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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Tue, 29 Jan 2019 10:05:07 -0800
Message-ID: <CAJuCfpH3k0rrZZw_HhrjXYuFX31p283qg_5vQEnyd_wAMaxbqA@mail.gmail.com>
Subject: Re: [PATCH v3 5/5] psi: introduce psi monitor
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tejun Heo <tj@kernel.org>, lizefan@huawei.com,
        Johannes Weiner <hannes@cmpxchg.org>, axboe@kernel.dk,
        dennis@kernel.org, Dennis Zhou <dennisszhou@gmail.com>,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jonathan Corbet <corbet@lwn.net>, cgroups@vger.kernel.org,
        linux-mm <linux-mm@kvack.org>, linux-doc@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>, kernel-team@android.com
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Thanks for review Peter!

On Tue, Jan 29, 2019 at 2:44 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Jan 24, 2019 at 01:15:18PM -0800, Suren Baghdasaryan wrote:
> >  static void psi_update_work(struct work_struct *work)
> >  {
> >       struct delayed_work *dwork;
> >       struct psi_group *group;
> > +     bool first_pass = true;
> > +     u64 next_update;
> > +     u32 change_mask;
> > +     int polling;
> >       bool nonidle;
> > +     u64 now;
> >
> >       dwork = to_delayed_work(work);
> >       group = container_of(dwork, struct psi_group, clock_work);
> >
> > +     now = sched_clock();
> > +
> > +     mutex_lock(&group->update_lock);
>
> actually acquiring a mutex can take a fairly long while; would it not
> make more sense to take the @now timestanp _after_ it, instead of
> before?

Yes, that makes sense. As long as *now* is set before the *retry*
label, otherwise the retry mechanism would get even more complicated
to understand with floating *now* timestamp. Will move the assignment
right after the mutex_lock()

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