * Initial simple sketch that just deactivates the blocked task
+ * chosen by pick_next_task() so we can then pick something that
+ * isn't blocked.
+ */
+static struct task_struct *
+find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
+{
+	struct mutex *mutex;
+
+	mutex = donor->blocked_on;
+	/* Something changed in the chain, so pick again */
+	if (!mutex)
+		return NULL;
+	/*
+	 * By taking mutex->wait_lock we hold off concurrent mutex_unlock()
+	 * and ensure @owner sticks around.
+	 */
+	guard(raw_spinlock)(&mutex->wait_lock);
+
+	/* Check again that donor is blocked with blocked_lock held */
+	if (!task_is_blocked(donor) || mutex != __get_task_blocked_on(donor)) {
+		/*
+		 * Something changed in the blocked_on chain and
+		 * we don't know if only at this level. So, let's
+		 * just bail out completely and let __schedule()
+		 * figure things out (pick_again loop).
+		 */
+		return NULL; /* do pick_next_task() again */
+	}
+	return proxy_deactivate(rq, donor);
+}
+#else /* SCHED_PROXY_EXEC */
+static struct task_struct *
+find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
+{
+	WARN_ONCE(1, "This should never be called in the !SCHED_PROXY_EXEC case\n");
+	return donor;
+}
+#endif /* SCHED_PROXY_EXEC */
+
 /*
  * __schedule() is the main scheduler function.
  *
@@ -6760,12 +6847,19 @@ static void __sched notrace __schedule(int sched_mode)
 			goto picked;
 		}
 	} else if (!preempt && prev_state) {
-		try_to_block_task(rq, prev, &prev_state);
+		try_to_block_task(rq, prev, &prev_state,
+				  !task_is_blocked(prev));
 		switch_count = &prev->nvcsw;
 	}
 
-	next = pick_next_task(rq, prev, &rf);
+pick_again:
+	next = pick_next_task(rq, rq->donor, &rf);
 	rq_set_donor(rq, next);
+	if (unlikely(task_is_blocked(next))) {
+		next = find_proxy_task(rq, next, &rf);
+		if (!next)
+			goto pick_again;
+	}
 picked:
 	clear_tsk_need_resched(prev);
 	clear_preempt_need_resched();
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 6b61e0c7f6e78..590a44c0215fb 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2290,6 +2290,14 @@ static inline int task_current_donor(struct rq *rq, struct task_struct *p)
 	return rq->donor == p;
 }
 
+static inline bool task_is_blocked(struct task_struct *p)
+{
+	if (!sched_proxy_exec())
+		return false;
+
+	return !!p->blocked_on;
+}
+
 static inline int task_on_cpu(struct rq *rq, struct task_struct *p)
 {
 #ifdef CONFIG_SMP
@@ -2499,7 +2507,7 @@ static inline void put_prev_set_next_task(struct rq *rq,
 					  struct task_struct *prev,
 					  struct task_struct *next)
 {
-	WARN_ON_ONCE(rq->curr != prev);
+	WARN_ON_ONCE(rq->donor != prev);
 
 	__put_prev_set_next_dl_server(rq, prev, next);
 
-- 
2.50.0.727.gbf7dc18ff4-goog

[RESEND][PATCH v18 7/8] sched: Fix proxy/current (push,pull)ability

[John Stultz]

From: Valentin Schneider <valentin.schneider@arm.com>

Proxy execution forms atomic pairs of tasks: The waiting donor
task (scheduling context) and a proxy (execution context). The
donor task, along with the rest of the blocked chain, follows
the proxy wrt CPU placement.

They can be the same task, in which case push/pull doesn't need any
modification. When they are different, however,
FIFO1 & FIFO42:

	      ,->  RT42
	      |     | blocked-on
	      |     v
blocked_donor |   mutex
	      |     | owner
	      |     v
	      `--  RT1

   RT1
   RT42

  CPU0            CPU1
   ^                ^
   |                |
  overloaded    !overloaded
  rq prio = 42  rq prio = 0

RT1 is eligible to be pushed to CPU1, but should that happen it will
"carry" RT42 along. Clearly here neither RT1 nor RT42 must be seen as
push/pullable.

Unfortunately, only the donor task is usually dequeued from the rq,
and the proxy'ed execution context (rq->curr) remains on the rq.
This can cause RT1 to be selected for migration from logic like the
rt pushable_list.

Thus, adda a dequeue/enqueue cycle on the proxy task before __schedule
returns, which allows the sched class logic to avoid adding the now
current task to the pushable_list.

Furthermore, tasks becoming blocked on a mutex don't need an explicit
dequeue/enqueue cycle to be made (push/pull)able: they have to be running
to block on a mutex, thus they will eventually hit put_prev_task().

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Connor O'Brien <connoro@google.com>
Signed-off-by: John Stultz <jstultz@google.com>
---
v3:
* Tweaked comments & commit message
v5:
* Minor simplifications to utilize the fix earlier
  in the patch series.
* Rework the wording of the commit message to match selected/
  proxy terminology and expand a bit to make it more clear how
  it works.
v6:
* Dropped now-unused proxied value, to be re-added later in the
  series when it is used, as caught by Dietmar
v7:
* Unused function argument fixup
* Commit message nit pointed out by Metin Kaya
* Dropped unproven unlikely() and use sched_proxy_exec()
  in proxy_tag_curr, suggested by Metin Kaya
v8:
* More cleanups and typo fixes suggested by Metin Kaya
v11:
* Cleanup of comimt message suggested by Metin
v12:
* Rework for rq_selected -> rq->donor renaming
v16:
* Pulled logic from later patch in to avoid sched_balance
  migrating blocked tasks.
* Moved enqueue_task_rt() logic from earlier into this patch
  as suggested by K Prateek Nayak
* Simplified changes to enqueue_task_rt to match deadline's
  logic, as pointed out by Peter
v17:
* Add similar logic to put_prev_task_dl as pointed out by
  K Prateek Nayak

