Re: [PATCH v12 09/18] vmscan: shrink slab on memcg pressure

[Vladimir Davydov <vdavydov@parallels.com>]

On 12/04/2013 08:51 AM, Dave Chinner wrote:
> On Tue, Dec 03, 2013 at 04:15:57PM +0400, Vladimir Davydov wrote:
>> On 12/03/2013 02:48 PM, Dave Chinner wrote:
>>>> @@ -236,11 +236,17 @@ shrink_slab_node(struct shrink_control *shrinkctl, struct shrinker *shrinker,
>>>>  		return 0;
>>>>  
>>>>  	/*
>>>> -	 * copy the current shrinker scan count into a local variable
>>>> -	 * and zero it so that other concurrent shrinker invocations
>>>> -	 * don't also do this scanning work.
>>>> +	 * Do not touch global counter of deferred objects on memcg pressure to
>>>> +	 * avoid isolation issues. Ideally the counter should be per-memcg.
>>>>  	 */
>>>> -	nr = atomic_long_xchg(&shrinker->nr_deferred[nid], 0);
>>>> +	if (!shrinkctl->target_mem_cgroup) {
>>>> +		/*
>>>> +		 * copy the current shrinker scan count into a local variable
>>>> +		 * and zero it so that other concurrent shrinker invocations
>>>> +		 * don't also do this scanning work.
>>>> +		 */
>>>> +		nr = atomic_long_xchg(&shrinker->nr_deferred[nid], 0);
>>>> +	}
>>> That's ugly. Effectively it means that memcg reclaim is going to be
>>> completely ineffective when large numbers of allocations and hence
>>> reclaim attempts are done under GFP_NOFS context.
>>>
>>> The only thing that keeps filesystem caches in balance when there is
>>> lots of filesystem work going on (i.e. lots of GFP_NOFS allocations)
>>> is the deferal of reclaim work to a context that can do something
>>> about it.
>> Imagine the situation: a memcg issues a GFP_NOFS allocation and goes to
>> shrink_slab() where it defers them to the global counter; then another
>> memcg issues a GFP_KERNEL allocation, also goes to shrink_slab() where
>> it sees a huge number of deferred objects and starts shrinking them,
>> which is not good IMHO.
> That's exactly what the deferred mechanism is for - we know we have
> to do the work, but we can't do it right now so let someone else do
> it who can.
>
> In most cases, deferral is handled by kswapd, because when a
> filesystem workload is causing memory pressure then most allocations
> are done in GFP_NOFS conditions. Hence the only memory reclaim that
> can make progress here is kswapd.
>
> Right now, you aren't deferring any of this memory pressure to some
> other agent, so it just does not get done. That's a massive problem
> - it's a design flaw - and instead I see lots of crazy hacks being
> added to do stuff that should simply be deferred to kswapd like is
> done for global memory pressure.
>
> Hell, kswapd shoul dbe allowed to walk memcg LRU lists and trim
> them, just like it does for the global lists. We only need a single
> "deferred work" counter per node for that - just let kswapd
> proportion the deferred work over the per-node LRU and the
> memcgs....

Seems I misunderstand :-(

Let me try. You mean we have the only nr_deferred counter per-node, and
kswapd scans

nr_deferred*memcg_kmem_size/total_kmem_size

objects in each memcg, right?

Then if there were a lot of objects deferred on memcg (not global)
pressure due to a memcg issuing a lot of GFP_NOFS allocations, kswapd
will reclaim objects from all, even unlimited, memcgs. This looks like
an isolation issue :-/

Currently we have a per-node nr_deferred counter for each shrinker. If
we add per-memcg reclaim, we have to make it per-memcg per-node, don't we?

Thanks.

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