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SubjectRe: [PATCH] sched: RCU-protect __set_task_cpu() in set_task_cpu()
On 06/06, Peter Zijlstra wrote:
>
> You're right, p->pi_lock for wakeups, rq->lock for runnable tasks.

Good, thanks.

Help! I have another question.

try_to_wake_up:

raw_spin_lock_irqsave(&p->pi_lock, flags);
if (!(p->state & state))
goto out;
cpu = task_cpu(p);

if (p->on_rq && ttwu_remote(p, wake_flags))
goto stat;
This doesn't look a bit confusing, we can't trust "cpu = task_cpu" before
we check ->on_rq. OK, not a problem, this cpu number can only be used in
ttwu_stat(cpu).

But ttwu_stat(cpu) in turn does

if (cpu != task_cpu(p))
schedstat_inc(p, se.statistics.nr_wakeups_migrate);
Ignoring the theoretical races with pull_task/etc, how it is possible
that cpu != task_cpu(p) ? Another caller is try_to_wake_up_local(), it
obviously can't trigger this case.

This looks broken to me. Looking at its name, I guess nr_wakeups_migrate
should be incremented if ttwu does set_task_cpu(), correct?

IOW. Don't we need something like the (untested/ucompiled) patch below?
_If_ I am right, I can resend it with the changelog/etc but please feel
free to make another fix.

Oleg.

--- x/kernel/sched.c
+++ x/kernel/sched.c
@@ -2423,13 +2423,14 @@ static void update_avg(u64 *avg, u64 sam
#endif

static void
-ttwu_stat(struct task_struct *p, int cpu, int wake_flags)
+ttwu_stat(struct task_struct *p, bool migrate, int wake_flags)
{
#ifdef CONFIG_SCHEDSTATS
struct rq *rq = this_rq();

#ifdef CONFIG_SMP
int this_cpu = smp_processor_id();
+ int cpu = task_cpu(p);

if (cpu == this_cpu) {
schedstat_inc(rq, ttwu_local);
@@ -2455,7 +2456,7 @@ ttwu_stat(struct task_struct *p, int cpu
if (wake_flags & WF_SYNC)
schedstat_inc(p, se.statistics.nr_wakeups_sync);

- if (cpu != task_cpu(p))
+ if (migrate)
schedstat_inc(p, se.statistics.nr_wakeups_migrate);

#endif /* CONFIG_SCHEDSTATS */
@@ -2630,6 +2631,7 @@ try_to_wake_up(struct task_struct *p, un
{
unsigned long flags;
int cpu, success = 0;
+ bool migrate = false;

smp_wmb();
raw_spin_lock_irqsave(&p->pi_lock, flags);
@@ -2637,7 +2639,6 @@ try_to_wake_up(struct task_struct *p, un
goto out;

success = 1; /* we're going to change ->state */
- cpu = task_cpu(p);

if (p->on_rq && ttwu_remote(p, wake_flags))
goto stat;
@@ -2674,13 +2675,15 @@ try_to_wake_up(struct task_struct *p, un
p->sched_class->task_waking(p);

cpu = select_task_rq(p, SD_BALANCE_WAKE, wake_flags);
- if (task_cpu(p) != cpu)
+ if (task_cpu(p) != cpu) {
set_task_cpu(p, cpu);
+ migrate = true;
+ }
#endif /* CONFIG_SMP */

ttwu_queue(p, cpu);
stat:
- ttwu_stat(p, cpu, wake_flags);
+ ttwu_stat(p, migrate, wake_flags);
out:
raw_spin_unlock_irqrestore(&p->pi_lock, flags);

@@ -2716,7 +2719,7 @@ static void try_to_wake_up_local(struct
ttwu_activate(rq, p, ENQUEUE_WAKEUP);

ttwu_do_wakeup(rq, p, 0);
- ttwu_stat(p, smp_processor_id(), 0);
+ ttwu_stat(p, false, 0);
out:
raw_spin_unlock(&p->pi_lock);
}



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