Date:   Mon, 8 Jun 2020 19:56:49 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Benjamin Segall <bsegall@...gle.com>, mgorman@...e.de,
        Shakeel Butt <shakeelb@...gle.com>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH 1/1] psi: eliminate kthread_worker from psi trigger
 scheduling mechanism

On Thu, Jun 4, 2020 at 12:20 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Thu, Jun 4, 2020 at 6:12 AM Peter Zijlstra <peterz@...radead.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@...gle.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.

After some more staring into kthread code I think I understand what
Peter's comment meant about delayed_work_list.
worker->delayed_work_list seems to be unnecessary because each
kthread_delayed_work has its own timer which will add the work into
worker->work_list when the time comes. So there is no need to store
the delayed work in an intermediate worker->delayed_work_list.
However I think kthread_destroy_worker() has an issue if it's called
while worker->delayed_work_list is non-empty. The issue is that
kthread_destroy_worker() does not stop all the
kthread_delayed_work->timers scheduled on the
worker->delayed_work_list. So if such a timer fires after a call to
kthread_destroy_worker(), timer's handler will dereference the already
destroyed worker.

If I'm right and this is indeed an issue then I think we do need
worker->delayed_work_list to cancel all the scheduled timers. The
issue can be avoided if we assume that the caller will alway call
kthread_cancel_delayed_work_sync() for each delayed_work scheduled on
worker->delayed_work_list before calling kthread_destroy_worker(). If
that's what we expect I think this expectation should be reflected in
the comments and a WARN_ON(!list_empty(&worker->delayed_work_list)) be
added in kthread_destroy_worker(). WDYT?

>
> 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.
>
> >
> > --
> > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@...roid.com.
> >

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