linux-kernel - Re: [PATCH v3 5/5] psi: introduce psi monitor

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Date:   Tue, 29 Jan 2019 20:08:51 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>, lizefan@...wei.com,
        Johannes Weiner <hannes@...xchg.org>, axboe@...nel.dk,
        dennis@...nel.org, Dennis Zhou <dennisszhou@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, cgroups@...r.kernel.org,
        linux-mm <linux-mm@...ck.org>, linux-doc@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@...roid.com
Subject: Re: [PATCH v3 5/5] psi: introduce psi monitor

On Tue, Jan 29, 2019 at 10:18:20AM -0800, Suren Baghdasaryan wrote:
> On Tue, Jan 29, 2019 at 4:38 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Thu, Jan 24, 2019 at 01:15:18PM -0800, Suren Baghdasaryan wrote:
> > > +                     atomic_set(&group->polling, polling);
> > > +                     /*
> > > +                      * Memory barrier is needed to order group->polling
> > > +                      * write before times[] read in collect_percpu_times()
> > > +                      */
> > > +                     smp_mb__after_atomic();
> >
> > That's broken, smp_mb__{before,after}_atomic() can only be used on
> > atomic RmW operations, something atomic_set() is _not_.
> 
> Oh, I didn't realize that. After reading the following example from
> atomic_ops.txt 

That document it woefully out of date (and I should double check, but I
think we can actually delete it now). Please see
Documentation/atomic_t.txt

> I was under impression that smp_mb__after_atomic()
> would make changes done by atomic_set() visible:
> 
> /* All memory operations before this call will
> * be globally visible before the clear_bit().
> */
> smp_mb__before_atomic();
> clear_bit( ... );
> /* The clear_bit() will be visible before all
> * subsequent memory operations.
> */
> smp_mb__after_atomic();
> 
> but I'm probably missing something. Is there a more detailed
> description of these rules anywhere else?

See atomic_t.txt; but the difference is that clear_bit() is a RmW, while
atomic_set() is just a plain store.

> Meanwhile I'll change smp_mb__after_atomic() into smp_mb(). Would that
> fix the ordering?

It would work here; but I'm still trying to actually understand all
this. So while the detail would be fine, I'm not ready to judge the
over-all thing.