From: Hugh Dickins <hughd@google.com>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: hughd@google.com, mhocko@kernel.org, akpm@linux-foundation.org,
mgorman@suse.de, riel@redhat.com, hannes@cmpxchg.org,
vbabka@suse.cz, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
mhocko@suse.com
Subject: Re: [PATCH] mm, vmscan: do not loop on too_many_isolated for ever
Date: Mon, 24 Jul 2017 00:01:13 -0700 (PDT) [thread overview]
Message-ID: <alpine.LSU.2.11.1707232339430.2154@eggly.anvils> (raw)
In-Reply-To: <201707201944.IJI05796.VLFJFFtSQMOOOH@I-love.SAKURA.ne.jp>
On Thu, 20 Jul 2017, Tetsuo Handa wrote:
> Hugh Dickins wrote:
> > You probably won't welcome getting into alternatives at this late stage;
> > but after hacking around it one way or another because of its pointless
> > lockups, I lost patience with that too_many_isolated() loop a few months
> > back (on realizing the enormous number of pages that may be isolated via
> > migrate_pages(2)), and we've been running nicely since with something like:
> >
> > bool got_mutex = false;
> >
> > if (unlikely(too_many_isolated(pgdat, file, sc))) {
> > if (mutex_lock_killable(&pgdat->too_many_isolated))
> > return SWAP_CLUSTER_MAX;
> > got_mutex = true;
> > }
> > ...
> > if (got_mutex)
> > mutex_unlock(&pgdat->too_many_isolated);
> >
> > Using a mutex to provide the intended throttling, without an infinite
> > loop or an arbitrary delay; and without having to worry (as we often did)
> > about whether those numbers in too_many_isolated() are really appropriate.
> > No premature OOMs complained of yet.
>
> Roughly speaking, there is a moment where shrink_inactive_list() acts
> like below.
>
> bool got_mutex = false;
>
> if (!current_is_kswapd()) {
> if (mutex_lock_killable(&pgdat->too_many_isolated))
> return SWAP_CLUSTER_MAX;
> got_mutex = true;
> }
>
> // kswapd is blocked here waiting for !current_is_kswapd().
That would be a shame, for kswapd to wait for !current_is_kswapd()!
But seriously, I think I understand what you mean by that, you're
thinking that kswapd would be waiting on some other task to clear
the too_many_isolated() condition?
No, it does not work that way: kswapd (never seeing too_many_isolated()
because that always says false when current_is_kswapd()) never tries to
take the pgdat->too_many_isolated mutex itself: it does not wait there
at all, although other tasks may be waiting there at the time.
Perhaps my naming the mutex "too_many_isolated", same as the function,
is actually confusing, when I had intended it to be helpful.
>
> if (got_mutex)
> mutex_unlock(&pgdat->too_many_isolated);
>
> >
> > But that was on a different kernel, and there I did have to make sure
> > that PF_MEMALLOC always prevented us from nesting: I'm not certain of
> > that in the current kernel (but do remember Johannes changing the memcg
> > end to make it use PF_MEMALLOC too). I offer the preview above, to see
> > if you're interested in that alternative: if you are, then I'll go ahead
> > and make it into an actual patch against v4.13-rc.
>
> I don't know what your actual patch looks like, but the problem is that
> pgdat->too_many_isolated waits for kswapd while kswapd waits for
> pgdat->too_many_isolated; nobody can unlock pgdat->too_many_isolated if
> once we hit it.
Not so (and we'd hardly be finding it a useful patch if that were so).
Hugh
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next prev parent reply other threads:[~2017-07-24 7:01 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-07-10 7:48 [PATCH] mm, vmscan: do not loop on too_many_isolated for ever Michal Hocko
2017-07-10 13:16 ` Vlastimil Babka
2017-07-10 13:58 ` Rik van Riel
2017-07-10 16:58 ` Johannes Weiner
2017-07-10 17:09 ` Michal Hocko
2017-07-19 22:20 ` Andrew Morton
2017-07-20 6:56 ` Michal Hocko
2017-07-21 23:01 ` Andrew Morton
2017-07-24 6:50 ` Michal Hocko
2017-07-20 1:54 ` Hugh Dickins
2017-07-20 10:44 ` Tetsuo Handa
2017-07-24 7:01 ` Hugh Dickins [this message]
2017-07-24 11:12 ` Tetsuo Handa
2017-07-20 13:22 ` Michal Hocko
2017-07-24 7:03 ` Hugh Dickins
-- strict thread matches above, loose matches on Subject: below --
2017-03-07 13:30 Michal Hocko
2017-03-07 19:52 ` Rik van Riel
2017-03-08 9:21 ` Michal Hocko
2017-03-08 15:54 ` Rik van Riel
2017-03-09 9:12 ` Michal Hocko
2017-03-09 14:16 ` Rik van Riel
2017-03-09 14:59 ` Michal Hocko
2017-03-09 18:05 ` Johannes Weiner
2017-03-09 22:18 ` Rik van Riel
2017-03-10 10:27 ` Michal Hocko
2017-03-10 10:20 ` Michal Hocko
2017-03-10 11:44 ` Tetsuo Handa
2017-03-21 10:37 ` Tetsuo Handa
2017-04-23 10:24 ` Tetsuo Handa
2017-04-24 12:39 ` Stanislaw Gruszka
2017-04-24 13:06 ` Tetsuo Handa
2017-04-25 6:33 ` Stanislaw Gruszka
2017-06-30 0:14 ` Tetsuo Handa
2017-06-30 13:32 ` Michal Hocko
2017-06-30 15:59 ` Tetsuo Handa
2017-06-30 16:19 ` Michal Hocko
2017-07-01 11:43 ` Tetsuo Handa
2017-07-05 8:19 ` Michal Hocko
2017-07-05 8:20 ` Michal Hocko
2017-07-06 10:48 ` Tetsuo Handa
2017-03-09 14:31 ` Mel Gorman
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