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

Andrew Morton
2012-11-07T07:16:31Z

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.