Cc: Joel Fernandes <joelagnelf@nvidia.com>
Cc: Qais Yousef <qyousef@layalina.io>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Zimuzo Ezeozue <zezeozue@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Metin Kaya <Metin.Kaya@arm.com>
Cc: Xuewen Yan <xuewen.yan94@gmail.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: kuyo chang <kuyo.chang@mediatek.com>
Cc: hupu <hupu.gm@gmail.com>
Cc: kernel-team@android.com
---
 kernel/sched/core.c     | 25 +++++++++++++++++++++++++
 kernel/sched/deadline.c |  7 +++++++
 kernel/sched/rt.c       |  5 +++++
 3 files changed, 37 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index dc82d9b8bee2c..98a802e0a3f36 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6735,6 +6735,23 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
 }
 #endif /* SCHED_PROXY_EXEC */
 
+static inline void proxy_tag_curr(struct rq *rq, struct task_struct *owner)
+{
+	if (!sched_proxy_exec())
+		return;
+	/*
+	 * pick_next_task() calls set_next_task() on the chosen task
+	 * at some point, which ensures it is not push/pullable.
+	 * However, the chosen/donor task *and* the mutex owner form an
+	 * atomic pair wrt push/pull.
+	 *
+	 * Make sure owner we run is not pushable. Unfortunately we can
+	 * only deal with that by means of a dequeue/enqueue cycle. :-/
+	 */
+	dequeue_task(rq, owner, DEQUEUE_NOCLOCK | DEQUEUE_SAVE);
+	enqueue_task(rq, owner, ENQUEUE_NOCLOCK | ENQUEUE_RESTORE);
+}
+
 /*
  * __schedule() is the main scheduler function.
  *
@@ -6873,6 +6890,10 @@ static void __sched notrace __schedule(int sched_mode)
 		 * changes to task_struct made by pick_next_task().
 		 */
 		RCU_INIT_POINTER(rq->curr, next);
+
+		if (!task_current_donor(rq, next))
+			proxy_tag_curr(rq, next);
+
 		/*
 		 * The membarrier system call requires each architecture
 		 * to have a full memory barrier after updating
@@ -6907,6 +6928,10 @@ static void __sched notrace __schedule(int sched_mode)
 		/* Also unlocks the rq: */
 		rq = context_switch(rq, prev, next, &rf);
 	} else {
+		/* In case next was already curr but just got blocked_donor */
+		if (!task_current_donor(rq, next))
+			proxy_tag_curr(rq, next);
+
 		rq_unpin_lock(rq, &rf);
 		__balance_callbacks(rq);
 		raw_spin_rq_unlock_irq(rq);
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index ad45a8fea245e..4757c29d0979e 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2166,6 +2166,9 @@ static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
 	if (dl_server(&p->dl))
 		return;
 
+	if (task_is_blocked(p))
+		return;
+
 	if (!task_current(rq, p) && !p->dl.dl_throttled && p->nr_cpus_allowed > 1)
 		enqueue_pushable_dl_task(rq, p);
 }
@@ -2465,6 +2468,10 @@ static void put_prev_task_dl(struct rq *rq, struct task_struct *p, struct task_s
 	update_curr_dl(rq);
 
 	update_dl_rq_load_avg(rq_clock_pelt(rq), rq, 1);
+
+	if (task_is_blocked(p))
+		return;
+
 	if (on_dl_rq(&p->dl) && p->nr_cpus_allowed > 1)
 		enqueue_pushable_dl_task(rq, p);
 }
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index e40422c370335..b1fc9e6cf4a60 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1488,6 +1488,9 @@ enqueue_task_rt(struct rq *rq, struct task_struct *p, int flags)
 
 	enqueue_rt_entity(rt_se, flags);
 
+	if (task_is_blocked(p))
+		return;
+
 	if (!task_current(rq, p) && p->nr_cpus_allowed > 1)
 		enqueue_pushable_task(rq, p);
 }
@@ -1768,6 +1771,8 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p, struct task_s
 
 	update_rt_rq_load_avg(rq_clock_pelt(rq), rq, 1);
 
+	if (task_is_blocked(p))
+		return;
 	/*
 	 * The previous task needs to be made eligible for pushing
 	 * if it is still active
-- 
2.50.0.727.gbf7dc18ff4-goog

[RESEND][PATCH v18 8/8] sched: Start blocked_on chain processing in find_proxy_task()

[John Stultz]

From: Peter Zijlstra <peterz@infradead.org>

Start to flesh out the real find_proxy_task() implementation,
but avoid the migration cases for now, in those cases just
deactivate the donor task and pick again.

To ensure the donor task or other blocked tasks in the chain
aren't migrated away while we're running the proxy, also tweak
the fair class logic to avoid migrating donor or mutex blocked
tasks.

