Re: [PATCH v2 1/1] psi: stop relying on timer_pending for poll_work rescheduling

[Johannes Weiner]

On Thu, Jul 01, 2021 at 10:58:24AM +0200, Peter Zijlstra wrote:
> On Wed, Jun 30, 2021 at 01:51:51PM -0700, Suren Baghdasaryan wrote:
> > +	/* cmpxchg should be called even when !force to set poll_scheduled */
> > +	if (atomic_cmpxchg(&group->poll_scheduled, 0, 1) && !force)
> >  		return;
> 
> Why is that a cmpxchg() ?

I now realize you had already pointed that out, but I dismissed it in
the context of poll_lock not being always taken after all.

But you're right, cmpxchg indeed seems inappropriate. xchg will do
just fine for this binary toggle.

When it comes to ordering, looking at it again, I think we actually
need ordering here that the seqcount doesn't provide. We have:

timer:
scheduled = 0
smp_rmb()
x = state

scheduler:
state = y
smp_wmb()
if xchg(scheduled, 1) == 0
  mod_timer()

Again, the requirement is that when the scheduler sees the timer as
already or still pending, the timer must observe its state updates -
otherwise we miss poll events.

The seqcount provides the wmb and rmb, but the scheduler-side read of
@scheduled mustn't be reordered before the write to @state. Likewise,
the timer-side read of @state also mustn't occur before the write to
@scheduled.

AFAICS this is broken, not just in the patch, but also in the current
code when timer_pending() on the scheduler side gets reordered. (Not
sure if timer reading state can be reordered before the detach_timer()
of its own expiration, but I don't see full ordering between them.)

So it seems to me we need the ordered atomic_xchg() on the scheduler
side, and on the timer side an smp_mb() after we set scheduled to 0.