[PATCH v3] posix-cpu-timers: clear TICK_DEP_BIT_POSIX_TIMER on clone

[PATCH v3] posix-cpu-timers: clear TICK_DEP_BIT_POSIX_TIMER on clone

[Benjamin Segall]

When cloning a new thread, its posix_cputimers are not inherited, and
are cleared by posix_cputimers_init(). However, this does not clear the
tick dependency it creates in tsk->tick_dep_mask, and the handler does
not reach the code to clear the dependency if there were no timers to
begin with.

Thus if a thread has a cputimer running before clone/fork, all
descendants will prevent nohz_full unless they create a cputimer of
their own.

Fix this by entirely clearing the tick_dep_mask in copy_process().
(There is currently no inherited state that needs a tick dependency)

Process-wide timers do not have this problem because fork does not copy
signal_struct as a baseline, it creates one from scratch.

Fixes: b78783000d5c ("posix-cpu-timers: Migrate to use new tick dependency mask model")
Signed-off-by: Ben Segall <bsegall@google.com>
Cc: stable@vger.kernel.org
---
 include/linux/tick.h | 8 ++++++++
 kernel/fork.c        | 2 ++
 2 files changed, 10 insertions(+)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 72744638c5b0..99c9c5a7252a 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -249,16 +249,23 @@ static inline void tick_dep_set_task(struct task_struct *tsk,
 				     enum tick_dep_bits bit)
 {
 	if (tick_nohz_full_enabled())
 		tick_nohz_dep_set_task(tsk, bit);
 }
+
 static inline void tick_dep_clear_task(struct task_struct *tsk,
 				       enum tick_dep_bits bit)
 {
 	if (tick_nohz_full_enabled())
 		tick_nohz_dep_clear_task(tsk, bit);
 }
+
+static inline void tick_dep_init_task(struct task_struct *tsk)
+{
+	atomic_set(&tsk->tick_dep_mask, 0);
+}
+
 static inline void tick_dep_set_signal(struct task_struct *tsk,
 				       enum tick_dep_bits bit)
 {
 	if (tick_nohz_full_enabled())
 		tick_nohz_dep_set_signal(tsk, bit);
@@ -288,10 +295,11 @@ static inline void tick_dep_set_cpu(int cpu, enum tick_dep_bits bit) { }
 static inline void tick_dep_clear_cpu(int cpu, enum tick_dep_bits bit) { }
 static inline void tick_dep_set_task(struct task_struct *tsk,
 				     enum tick_dep_bits bit) { }
 static inline void tick_dep_clear_task(struct task_struct *tsk,
 				       enum tick_dep_bits bit) { }
+static inline void tick_dep_init_task(struct task_struct *tsk) { }
 static inline void tick_dep_set_signal(struct task_struct *tsk,
 				       enum tick_dep_bits bit) { }
 static inline void tick_dep_clear_signal(struct signal_struct *signal,
 					 enum tick_dep_bits bit) { }
 
diff --git a/kernel/fork.c b/kernel/fork.c
index df8e4575ff01..9adad1ad202e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -103,10 +103,11 @@
 #include <linux/user_events.h>
 #include <linux/iommu.h>
 #include <linux/rseq.h>
 #include <uapi/linux/pidfd.h>
 #include <linux/pidfs.h>
+#include <linux/tick.h>
 
 #include <asm/pgalloc.h>
 #include <linux/uaccess.h>
 #include <asm/mmu_context.h>
 #include <asm/cacheflush.h>
@@ -2290,10 +2291,11 @@ __latent_entropy struct task_struct *copy_process(
 
 	task_io_accounting_init(&p->ioac);
 	acct_clear_integrals(p);
 
 	posix_cputimers_init(&p->posix_cputimers);
+	tick_dep_init_task(p);
 
 	p->io_context = NULL;
 	audit_set_context(p, NULL);
 	cgroup_fork(p);
 	if (args->kthread) {
-- 
2.47.0.163.g1226f6d8fa-goog

Re: [PATCH v3] posix-cpu-timers: clear TICK_DEP_BIT_POSIX_TIMER on clone

[Frederic Weisbecker]

