Re: [PATCH 1/1] psi: eliminate kthread_worker from psi trigger scheduling mechanism

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Jun 4, 2020 at 6:12 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, May 28, 2020 at 12:54:42PM -0700, Suren Baghdasaryan wrote:
> > Each psi group requires a dedicated kthread_delayed_work and
> > kthread_worker. Since no other work can be performed using psi_group's
> > kthread_worker, the same result can be obtained using a task_struct and
> > a timer directly. This makes psi triggering simpler by removing lists
> > and locks involved with kthread_worker usage and eliminates the need for
> > poll_scheduled atomic use in the hot path.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> > This patch is meant to address Peter's request in [1] to pull
> > kthread_queue_delayed_work() out from under rq->lock. This should also address
> > the lockdep warning about possibility of a circular dependency described in [2]
>
> I think you could've just fixed kthread_queue_delayed_work(), that code
> is sub-optimal.

Ok, let me look into it some more. My understanding was that the
worker->lock in kthread_queue_delayed_work() was needed to synchronize
worker->delayed_work_list access. But maybe I'm missing something... I
assume you are talking about optimizing this beyond what
https://lkml.org/lkml/2020/5/4/1148 was doing?

BTW, any objections against taking https://lkml.org/lkml/2020/5/4/1148
? It's not the ultimate fix but it is an improvement since it gets
some of the operations that were unnecessarily under worker->lock out
of it.

>
> But I suppose this works too.

In PSI's case there is always one work for each worker, so the
delayed_work_list and work_list are not needed and therefore I can
replace kthread_worker machinery with a task and a timer.
I think I can simplify this a bit further. For example
group->poll_wakeup doesn't have to be an atomic. Originally I wanted
to avoid a possibility of a race when poll_timer_fn sets it and
psi_poll_worker resets it and as a result misses a wakeup, however if
psi_poll_worker resets it before calling psi_poll_work then there is
no harm in missing a wakeup because we called psi_poll_work and did
the required work anyway.

One question about this patch I'm not sure about and wanted to ask you
Peter is whether it's ok to call mod_timer from within a hotpath
(while holding rq->lock). As I described in the additional comment,
there is a possibility of a race between when I check timer_pending
and the call to mod_timer, so it's possible that mod_timer might be
called both from psi_poll_work (psi poll work handler) and from
psi_task_change (hotpath under rq->lock). I see that mod_timer takes
base->lock spinlock, and IIUC such a race might block the hotpath and
therefore is unacceptable. If this is true I'll need to revive the
poll_scheduled atomic to close this race and then I can change
mod_timer into add_timer.
WDYT? And sorry for my ignorance if this is a trivial question. I'm
not sure about the rules when it comes to rq->locks.

Thanks,
Suren.

>
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