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Connor O'Brien <connoro@google.com>
[jstultz: This change was split out from the larger proxy patch]
Signed-off-by: John Stultz <jstultz@google.com>
---
v5:
* Split this out from larger proxy patch
v7:
* Minor refactoring of core find_proxy_task() function
* Minor spelling and corrections suggested by Metin Kaya
* Dropped an added BUG_ON that was frequently tripped
v8:
* Fix issue if proxy_deactivate fails, we don't leave task
  BO_BLOCKED
* Switch to WARN_ON from BUG_ON checks
v9:
* Improve comments suggested by Metin
* Minor cleanups
v11:
* Previously we checked next==rq->idle && prev==rq->idle, but I
  think we only really care if next==rq->idle from find_proxy_task,
  as we will still want to resched regardless of what prev was.
v12:
* Commit message rework for selected -> donor rewording
v13:
* Address new delayed dequeue condition (deactivate donor for now)
* Next to donor renaming in find_proxy_task
* Improved comments for find_proxy_task
* Rework for proxy_deactivate cleanup
v14:
* Fix build error from __mutex_owner() with CONFIG_PREEMPT_RT
v15:
* Reworks for moving blocked_on_state to later in the series
v16:
* Pull down fix from later in the series where a deactivated
  task could pass the (task_cpu(owner) == this_cpu) check
  then have it be activated on a different cpu, so it passes
  the on_rq check. Thus double check the values in the opposite
  order to make sure nothing slips by.
* Add resched_idle label to simplify common exit path
* Get rid of preserve_need_resched flag and rework per Peter's
  suggestion
* Rework find_proxy_task() to use guard to cleanup the exit gotos
  as Peter suggested.
v18:
* Minor tweak to order of logic in find_proxy_task() to enable
  an optimization later in the full series suggested by Suleiman
* Use READ_ONCE calls to on_rq references as I hit a bug where
  the compiler was optimizing out some of the reads.

Cc: Joel Fernandes <joelagnelf@nvidia.com>
Cc: Qais Yousef <qyousef@layalina.io>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Zimuzo Ezeozue <zezeozue@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Metin Kaya <Metin.Kaya@arm.com>
Cc: Xuewen Yan <xuewen.yan94@gmail.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: kuyo chang <kuyo.chang@mediatek.com>
Cc: hupu <hupu.gm@gmail.com>
Cc: kernel-team@android.com
---
 kernel/locking/mutex.h |   3 +-
 kernel/sched/core.c    | 143 ++++++++++++++++++++++++++++++++++-------
 kernel/sched/fair.c    |  11 +++-
 3 files changed, 132 insertions(+), 25 deletions(-)

diff --git a/kernel/locking/mutex.h b/kernel/locking/mutex.h
index cbff35b9b7ae3..2e8080a9bee37 100644
--- a/kernel/locking/mutex.h
+++ b/kernel/locking/mutex.h
@@ -6,7 +6,7 @@
  *
  *  Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
  */
-
+#ifndef CONFIG_PREEMPT_RT
 /*
  * This is the control structure for tasks blocked on mutex, which resides
  * on the blocked task's kernel stack:
@@ -70,3 +70,4 @@ extern void debug_mutex_init(struct mutex *lock, const char *name,
 # define debug_mutex_unlock(lock)			do { } while (0)
 # define debug_mutex_init(lock, name, key)		do { } while (0)
 #endif /* !CONFIG_DEBUG_MUTEXES */
+#endif /* CONFIG_PREEMPT_RT */
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 98a802e0a3f36..1c7573d0cd312 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -96,6 +96,7 @@
 #include "../workqueue_internal.h"
 #include "../../io_uring/io-wq.h"
 #include "../smpboot.h"
+#include "../locking/mutex.h"
 
 EXPORT_TRACEPOINT_SYMBOL_GPL(ipi_send_cpu);
 EXPORT_TRACEPOINT_SYMBOL_GPL(ipi_send_cpumask);
@@ -2965,8 +2966,15 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
 	struct set_affinity_pending my_pending = { }, *pending = NULL;
 	bool stop_pending, complete = false;
 
-	/* Can the task run on the task's current CPU? If so, we're done */
-	if (cpumask_test_cpu(task_cpu(p), &p->cpus_mask)) {
+	/*
+	 * Can the task run on the task's current CPU? If so, we're done
+	 *
+	 * We are also done if the task is the current donor, boosting a lock-
+	 * holding proxy, (and potentially has been migrated outside its
+	 * current or previous affinity mask)
+	 */
+	if (cpumask_test_cpu(task_cpu(p), &p->cpus_mask) ||
+	    (task_current_donor(rq, p) && !task_current(rq, p))) {
 		struct task_struct *push_task = NULL;
 
 		if ((flags & SCA_MIGRATE_ENABLE) &&
@@ -6695,36 +6703,124 @@ static struct task_struct *proxy_deactivate(struct rq *rq, struct task_struct *d
 }
 
 /*
- * Initial simple sketch that just deactivates the blocked task
- * chosen by pick_next_task() so we can then pick something that
- * isn't blocked.
+ * Find runnable lock owner to proxy for mutex blocked donor
+ *
+ * Follow the blocked-on relation:
+ *   task->blocked_on -> mutex->owner -> task...
+ *
+ * Lock order:
+ *
+ *   p->pi_lock
+ *     rq->lock
+ *       mutex->wait_lock
+ *
+ * Returns the task that is going to be used as execution context (the one
+ * that is actually going to be run on cpu_of(rq)).
  */
 static struct task_struct *
 find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
 {
+	struct task_struct *owner = NULL;
+	int this_cpu = cpu_of(rq);
+	struct task_struct *p;
 	struct mutex *mutex;
 
-	mutex = donor->blocked_on;
-	/* Something changed in the chain, so pick again */
-	if (!mutex)
-		return NULL;
-	/*
-	 * By taking mutex->wait_lock we hold off concurrent mutex_unlock()
-	 * and ensure @owner sticks around.
-	 */
-	guard(raw_spinlock)(&mutex->wait_lock);
+	/* Follow blocked_on chain. */
+	for (p = donor; task_is_blocked(p); p = owner) {
+		mutex = p->blocked_on;
+		/* Something changed in the chain, so pick again */
+		if (!mutex)
+			return NULL;
+		/*
+		 * By taking mutex->wait_lock we hold off concurrent mutex_unlock()
+		 * and ensure @owner sticks around.
+		 */
+		guard(raw_spinlock)(&mutex->wait_lock);
 
