Date: Fri, 11 Mar 2016 11:34:40 +0300
From: Vladimir Davydov <vdavydov@virtuozzo.com>
To: Johannes Weiner <hannes@cmpxchg.org>
CC: Andrew Morton <akpm@linux-foundation.org>, Michal Hocko <mhocko@suse.cz>,
        <linux-mm@kvack.org>, <cgroups@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <kernel-team@fb.com>
Subject: Re: [PATCH] mm: memcontrol: reclaim when shrinking memory.high below
 usage
Message-ID: <20160311083440.GI1946@esperanza>
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On Thu, Mar 10, 2016 at 03:50:13PM -0500, Johannes Weiner wrote:
> When setting memory.high below usage, nothing happens until the next
> charge comes along, and then it will only reclaim its own charge and
> not the now potentially huge excess of the new memory.high. This can
> cause groups to stay in excess of their memory.high indefinitely.
> 
> To fix that, when shrinking memory.high, kick off a reclaim cycle that
> goes after the delta.

I agree that we should reclaim the high excess, but I don't think it's a
good idea to do it synchronously. Currently, memory.low and memory.high
knobs can be easily used by a single-threaded load manager implemented
in userspace, because it doesn't need to care about potential stalls
caused by writes to these files. After this change it might happen that
a write to memory.high would take long, seconds perhaps, so in order to
react quickly to changes in other cgroups, a load manager would have to
spawn a thread per each write to memory.high, which would complicate its
implementation significantly.

Since, in contrast to memory.max, memory.high definition allows cgroup
to breach it, I believe it would be better if we spawned an asynchronous
reclaim work from the kernel on write to memory.high instead of doing
this synchronously. I guess we could reuse mem_cgroup->high_work for
that.

Thanks,
Vladimir