Date: Mon, 2 Nov 2015 16:01:37 +0100
From: Michal Hocko <mhocko@kernel.org>
To: Tejun Heo <htejun@gmail.com>
Cc: Christoph Lameter <cl@linux.com>,
        Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
        David Rientjes <rientjes@google.com>, oleg@redhat.com,
        kwalker@redhat.com, akpm@linux-foundation.org, hannes@cmpxchg.org,
        vdavydov@parallels.com, skozina@redhat.com, mgorman@suse.de,
        riel@redhat.com
Subject: Re: [PATCH] mm,vmscan: Use accurate values for zone_reclaimable()
 checks
Message-ID: <20151102150137.GB3442@dhcp22.suse.cz>
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On Fri 23-10-15 13:26:49, Tejun Heo wrote:
> Hello,
> 
> So, something like the following.  Just compile tested but this is
> essentially partial revert of 3270476a6c0c ("workqueue: reimplement
> WQ_HIGHPRI using a separate worker_pool") - resurrecting the old
> WQ_HIGHPRI implementation under WQ_IMMEDIATE, so we know this works.
> If for some reason, it gets decided against simply adding one jiffy
> sleep, please let me know.  I'll verify the operation and post a
> proper patch.  That said, given that this prolly needs -stable
> backport and vmstat is likely to be the only user (busy loops are
> really rare in the kernel after all), I think the better approach
> would be reinstating the short sleep.

As already pointed out I really detest a short sleep and would prefer
a way to tell WQ what we really need. vmstat is not the only user. OOM
sysrq will need this special treatment as well. While the
zone_reclaimable can be fixed in an easy patch
(http://lkml.kernel.org/r/201510212126.JIF90648.HOOFJVFQLMStOF%40I-love.SAKURA.ne.jp)
which is perfectly suited for the stable backport, OOM sysrq resp. any
sysrq which runs from the WQ context should be as robust as possible and
shouldn't rely on all the code running from WQ context to issue a sleep
to get unstuck. So I definitely support something like this patch.

I am still not sure whether other WQ_MEM_RECLAIM users needs this flag
as well because I am not familiar with their implementation but at
vmstat and sysrq should use it and should be safe to do so without risk
of breaking anything AFAICS.

Thanks!
-- 
Michal Hocko
SUSE Labs
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