-	/* Check again that donor is blocked with blocked_lock held */
-	if (!task_is_blocked(donor) || mutex != __get_task_blocked_on(donor)) {
+		/* Check again that p is blocked with wait_lock held */
+		if (mutex != __get_task_blocked_on(p)) {
+			/*
+			 * Something changed in the blocked_on chain and
+			 * we don't know if only at this level. So, let's
+			 * just bail out completely and let __schedule()
+			 * figure things out (pick_again loop).
+			 */
+			return NULL;
+		}
+
+		owner = __mutex_owner(mutex);
+		if (!owner) {
+			__clear_task_blocked_on(p, mutex);
+			return p;
+		}
+
+		if (!READ_ONCE(owner->on_rq) || owner->se.sched_delayed) {
+			/* XXX Don't handle blocked owners/delayed dequeue yet */
+			return proxy_deactivate(rq, donor);
+		}
+
+		if (task_cpu(owner) != this_cpu) {
+			/* XXX Don't handle migrations yet */
+			return proxy_deactivate(rq, donor);
+		}
+
+		if (task_on_rq_migrating(owner)) {
+			/*
+			 * One of the chain of mutex owners is currently migrating to this
+			 * CPU, but has not yet been enqueued because we are holding the
+			 * rq lock. As a simple solution, just schedule rq->idle to give
+			 * the migration a chance to complete. Much like the migrate_task
+			 * case we should end up back in find_proxy_task(), this time
+			 * hopefully with all relevant tasks already enqueued.
+			 */
+			return proxy_resched_idle(rq);
+		}
+
+		/*
+		 * Its possible to race where after we check owner->on_rq
+		 * but before we check (owner_cpu != this_cpu) that the
+		 * task on another cpu was migrated back to this cpu. In
+		 * that case it could slip by our  checks. So double check
+		 * we are still on this cpu and not migrating. If we get
+		 * inconsistent results, try again.
+		 */
+		if (!task_on_rq_queued(owner) || task_cpu(owner) != this_cpu)
+			return NULL;
+
+		if (owner == p) {
+			/*
+			 * It's possible we interleave with mutex_unlock like:
+			 *
+			 *				lock(&rq->lock);
+			 *				  find_proxy_task()
+			 * mutex_unlock()
+			 *   lock(&wait_lock);
+			 *   donor(owner) = current->blocked_donor;
+			 *   unlock(&wait_lock);
+			 *
+			 *   wake_up_q();
+			 *     ...
+			 *       ttwu_runnable()
+			 *         __task_rq_lock()
+			 *				  lock(&wait_lock);
+			 *				  owner == p
+			 *
+			 * Which leaves us to finish the ttwu_runnable() and make it go.
+			 *
+			 * So schedule rq->idle so that ttwu_runnable() can get the rq
+			 * lock and mark owner as running.
+			 */
+			return proxy_resched_idle(rq);
+		}
 		/*
-		 * Something changed in the blocked_on chain and
-		 * we don't know if only at this level. So, let's
-		 * just bail out completely and let __schedule()
-		 * figure things out (pick_again loop).
+		 * OK, now we're absolutely sure @owner is on this
+		 * rq, therefore holding @rq->lock is sufficient to
+		 * guarantee its existence, as per ttwu_remote().
 		 */
-		return NULL; /* do pick_next_task() again */
 	}
-	return proxy_deactivate(rq, donor);
+
+	WARN_ON_ONCE(owner && !owner->on_rq);
+	return owner;
 }
 #else /* SCHED_PROXY_EXEC */
 static struct task_struct *
@@ -6876,10 +6972,13 @@ static void __sched notrace __schedule(int sched_mode)
 		next = find_proxy_task(rq, next, &rf);
 		if (!next)
 			goto pick_again;
+		if (next == rq->idle)
+			goto keep_resched;
 	}
 picked:
 	clear_tsk_need_resched(prev);
 	clear_preempt_need_resched();
+keep_resched:
 	rq->last_seen_need_resched_ns = 0;
 
 	is_switch = prev != next;
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c34e0891193a7..ea4331ed6bc23 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9415,7 +9415,8 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 	 * 2) throttled_lb_pair, or
 	 * 3) cannot be migrated to this CPU due to cpus_ptr, or
 	 * 4) running (obviously), or
-	 * 5) are cache-hot on their current CPU.
+	 * 5) are cache-hot on their current CPU or
+	 * 6) are blocked on mutexes (if SCHED_PROXY_EXEC is enabled)
 	 */
 	if ((p->se.sched_delayed) && (env->migration_type != migrate_load))
 		return 0;
@@ -9437,6 +9438,9 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 	if (kthread_is_per_cpu(p))
 		return 0;
 
+	if (task_is_blocked(p))
+		return 0;
+
 	if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) {
 		int cpu;
 
@@ -9472,7 +9476,8 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 	/* Record that we found at least one task that could run on dst_cpu */
 	env->flags &= ~LBF_ALL_PINNED;
 
-	if (task_on_cpu(env->src_rq, p)) {
+	if (task_on_cpu(env->src_rq, p) ||
+	    task_current_donor(env->src_rq, p)) {
 		schedstat_inc(p->stats.nr_failed_migrations_running);
 		return 0;
 	}
@@ -9515,6 +9520,8 @@ static void detach_task(struct task_struct *p, struct lb_env *env)
 		schedstat_inc(env->sd->lb_hot_gained[env->idle]);
 		schedstat_inc(p->stats.nr_forced_migrations);
 	}
+	WARN_ON(task_current(env->src_rq, p));
+	WARN_ON(task_current_donor(env->src_rq, p));
 
 	deactivate_task(env->src_rq, p, DEQUEUE_NOCLOCK);
 	set_task_cpu(p, env->dst_cpu);
-- 
2.50.0.727.gbf7dc18ff4-goog

