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Date:   Wed, 20 Dec 2017 12:34:04 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm/memcg: try harder to decrease
 [memory,memsw].limit_in_bytes

On Wed 20-12-17 14:32:19, Andrey Ryabinin wrote:
> On 12/20/2017 01:33 PM, Michal Hocko wrote:
> > On Wed 20-12-17 13:24:28, Andrey Ryabinin wrote:
> >> mem_cgroup_resize_[memsw]_limit() tries to free only 32 (SWAP_CLUSTER_MAX)
> >> pages on each iteration. This makes practically impossible to decrease
> >> limit of memory cgroup. Tasks could easily allocate back 32 pages,
> >> so we can't reduce memory usage, and once retry_count reaches zero we return
> >> -EBUSY.
> >>
> >> It's easy to reproduce the problem by running the following commands:
> >>
> >>   mkdir /sys/fs/cgroup/memory/test
> >>   echo $$ >> /sys/fs/cgroup/memory/test/tasks
> >>   cat big_file > /dev/null &
> >>   sleep 1 && echo $((100*1024*1024)) > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
> >>   -bash: echo: write error: Device or resource busy
> >>
> >> Instead of trying to free small amount of pages, it's much more
> >> reasonable to free 'usage - limit' pages.
> > 
> > But that only makes the issue less probable. It doesn't fix it because 
> > 		if (curusage >= oldusage)
> > 			retry_count--;
> > can still be true because allocator might be faster than the reclaimer.
> > Wouldn't it be more reasonable to simply remove the retry count and keep
> > trying until interrupted or we manage to update the limit.
> 
> But does it makes sense to continue reclaiming even if reclaimer can't
> make any progress? I'd say no. "Allocator is faster than reclaimer"
> may be not the only reason for failed reclaim. E.g. we could try to
> set limit lower than amount of mlock()ed memory in cgroup, retrying
> reclaim would be just a waste of machine's resources.  Or we simply
> don't have any swap, and anon > new_limit. Should be burn the cpu in
> that case?

We can check the number of reclaimed pages and go EBUSY if it is 0.
 
> > Another option would be to commit the new limit and allow temporal overcommit
> > of the hard limit. New allocations and the limit update paths would
> > reclaim to the hard limit.
> > 
> 
> It sounds a bit fragile and tricky to me. I wouldn't go that way
> without unless we have a very good reason for this.

I haven't explored this, to be honest, so there may be dragons that way.
I've just mentioned that option for completness.
-- 
Michal Hocko
SUSE Labs

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