Re: [PATCH v6 25/29] memcg/sl[au]b: shrink dead caches

[Glauber Costa <glommer@parallels.com>]

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On 11/07/2012 08:16 AM, Andrew Morton wrote:
> On Wed, 7 Nov 2012 08:13:08 +0100 Glauber Costa <glommer@parallels.com> wrote:
> 
>> On 11/06/2012 01:48 AM, Andrew Morton wrote:
>>> On Thu,  1 Nov 2012 16:07:41 +0400
>>> Glauber Costa <glommer@parallels.com> wrote:
>>>
>>>> This means that when we destroy a memcg cache that happened to be empty,
>>>> those caches may take a lot of time to go away: removing the memcg
>>>> reference won't destroy them - because there are pending references, and
>>>> the empty pages will stay there, until a shrinker is called upon for any
>>>> reason.
>>>>
>>>> In this patch, we will call kmem_cache_shrink for all dead caches that
>>>> cannot be destroyed because of remaining pages. After shrinking, it is
>>>> possible that it could be freed. If this is not the case, we'll schedule
>>>> a lazy worker to keep trying.
>>>
>>> This patch is really quite nasty.  We poll the cache once per minute
>>> trying to shrink then free it?  a) it gives rise to concerns that there
>>> will be scenarios where the system could suffer unlimited memory windup
>>> but mainly b) it's just lame.
>>>
>>> The kernel doesn't do this sort of thing.  The kernel tries to be
>>> precise: in a situation like this we keep track of the number of
>>> outstanding objects and when that falls to zero, we free their
>>> container synchronously.  If those objects are normally left floating
>>> around in an allocated but reclaimable state then we can address that
>>> by synchronously freeing them if their container has been destroyed.
>>>
>>> Or something like that.  If it's something else then fine, but not this.
>>>
>>> What do we need to do to fix this?
>>>
>> The original patch had a unlikely() test in the free path, conditional
>> on whether or not the cache is dead, that would then call this is the
>> cache would now be empty.
>>
>> I got several requests to remove it and change it to something like
>> this, because that is a fast path (I myself think an unlikely branch is
>> not that bad)
>>
>> If you think such a test is acceptable, I can bring it back and argue in
>> the basis of "akpm made me do it!". But meanwhile I will give this extra
>> though to see if there is any alternative way I can do it...
> 
> OK, thanks, please do take a look at it.
> 
> I'd be interested in seeing the old version of the patch which had this
> test-n-branch.  Perhaps there's some trick we can pull to lessen its cost.
> 
Attached.

This is the last version that used it (well, I believe it is). There is
other unrelated things in this patch, that I got rid of. Look for
kmem_cache_verify_dead().

In a summary, all calls to the free function would as a last step do:
kmem_cache_verify_dead() that would either be an empty placeholder, or:

+static inline void kmem_cache_verify_dead(struct kmem_cache *s)
+{
+       if (unlikely(s->memcg_params.dead))
+               schedule_work(&s->memcg_params.cache_shrinker);
+}


cache_shrinker got changed to the destroy worker. So if we are freeing
an object from a cache that is dead, we try to schedule a worker that
will eventually call kmem_cache_srhink(), and hopefully
kmem_cache_destroy() - if last object.


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