Re: [RESEND][PATCH v18 3/8] locking/mutex: Add p->blocked_on wrappers for correctness checks

[K Prateek Nayak]

Hello John,

On 7/8/2025 2:13 AM, John Stultz wrote:
> @@ -2177,6 +2178,57 @@ extern int __cond_resched_rwlock_write(rwlock_t *lock);
>  	__cond_resched_rwlock_write(lock);					\
>  })
>  
> +static inline void __set_task_blocked_on(struct task_struct *p, struct mutex *m)
> +{
> +	WARN_ON_ONCE(!m);
> +	/* The task should only be setting itself as blocked */
> +	WARN_ON_ONCE(p != current);
> +	/* Currently we serialize blocked_on under the mutex::wait_lock */
> +	lockdep_assert_held_once(&m->wait_lock);

Sorry I didn't catch this earlier but building this with PREEMPT_RT fails
with the following error:

    ./include/linux/sched.h: In function ‘__set_task_blocked_on’:
    ./include/linux/sched.h:2187:36: error: ‘struct mutex’ has no member named ‘wait_lock’
     2187 |         lockdep_assert_held_once(&m->wait_lock);


Putting all these helpers behind a "#ifdef CONFIG_PREEMPT_RT" will then
lead to the this error:

    kernel/locking/ww_mutex.h:292:17: error: implicit declaration of function ‘__clear_task_blocked_on’ [-Werror=implicit-function-declaration]
      292 |                 __clear_task_blocked_on(waiter->task, lock);


Putting the "__clear_task_blocked_on()" calls in kernel/locking/ww_mutex.h
behind "#ifndef WW_RT" (which should start from Patch 2 since MUTEX and
MUTEX_WAITER for ww_mutex will resolve to rt_mutex and rt_mutex_waiter in
presence of WW_RT) solves all build issues with PREEMPT_RT. I'll let others
comment on the correctness tho :)


Following is the diff I used on top of this series for reference:

(build and boot tested only)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1d7f625adbb5..d47c9e4edf4f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2178,6 +2178,8 @@ extern int __cond_resched_rwlock_write(rwlock_t *lock);
 	__cond_resched_rwlock_write(lock);					\
 })
 
+#ifndef CONFIG_PREEMPT_RT
+
 static inline void __set_task_blocked_on(struct task_struct *p, struct mutex *m)
 {
 	WARN_ON_ONCE(!m);
@@ -2229,6 +2231,8 @@ static inline struct mutex *__get_task_blocked_on(struct task_struct *p)
 	return m;
 }
 
+#endif
+
 static __always_inline bool need_resched(void)
 {
 	return unlikely(tif_need_resched());
diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
index 086fd5487ca7..fd67a4a49892 100644
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -283,13 +283,14 @@ __ww_mutex_die(struct MUTEX *lock, struct MUTEX_WAITER *waiter,
 	if (waiter->ww_ctx->acquired > 0 && __ww_ctx_less(waiter->ww_ctx, ww_ctx)) {
 #ifndef WW_RT
 		debug_mutex_wake_waiter(lock, waiter);
-#endif
+
 		/*
 		 * When waking up the task to die, be sure to clear the
 		 * blocked_on pointer. Otherwise we can see circular
 		 * blocked_on relationships that can't resolve.
 		 */
 		__clear_task_blocked_on(waiter->task, lock);
+#endif
 		wake_q_add(wake_q, waiter->task);
 	}
 
@@ -338,12 +339,14 @@ static bool __ww_mutex_wound(struct MUTEX *lock,
 		 * wakeup pending to re-read the wounded state.
 		 */
 		if (owner != current) {
+#ifndef WW_RT
 			/*
 			 * When waking up the task to wound, be sure to clear the
 			 * blocked_on pointer. Otherwise we can see circular
 			 * blocked_on relationships that can't resolve.
 			 */
 			__clear_task_blocked_on(owner, lock);
+#endif
 			wake_q_add(wake_q, owner);
 		}
 		return true;
---

I'll make sure to give the PREEMPT_RT build a spin next time around.
Sorry for the oversight.

> +	/*
> +	 * Check ensure we don't overwrite existing mutex value
> +	 * with a different mutex. Note, setting it to the same
> +	 * lock repeatedly is ok.
> +	 */
> +	WARN_ON_ONCE(p->blocked_on && p->blocked_on != m);
> +	p->blocked_on = m;
> +}
> +
> +static inline void set_task_blocked_on(struct task_struct *p, struct mutex *m)
> +{
> +	guard(raw_spinlock_irqsave)(&m->wait_lock);
> +	__set_task_blocked_on(p, m);
> +}
> +
> +static inline void __clear_task_blocked_on(struct task_struct *p, struct mutex *m)
> +{
> +	WARN_ON_ONCE(!m);
> +	/* Currently we serialize blocked_on under the mutex::wait_lock */
> +	lockdep_assert_held_once(&m->wait_lock);
> +	/*
> +	 * There may be cases where we re-clear already cleared
> +	 * blocked_on relationships, but make sure we are not
> +	 * clearing the relationship with a different lock.
> +	 */
> +	WARN_ON_ONCE(m && p->blocked_on && p->blocked_on != m);
> +	p->blocked_on = NULL;
> +}
> +
> +static inline void clear_task_blocked_on(struct task_struct *p, struct mutex *m)
> +{
> +	guard(raw_spinlock_irqsave)(&m->wait_lock);
> +	__clear_task_blocked_on(p, m);
> +}
> +
> +static inline struct mutex *__get_task_blocked_on(struct task_struct *p)
> +{
> +	struct mutex *m = p->blocked_on;
> +
> +	if (m)
> +		lockdep_assert_held_once(&m->wait_lock);
> +	return m;
> +}
> +
>  static __always_inline bool need_resched(void)
>  {
>  	return unlikely(tif_need_resched());


