Re: [PATCH RFC v2 tip/core/rcu 01/22] sched/core: Add function to sample state of locked-down task

Paul E. McKenney
2020-03-20T02:49:46Z

On Thu, Mar 19, 2020 at 10:35:25AM -0700, Paul E. McKenney wrote:
> On Thu, Mar 19, 2020 at 01:22:38PM -0400, Steven Rostedt wrote:
> > On Wed, 18 Mar 2020 17:10:39 -0700
> > paulmck@kernel.org wrote:

[ . . . ]

> > > /**
> > > + * try_invoke_on_locked_down_task - Invoke a function on task in fixed state
> > > + * @p: Process for which the function is to be invoked.
> > > + * @func: Function to invoke.
> > > + * @arg: Argument to function.
> > > + *
> > > + * If the specified task can be quickly locked into a definite state
> > > + * (either sleeping or on a given runqueue), arrange to keep it in that
> > > + * state while invoking @func(@arg). This function can use ->on_rq and
> > > + * task_curr() to work out what the state is, if required. Given that
> > > + * @func can be invoked with a runqueue lock held, it had better be quite
> > > + * lightweight.
> > > + *
> > > + * Returns:
> > > + * @false if the task slipped out from under the locks.
> > > + * @true if the task was locked onto a runqueue or is sleeping.
> > > + * However, @func can override this by returning @false.
> >
> > Should probably state that it will return false if the state could be
> > locked, otherwise it returns the return code of the function.
>
> So like this?
>
> * Returns:
> * @false if the task state could not be locked.
> * Otherwise, the return value from @func(arg).
>
> > I'm wondering if we shouldn't have the function return code be something
> > passed in by the parameter, and have this return either true (locked and
> > function called), or false (not locked and function wasn't called).
>
> I was thinking of this as one of the possible uses of whatever arg
> points to, which allows the caller of try_invoke_on_locked_down_task()
> and the specified function to communicate whatever they wish. Then
> the specified function could (for example) unconditionally return true
> so that the return value from try_invoke_on_locked_down_task() indicated
> whether or not the specified function was called.
>
> The current setup is very convenient for the use cases thus far. It
> allows the function to say "Yeah, I was called, but I couldn't do
> anything", thus allowing the caller to make exactly one check to know
> that corrective action is required.

And here is another use case that led me to take this approach.
The trc_inspect_reader_notrunning() function in the patch below is passed
to try_invoke_on_locked_down_task() whose caller can continue testing
just the return value from try_invoke_on_locked_down_task() to work out
what to do next.

Thoughts? Other use cases?

Thanx, Paul

------------------------------------------------------------------------

commit e26a234c1205bf02b62b62cd7f15f8086fc0b13b
Author: Paul E. McKenney <paulmck@kernel.org>
Date: Thu Mar 19 15:33:12 2020 -0700

rcu-tasks: Avoid IPIing userspace/idle tasks if kernel is so built

Systems running CPU-bound real-time task do not want IPIs sent to CPUs
executing nohz_full userspace tasks. Battery-powered systems don't
want IPIs sent to idle CPUs in low-power mode. Unfortunately, RCU tasks
trace can and will send such IPIs in some cases.

Both of these situations occur only when the target CPU is in RCU
dyntick-idle mode, in other words, when RCU is not watching the
target CPU. This suggests that CPUs in dyntick-idle mode should use
memory barriers in outermost invocations of rcu_read_lock_trace()
and rcu_read_unlock_trace(), which would allow the RCU tasks trace
grace period to directly read out the target CPU's read-side state.
One challenge is that RCU tasks trace is not targeting a specific
CPU, but rather a task. And that task could switch from one CPU to
another at any time.

This commit therefore uses try_invoke_on_locked_down_task()
and checks for task_curr() in trc_inspect_reader_notrunning().
When this condition holds, the target task is running and cannot move.
If CONFIG_TASKS_TRACE_RCU_READ_MB=y, the new rcu_dynticks_zero_in_eqs()
function can be used to check if the specified integer (in this case,
t->trc_reader_nesting) is zero while the target CPU remains in that same
dyntick-idle sojourn. If so, the target task is in a quiescent state.
If not, trc_read_check_handler() must indicate failure so that the
grace-period kthread can take appropriate action or retry after an
appropriate delay, as the case may be.

With this change, given CONFIG_TASKS_TRACE_RCU_READ_MB=y, if a given
CPU remains idle or a given task continues executing in nohz_full mode,
the RCU tasks trace grace-period kthread will detect this without the
need to send an IPI.

Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index e1089fd..296f926 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -501,6 +501,7 @@ void srcutorture_get_gp_data(enum rcutorture_type test_type,
#endif