Le Fri, Oct 25, 2024 at 06:35:35PM -0700, Benjamin Segall a écrit :
> When cloning a new thread, its posix_cputimers are not inherited, and
> are cleared by posix_cputimers_init(). However, this does not clear the
> tick dependency it creates in tsk->tick_dep_mask, and the handler does
> not reach the code to clear the dependency if there were no timers to
> begin with.
> 
> Thus if a thread has a cputimer running before clone/fork, all
> descendants will prevent nohz_full unless they create a cputimer of
> their own.
> 
> Fix this by entirely clearing the tick_dep_mask in copy_process().
> (There is currently no inherited state that needs a tick dependency)
> 
> Process-wide timers do not have this problem because fork does not copy
> signal_struct as a baseline, it creates one from scratch.
> 
> Fixes: b78783000d5c ("posix-cpu-timers: Migrate to use new tick dependency mask model")
> Signed-off-by: Ben Segall <bsegall@google.com>
> Cc: stable@vger.kernel.org

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

Thanks!

[tip: timers/urgent] posix-cpu-timers: Clear TICK_DEP_BIT_POSIX_TIMER on clone

[tip-bot2 for Benjamin Segall]

The following commit has been merged into the timers/urgent branch of tip:

Commit-ID:     b5413156bad91dc2995a5c4eab1b05e56914638a
Gitweb:        https://git.kernel.org/tip/b5413156bad91dc2995a5c4eab1b05e56914638a
Author:        Benjamin Segall <bsegall@google.com>
AuthorDate:    Fri, 25 Oct 2024 18:35:35 -07:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Sun, 27 Oct 2024 10:36:04 +01:00

posix-cpu-timers: Clear TICK_DEP_BIT_POSIX_TIMER on clone

When cloning a new thread, its posix_cputimers are not inherited, and
are cleared by posix_cputimers_init(). However, this does not clear the
tick dependency it creates in tsk->tick_dep_mask, and the handler does
not reach the code to clear the dependency if there were no timers to
begin with.

Thus if a thread has a cputimer running before clone/fork, all
descendants will prevent nohz_full unless they create a cputimer of
their own.

Fix this by entirely clearing the tick_dep_mask in copy_process().
(There is currently no inherited state that needs a tick dependency)

Process-wide timers do not have this problem because fork does not copy
signal_struct as a baseline, it creates one from scratch.

Fixes: b78783000d5c ("posix-cpu-timers: Migrate to use new tick dependency mask model")
Signed-off-by: Ben Segall <bsegall@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/xm26o737bq8o.fsf@google.com

---
 include/linux/tick.h | 8 ++++++++
 kernel/fork.c        | 2 ++
 2 files changed, 10 insertions(+)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 7274463..99c9c5a 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -251,12 +251,19 @@ static inline void tick_dep_set_task(struct task_struct *tsk,
 	if (tick_nohz_full_enabled())
 		tick_nohz_dep_set_task(tsk, bit);
 }
+
 static inline void tick_dep_clear_task(struct task_struct *tsk,
 				       enum tick_dep_bits bit)
 {
 	if (tick_nohz_full_enabled())
 		tick_nohz_dep_clear_task(tsk, bit);
 }
+
+static inline void tick_dep_init_task(struct task_struct *tsk)
+{
+	atomic_set(&tsk->tick_dep_mask, 0);
+}
+
 static inline void tick_dep_set_signal(struct task_struct *tsk,
 				       enum tick_dep_bits bit)
 {
@@ -290,6 +297,7 @@ static inline void tick_dep_set_task(struct task_struct *tsk,
 				     enum tick_dep_bits bit) { }
 static inline void tick_dep_clear_task(struct task_struct *tsk,
 				       enum tick_dep_bits bit) { }
+static inline void tick_dep_init_task(struct task_struct *tsk) { }
 static inline void tick_dep_set_signal(struct task_struct *tsk,
 				       enum tick_dep_bits bit) { }
 static inline void tick_dep_clear_signal(struct signal_struct *signal,
diff --git a/kernel/fork.c b/kernel/fork.c
index 89ceb4a..6fa9fe6 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -105,6 +105,7 @@
 #include <linux/rseq.h>
 #include <uapi/linux/pidfd.h>
 #include <linux/pidfs.h>
+#include <linux/tick.h>
 
 #include <asm/pgalloc.h>
 #include <linux/uaccess.h>
@@ -2292,6 +2293,7 @@ __latent_entropy struct task_struct *copy_process(
 	acct_clear_integrals(p);
 
 	posix_cputimers_init(&p->posix_cputimers);
+	tick_dep_init_task(p);
 
 	p->io_context = NULL;
 	audit_set_context(p, NULL);