-- 
Thanks and Regards,
Prateek

Re: [RESEND][PATCH v18 3/8] locking/mutex: Add p->blocked_on wrappers for correctness checks

[John Stultz]

On Mon, Jul 7, 2025 at 11:44 PM K Prateek Nayak <kprateek.nayak@amd.com> wrote:
> On 7/8/2025 2:13 AM, John Stultz wrote:
> > @@ -2177,6 +2178,57 @@ extern int __cond_resched_rwlock_write(rwlock_t *lock);
> >       __cond_resched_rwlock_write(lock);                                      \
> >  })
> >
> > +static inline void __set_task_blocked_on(struct task_struct *p, struct mutex *m)
> > +{
> > +     WARN_ON_ONCE(!m);
> > +     /* The task should only be setting itself as blocked */
> > +     WARN_ON_ONCE(p != current);
> > +     /* Currently we serialize blocked_on under the mutex::wait_lock */
> > +     lockdep_assert_held_once(&m->wait_lock);
>
> Sorry I didn't catch this earlier but building this with PREEMPT_RT fails
> with the following error:
...
> @@ -338,12 +339,14 @@ static bool __ww_mutex_wound(struct MUTEX *lock,
>                  * wakeup pending to re-read the wounded state.
>                  */
>                 if (owner != current) {
> +#ifndef WW_RT
>                         /*
>                          * When waking up the task to wound, be sure to clear the
>                          * blocked_on pointer. Otherwise we can see circular
>                          * blocked_on relationships that can't resolve.
>                          */
>                         __clear_task_blocked_on(owner, lock);
> +#endif
>                         wake_q_add(wake_q, owner);
>                 }
>                 return true;
> ---
>
> I'll make sure to give the PREEMPT_RT build a spin next time around.
> Sorry for the oversight.

No no, thank you for letting me know! Very much appreciate the detailed testing!

Between PROXY_EXEC, SMP and PREEMPT_RT, there's a fair number of
relevant combinations to test, and I've only done so in an ad-hoc
fashion (and I'm sure I'm still missing some).
So I've added a few defconfigs along with a build script to my full
tree so I can better automate build testing across all the patches in
the series, so this hopefully doesn't bite me again.
(Though let me know if you find something else, as I probably will
need to add another defconfig to test).

I've also tried to rework your suggestion above a little more deeply
into the helper logic so we don't have as many ifdefs in the
functions.  It unfortunately touches a lot of the patch series so its
been slow going in getting all the changes in the most sensible spots.

I'll continue testing and refining the changes locally and if there's
no other feedback on the series I'll send out a v19 near the end of
the week.

thanks
-john

Re: [RESEND][PATCH v18 5/8] sched: Fix runtime accounting w/ split exec & sched contexts

[Peter Zijlstra]

On Mon, Jul 07, 2025 at 08:43:52PM +0000, John Stultz wrote:

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index edcc7d59ecc3b..c34e0891193a7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1143,30 +1143,40 @@ static void update_tg_load_avg(struct cfs_rq *cfs_rq)
>  }
>  #endif /* CONFIG_SMP */
>  
> +static s64 update_se(struct rq *rq, struct sched_entity *se)
>  {
>  	u64 now = rq_clock_task(rq);
>  	s64 delta_exec;
>  
> +	delta_exec = now - se->exec_start;
>  	if (unlikely(delta_exec <= 0))
>  		return delta_exec;
>  
> +	se->exec_start = now;
> +	if (entity_is_task(se)) {
> +		struct task_struct *donor = task_of(se);
> +		struct task_struct *running = rq->curr;
> +		/*
> +		 * If se is a task, we account the time against the running
> +		 * task, as w/ proxy-exec they may not be the same.
> +		 */
> +		running->se.exec_start = now;
> +		running->se.sum_exec_runtime += delta_exec;
>  
> +		trace_sched_stat_runtime(running, delta_exec);
> +		account_group_exec_runtime(running, delta_exec);
>  
> +		/* cgroup time is always accounted against the donor */
> +		cgroup_account_cputime(donor, delta_exec);
> +	} else {
> +		/* If not task, account the time against donor se  */
> +		se->sum_exec_runtime += delta_exec;
>  	}

Bah.. this is all terrible :-) But yeah, I suppose this wil do.

>  
>  	if (schedstat_enabled()) {
>  		struct sched_statistics *stats;
>  
> +		stats = __schedstats_from_se(se);
>  		__schedstat_set(stats->exec_max,
>  				max(delta_exec, stats->exec_max));
>  	}
> @@ -1213,7 +1223,7 @@ s64 update_curr_common(struct rq *rq)
>  {
>  	struct task_struct *donor = rq->donor;
>  
> +	return update_se(rq, &donor->se);
>  }

At this point this might as well read:

	return update_se(rq, &rq->donor->se);

Re: [RESEND][PATCH v18 6/8] sched: Add an initial sketch of the find_proxy_task() function

[Peter Zijlstra]

On Mon, Jul 07, 2025 at 08:43:53PM +0000, John Stultz wrote:

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 853157b27f384..dc82d9b8bee2c 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6614,7 +6614,8 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>   * Otherwise marks the task's __state as RUNNING
>   */
>  static bool try_to_block_task(struct rq *rq, struct task_struct *p,
> -			      unsigned long *task_state_p)
> +			      unsigned long *task_state_p,
> +			      bool deactivate_cond)
>  {
>  	unsigned long task_state = *task_state_p;
>  	int flags = DEQUEUE_NOCLOCK;
> @@ -6625,6 +6626,9 @@ static bool try_to_block_task(struct rq *rq, struct task_struct *p,
>  		return false;
>  	}
>  
> +	if (!deactivate_cond)
> +		return false;
> +
>  	p->sched_contributes_to_load =
>  		(task_state & TASK_UNINTERRUPTIBLE) &&
>  		!(task_state & TASK_NOLOAD) &&

I'm struggling with this; @deactivate_cond doesn't seem to adequately
cover what it actually does.