#ifdef CONFIG_TINY_RCU
+static inline bool rcu_dynticks_zero_in_eqs(int cpu, int *vp) { return false; }
static inline unsigned long rcu_get_gp_seq(void) { return 0; }
static inline unsigned long rcu_exp_batches_completed(void) { return 0; }
static inline unsigned long
@@ -510,6 +511,7 @@ static inline void show_rcu_gp_kthreads(void) { }
static inline int rcu_get_gp_kthreads_prio(void) { return 0; }
static inline void rcu_fwd_progress_check(unsigned long j) { }
#else /* #ifdef CONFIG_TINY_RCU */
+bool rcu_dynticks_zero_in_eqs(int cpu, int *vp);
unsigned long rcu_get_gp_seq(void);
unsigned long rcu_exp_batches_completed(void);
unsigned long srcu_batches_completed(struct srcu_struct *sp);
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index d31ed74..36f03d3 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -802,22 +802,38 @@ static void trc_read_check_handler(void *t_in)
/* Callback function for scheduler to check non-running) task. */
static bool trc_inspect_reader_notrunning(struct task_struct *t, void *arg)
{
- if (task_curr(t))
- return false; // It is running, so decline to inspect it.
+ int cpu = task_cpu(t);
+ bool in_qs = false;
+
+ if (task_curr(t)) {
+ // If no chance of heavyweight readers, do it the hard way.
+ if (!IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
+ return false;
+
+ // If heavyweight readers are enabled on the remote task,
+ // we can inspect its state despite its currently running.
+ // However, we cannot safely change its state.
+ if (!rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
+ return false; // No quiescent state, do it the hard way.
+ in_qs = true;
+ } else {
+ in_qs = likely(!t->trc_reader_nesting);
+ }

// Mark as checked. Because this is called from the grace-period
// kthread, also remove the task from the holdout list.
t->trc_reader_checked = true;
trc_del_holdout(t);

- // If the task is in a read-side critical section, set up its
+ if (in_qs)
+ return true; // Already in quiescent state, done!!!
+
+ // The task is in a read-side critical section, so set up its
// its state so that it will awaken the grace-period kthread upon
// exit from that critical section.
- if (unlikely(t->trc_reader_nesting)) {
- atomic_inc(&trc_n_readers_need_end); // One more to wait on.
- WARN_ON_ONCE(t->trc_reader_special.b.need_qs);
- WRITE_ONCE(t->trc_reader_special.b.need_qs, true);
- }
+ atomic_inc(&trc_n_readers_need_end); // One more to wait on.
+ WARN_ON_ONCE(t->trc_reader_special.b.need_qs);
+ WRITE_ONCE(t->trc_reader_special.b.need_qs, true);
return true;
}

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index de6228a..4eb424e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -239,6 +239,7 @@ static void rcu_dynticks_eqs_enter(void)
* critical sections, and we also must force ordering with the
* next idle sojourn.
*/
+ rcu_dynticks_task_trace_enter(); // Before ->dynticks update!
seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
// RCU is no longer watching. Better be in extended quiescent state!
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
@@ -265,6 +266,7 @@ static void rcu_dynticks_eqs_exit(void)
*/
seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
// RCU is now watching. Better not be in an extended quiescent state!
+ rcu_dynticks_task_trace_exit(); // After ->dynticks update!
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
!(seq & RCU_DYNTICK_CTRL_CTR));
if (seq & RCU_DYNTICK_CTRL_MASK) {
@@ -337,6 +339,28 @@ static bool rcu_dynticks_in_eqs_since(struct rcu_data *rdp, int snap)
}

/*
+ * Return true if the referenced integer is zero while the specified
+ * CPU remains within a single extended quiescent state.
+ */
+bool rcu_dynticks_zero_in_eqs(int cpu, int *vp)
+{
+ struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
+ int snap;
+
+ // If not quiescent, force back to earlier extended quiescent state.
+ snap = atomic_read(&rdp->dynticks) & ~(RCU_DYNTICK_CTRL_MASK |
+ RCU_DYNTICK_CTRL_CTR);
+
+ smp_rmb(); // Order ->dynticks and *vp reads.
+ if (READ_ONCE(*vp))
+ return false; // Non-zero, so report failure;
+ smp_rmb(); // Order *vp read and ->dynticks re-read.
+
+ // If still in the same extended quiescent state, we are good!
+ return snap == (atomic_read(&rdp->dynticks) & ~RCU_DYNTICK_CTRL_MASK);
+}
+
+/*
* Set the special (bottom) bit of the specified CPU so that it
* will take special action (such as flushing its TLB) on the
* next exit from an extended quiescent state. Returns true if
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 44edd0a..43991a4 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -455,6 +455,8 @@ static void rcu_bind_gp_kthread(void);
static bool rcu_nohz_full_cpu(void);
static void rcu_dynticks_task_enter(void);
static void rcu_dynticks_task_exit(void);
+static void rcu_dynticks_task_trace_enter(void);
+static void rcu_dynticks_task_trace_exit(void);

/* Forward declarations for tree_stall.h */
static void record_gp_stall_check_time(void);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 9355536..f4a344e 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2553,3 +2553,21 @@ static void rcu_dynticks_task_exit(void)
WRITE_ONCE(current->rcu_tasks_idle_cpu, -1);
#endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
}
+
+/* Turn on heavyweight RCU tasks trace readers on idle/user entry. */
+static void rcu_dynticks_task_trace_enter(void)
+{
+#ifdef CONFIG_TASKS_RCU_TRACE
+ if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
+ current->trc_reader_special.b.need_mb = true;
+#endif /* #ifdef CONFIG_TASKS_RCU_TRACE */
+}
+
+/* Turn off heavyweight RCU tasks trace readers on idle/user exit. */
+static void rcu_dynticks_task_trace_exit(void)
+{
+#ifdef CONFIG_TASKS_RCU_TRACE
+ if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
+ current->trc_reader_special.b.need_mb = false;
+#endif /* #ifdef CONFIG_TASKS_RCU_TRACE */
+}