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

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Date:   Mon, 28 Jan 2019 16:26:32 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     gregkh@...uxfoundation.org, tj@...nel.org, lizefan@...wei.com,
        axboe@...nel.dk, dennis@...nel.org, dennisszhou@...il.com,
        mingo@...hat.com, peterz@...radead.org, akpm@...ux-foundation.org,
        corbet@....net, cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v3 5/5] psi: introduce psi monitor

One thought on the v3 delta that I missed earlier:

On Thu, Jan 24, 2019 at 01:15:18PM -0800, Suren Baghdasaryan wrote:
> +/*
> + * psi_update_work represents slowpath accounting part while psi_group_change
> + * represents hotpath part. There are two potential races between them:
> + * 1. Changes to group->polling when slowpath checks for new stall, then hotpath
> + *    records new stall and then slowpath resets group->polling flag. This leads
> + *    to the exit from the polling mode while monitored state is still changing.
> + * 2. Slowpath overwriting an immediate update scheduled from the hotpath with
> + *    a regular update further in the future and missing the immediate update.
> + * Both races are handled with a retry cycle in the slowpath:
> + *
> + *    HOTPATH:                         |    SLOWPATH:
> + *                                     |
> + * A) times[cpu] += delta              | E) delta = times[*]
> + * B) start_poll = (delta[poll_mask] &&|    if delta[poll_mask]:
> + *      cmpxchg(g->polling, 0, 1) == 0)| F)   polling_until = now + grace_period
> + *    if start_poll:                   |    if now > polling_until:
> + * C)   mod_delayed_work(1)            |      if g->polling:

With the polling flag being atomic now, this "if g->polling" line
isn't accurate anymore. Since this diagram is specifically about
memory ordering, this should move the g->polling load up to where
delta is read and then refer to unordered local variables down here.