So far what it seems to do is when true, don't block. It still does the
signal thing -- but I can't tell if that is actually required or not.

Would 'should_block' be a better name? And maybe stick a little
something in the comment above try_to_block_task() or near the:

	if (!should_block)
		return false;

lines about why the signal bits are important to have done.

> @@ -6648,6 +6652,89 @@ static bool try_to_block_task(struct rq *rq, struct task_struct *p,
>  	return true;
>  }
>  
> +#ifdef CONFIG_SCHED_PROXY_EXEC
> +static inline struct task_struct *proxy_resched_idle(struct rq *rq)
> +{
> +	put_prev_set_next_task(rq, rq->donor, rq->idle);
> +	rq_set_donor(rq, rq->idle);
> +	set_tsk_need_resched(rq->idle);
> +	return rq->idle;
> +}

Nothing cares about the return value.

Re: [RESEND][PATCH v18 5/8] sched: Fix runtime accounting w/ split exec & sched contexts

[John Stultz]

On Thu, Jul 10, 2025 at 2:45 AM Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, Jul 07, 2025 at 08:43:52PM +0000, John Stultz wrote:
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index edcc7d59ecc3b..c34e0891193a7 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -1143,30 +1143,40 @@ static void update_tg_load_avg(struct cfs_rq *cfs_rq)
> >  }
> >  #endif /* CONFIG_SMP */
> >
> > +static s64 update_se(struct rq *rq, struct sched_entity *se)
> >  {
> >       u64 now = rq_clock_task(rq);
> >       s64 delta_exec;
> >
> > +     delta_exec = now - se->exec_start;
> >       if (unlikely(delta_exec <= 0))
> >               return delta_exec;
> >
> > +     se->exec_start = now;
> > +     if (entity_is_task(se)) {
> > +             struct task_struct *donor = task_of(se);
> > +             struct task_struct *running = rq->curr;
> > +             /*
> > +              * If se is a task, we account the time against the running
> > +              * task, as w/ proxy-exec they may not be the same.
> > +              */
> > +             running->se.exec_start = now;
> > +             running->se.sum_exec_runtime += delta_exec;
> >
> > +             trace_sched_stat_runtime(running, delta_exec);
> > +             account_group_exec_runtime(running, delta_exec);
> >
> > +             /* cgroup time is always accounted against the donor */
> > +             cgroup_account_cputime(donor, delta_exec);
> > +     } else {
> > +             /* If not task, account the time against donor se  */
> > +             se->sum_exec_runtime += delta_exec;
> >       }
>
> Bah.. this is all terrible :-) But yeah, I suppose this wil do.

Yeah, :/ I'm happy to rework it further if you have ideas.

> > @@ -1213,7 +1223,7 @@ s64 update_curr_common(struct rq *rq)
> >  {
> >       struct task_struct *donor = rq->donor;
> >
> > +     return update_se(rq, &donor->se);
> >  }
>
> At this point this might as well read:
>
>         return update_se(rq, &rq->donor->se);

Good point. Done.

Thanks so much for the review! I'll get the next iteration ready to go out soon.
-john

Re: [RESEND][PATCH v18 6/8] sched: Add an initial sketch of the find_proxy_task() function

[John Stultz]

On Thu, Jul 10, 2025 at 3:02 AM Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, Jul 07, 2025 at 08:43:53PM +0000, John Stultz wrote:
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 853157b27f384..dc82d9b8bee2c 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -6614,7 +6614,8 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> >   * Otherwise marks the task's __state as RUNNING
> >   */
> >  static bool try_to_block_task(struct rq *rq, struct task_struct *p,
> > -                           unsigned long *task_state_p)
> > +                           unsigned long *task_state_p,
> > +                           bool deactivate_cond)
> >  {
> >       unsigned long task_state = *task_state_p;
> >       int flags = DEQUEUE_NOCLOCK;
> > @@ -6625,6 +6626,9 @@ static bool try_to_block_task(struct rq *rq, struct task_struct *p,
> >               return false;
> >       }
> >
> > +     if (!deactivate_cond)
> > +             return false;
> > +
> >       p->sched_contributes_to_load =
> >               (task_state & TASK_UNINTERRUPTIBLE) &&
> >               !(task_state & TASK_NOLOAD) &&
>
> I'm struggling with this; @deactivate_cond doesn't seem to adequately
> cover what it actually does.
>

So initially, there was just this extra logic to exit early if the
prev task was blocked_on so we don't deactivate it (keeping it on the
rq). However, in cases (later in the series) where we later decide to
bail on proxying and just deactivate the task even if we are
blocked_on, I wanted to re-use this for that fallback case, so passing
the !blocked_on state as an argument (which could be forced to true
when we bail) made sense to me. Thus a "deactivation condition".

> So far what it seems to do is when true, don't block. It still does the
> signal thing -- but I can't tell if that is actually required or not.

So, preserving the signal check seemed important, as if we do get a
signal we probably want to make sure we don't deactivate and that the
task so it can be selected to run.
I'm curious as to your thinking that makes you skeptical that it is required?

That said, I appreciate you raising the question, as looking at and
thinking about it some more, I do see that we should be clearing the
blocked_on state when we do set ourselves as runnable in that case,
otherwise if we are blocked_on we might get stuck waiting for a wakeup
(likely from the lock release).

