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

Glauber Costa
2012-11-07T07:13:22Z

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...