On Tue, Nov 17, 2020 at 09:30:16AM +0100, Peter Zijlstra wrote:
> On Mon, Nov 16, 2020 at 07:31:49PM +0000, Mel Gorman wrote:
>
> > And this works.
>
> Yay!
>
> > sched_psi_wake_requeue can probably stay with the other three fields
> > given they are under the rq lock but sched_remote_wakeup needs to move
> > out.
>
> I _think_ we can move the bit into the unserialized section below.
>
> It's a bit cheecky, but it should work I think because the only time we
> actually use this bit, we're guaranteed the task isn't actually running,
> so current doesn't exist.
>
> I suppose the question is wether this is worth saving 31 bits over...
>
> How's this?
>
> ---
> Subject: sched: Fix data-race in wakeup
> From: Peter Zijlstra <pet...@infradead.org>
> Date: Tue Nov 17 09:08:41 CET 2020
>
> Mel reported that on some ARM64 platforms loadavg goes bananas and
> tracked it down to the following data race:
>
> CPU0 CPU1
>
> schedule()
> prev->sched_contributes_to_load = X;
> deactivate_task(prev);
>
> try_to_wake_up()
> if (p->on_rq &&) // false
> if (smp_load_acquire(&p->on_cpu) &&
> // true
> ttwu_queue_wakelist())
> p->sched_remote_wakeup = Y;
>
> smp_store_release(prev->on_cpu, 0);
(nit: I suggested this race over at [1] ;)
> where both p->sched_contributes_to_load and p->sched_remote_wakeup are
> in the same word, and thus the stores X and Y race (and can clobber
> one another's data).
>
> Whereas prior to commit c6e7bd7afaeb ("sched/core: Optimize ttwu()
> spinning on p->on_cpu") the p->on_cpu handoff serialized access to
> p->sched_remote_wakeup (just as it still does with
> p->sched_contributes_to_load) that commit broke that by calling
> ttwu_queue_wakelist() with p->on_cpu != 0.
>
> However, due to
>
> p->XXX ttwu()
> schedule() if (p->on_rq && ...) // false
> smp_mb__after_spinlock() if (smp_load_acquire(&p->on_cpu) &&
> deactivate_task() ttwu_queue_wakelist())
> p->on_rq = 0; p->sched_remote_wakeup = X;
>
> We can be sure any 'current' store is complete and 'current' is
> guaranteed asleep. Therefore we can move p->sched_remote_wakeup into
> the current flags word.
>
> Note: while the observed failure was loadavg accounting gone wrong due
> to ttwu() cobbering p->sched_contributes_to_load, the reverse problem
> is also possible where schedule() clobbers p->sched_remote_wakeup,
> this could result in enqueue_entity() wrecking ->vruntime and causing
> scheduling artifacts.
>
> Fixes: c6e7bd7afaeb ("sched/core: Optimize ttwu() spinning on p->on_cpu")
> Reported-by: Mel Gorman <mgor...@techsingularity.net>
> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
> ---
> include/linux/sched.h | 13 ++++++++++++-
> 1 file changed, 12 insertions(+), 1 deletion(-)
>
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -775,7 +775,6 @@ struct task_struct {
> unsigned sched_reset_on_fork:1;
> unsigned sched_contributes_to_load:1;
> unsigned sched_migrated:1;
> - unsigned sched_remote_wakeup:1;
> #ifdef CONFIG_PSI
> unsigned sched_psi_wake_requeue:1;
> #endif
> @@ -785,6 +784,18 @@ struct task_struct {
>
> /* Unserialized, strictly 'current' */
>
> + /*
> + * p->in_iowait = 1; ttwu()
> + * schedule() if (p->on_rq && ..) // false
> + * smp_mb__after_spinlock(); if (smp_load_acquire(&p->on_cpu) &&
> //true
> + * deactivate_task() ttwu_queue_wakelist())
> + * p->on_rq = 0; p->sched_remote_wakeup = X;
> + *
> + * Guarantees all stores of 'current' are visible before
> + * ->sched_remote_wakeup gets used.
I'm still not sure this is particularly clear -- don't we want to highlight
that the store of p->on_rq is unordered wrt the update to
p->sched_contributes_to_load() in deactivate_task()?
I dislike bitfields with a passion, but the fix looks good:
Acked-by: Will Deacon <w...@kernel.org>
Now the million dollar question is why KCSAN hasn't run into this. Hrmph.
Will
[1] https://lore.kernel.org/r/20201116131102.GA29992@willie-the-truck