I'll fix that up here.

> Would 'should_block' be a better name? And maybe stick a little
> something in the comment above try_to_block_task() or near the:
>
>         if (!should_block)
>                 return false;
>
> lines about why the signal bits are important to have done.

Sure, I'm happy to use that if should_block is more clear. I'll try to
add some better comments as well.

>
> > @@ -6648,6 +6652,89 @@ static bool try_to_block_task(struct rq *rq, struct task_struct *p,
> >       return true;
> >  }
> >
> > +#ifdef CONFIG_SCHED_PROXY_EXEC
> > +static inline struct task_struct *proxy_resched_idle(struct rq *rq)
> > +{
> > +     put_prev_set_next_task(rq, rq->donor, rq->idle);
> > +     rq_set_donor(rq, rq->idle);
> > +     set_tsk_need_resched(rq->idle);
> > +     return rq->idle;
> > +}
>
> Nothing cares about the return value.

Well, nothing in this patch, but the last patch in this series I sent
here uses it.  But if you think it makes the series easier to grok and
I can drop it here and add the return in that later patch.

Thanks so much again for the review and feedback! I really appreciate it!
-john

Re: [RESEND][PATCH v18 5/8] sched: Fix runtime accounting w/ split exec & sched contexts

[Peter Zijlstra]

On Thu, Jul 10, 2025 at 10:25:46AM -0700, John Stultz wrote:
> On Thu, Jul 10, 2025 at 2:45 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > On Mon, Jul 07, 2025 at 08:43:52PM +0000, John Stultz wrote:
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index edcc7d59ecc3b..c34e0891193a7 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -1143,30 +1143,40 @@ static void update_tg_load_avg(struct cfs_rq *cfs_rq)
> > >  }
> > >  #endif /* CONFIG_SMP */
> > >
> > > +static s64 update_se(struct rq *rq, struct sched_entity *se)
> > >  {
> > >       u64 now = rq_clock_task(rq);
> > >       s64 delta_exec;
> > >
> > > +     delta_exec = now - se->exec_start;
> > >       if (unlikely(delta_exec <= 0))
> > >               return delta_exec;
> > >
> > > +     se->exec_start = now;
> > > +     if (entity_is_task(se)) {
> > > +             struct task_struct *donor = task_of(se);
> > > +             struct task_struct *running = rq->curr;
> > > +             /*
> > > +              * If se is a task, we account the time against the running
> > > +              * task, as w/ proxy-exec they may not be the same.
> > > +              */
> > > +             running->se.exec_start = now;
> > > +             running->se.sum_exec_runtime += delta_exec;
> > >
> > > +             trace_sched_stat_runtime(running, delta_exec);
> > > +             account_group_exec_runtime(running, delta_exec);
> > >
> > > +             /* cgroup time is always accounted against the donor */
> > > +             cgroup_account_cputime(donor, delta_exec);
> > > +     } else {
> > > +             /* If not task, account the time against donor se  */
> > > +             se->sum_exec_runtime += delta_exec;
> > >       }
> >
> > Bah.. this is all terrible :-) But yeah, I suppose this wil do.
> 
> Yeah, :/ I'm happy to rework it further if you have ideas.

Not really; I stared at this for a bit yesterday. Its a confusing mess,
but I didn't come up with anything better.

Re: [RESEND][PATCH v18 6/8] sched: Add an initial sketch of the find_proxy_task() function

[Peter Zijlstra]

On Thu, Jul 10, 2025 at 05:43:36PM -0700, John Stultz wrote:

> > > @@ -6648,6 +6652,89 @@ static bool try_to_block_task(struct rq *rq, struct task_struct *p,
> > >       return true;
> > >  }
> > >
> > > +#ifdef CONFIG_SCHED_PROXY_EXEC
> > > +static inline struct task_struct *proxy_resched_idle(struct rq *rq)
> > > +{
> > > +     put_prev_set_next_task(rq, rq->donor, rq->idle);
> > > +     rq_set_donor(rq, rq->idle);
> > > +     set_tsk_need_resched(rq->idle);
> > > +     return rq->idle;
> > > +}
> >
> > Nothing cares about the return value.
> 
> Well, nothing in this patch, but the last patch in this series I sent
> here uses it.  But if you think it makes the series easier to grok and
> I can drop it here and add the return in that later patch.

I found it in the later patch; I meant to reply here, but promptly
forgot. It can stay here I suppose, no harm done.

Re: [RESEND][PATCH v18 6/8] sched: Add an initial sketch of the find_proxy_task() function

[John Stultz]

On Thu, Jul 10, 2025 at 5:43 PM John Stultz <jstultz@google.com> wrote:
> On Thu, Jul 10, 2025 at 3:02 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > So far what it seems to do is when true, don't block. It still does the
> > signal thing -- but I can't tell if that is actually required or not.
...
> That said, I appreciate you raising the question, as looking at and
> thinking about it some more, I do see that we should be clearing the
> blocked_on state when we do set ourselves as runnable in that case,
> otherwise if we are blocked_on we might get stuck waiting for a wakeup
> (likely from the lock release).
>
> I'll fix that up here.

So just FYI, I actually talked myself out of that position last night
before finishing up and sending out v19. If we are blocked_on, we're
not going to get back to userland to handle the signal, we're just
going to wake up, spin through the mutex lock code and head back into
__schedule(), so clearing the blocked_on state isn't the right answer
there. Leaving the blocked_on task on the runqueue (even if we set it
TASK_RUNNABLE) until the blocked_on state is cleared should be fine.

I did some stress testing, and never actually saw a case where we had
signals pending and were blocked on, so it seems like a rare corner
case, and I feel leaving the existing longer-tested behavior is right.

thanks
-john