linux-kernel - Re: [PATCH 2/2] mm, slab: Extend vm/drop

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Message-ID: <20190628173040.GA11971@tower.DHCP.thefacebook.com>
Date:   Fri, 28 Jun 2019 17:30:46 +0000
From:   Roman Gushchin <guro@...com>
To:     Yang Shi <shy828301@...il.com>
CC:     Christopher Lameter <cl@...ux.com>,
        Waiman Long <longman@...hat.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jonathan Corbet <corbet@....net>,
        Luis Chamberlain <mcgrof@...nel.org>,
        "Kees Cook" <keescook@...omium.org>,
        Johannes Weiner <hannes@...xchg.org>,
        "Michal Hocko" <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        "Andrea Arcangeli" <aarcange@...hat.com>
Subject: Re: [PATCH 2/2] mm, slab: Extend vm/drop_caches to shrink kmem slabs

On Fri, Jun 28, 2019 at 10:16:13AM -0700, Yang Shi wrote:
> On Fri, Jun 28, 2019 at 8:32 AM Christopher Lameter <cl@...ux.com> wrote:
> >
> > On Thu, 27 Jun 2019, Roman Gushchin wrote:
> >
> > > so that objects belonging to different memory cgroups can share the same page
> > > and kmem_caches.
> > >
> > > It's a fairly big change though.
> >
> > Could this be done at another level? Put a cgoup pointer into the
> > corresponding structures and then go back to just a single kmen_cache for
> > the system as a whole? You can still account them per cgroup and there
> > will be no cleanup problem anymore. You could scan through a slab cache
> > to remove the objects of a certain cgroup and then the fragmentation
> > problem that cgroups create here will be handled by the slab allocators in
> > the traditional way. The duplication of the kmem_cache was not designed
> > into the allocators but bolted on later.
> 
> I'm afraid this may bring in another problem for memcg page reclaim.
> When shrinking the slabs, the shrinker may end up scanning a very long
> list to find out the slabs for a specific memcg. Particularly for the
> count operation, it may have to scan the list from the beginning all
> the way down to the end. It may take unbounded time.
> 
> When I worked on THP deferred split shrinker problem, I used to do
> like this, but it turns out it may take milliseconds to count the
> objects on the list, but it may just need reclaim a few of them.

I don't think the shrinker mechanism should be altered. Shrinker lists
already contain individual objects, and I don't see any reasons, why
these objects can't reside on a shared set of pages.

What we're discussing is that it's way too costly (under some conditions)
to have many sets of kmem_caches, if each of them is containing only
few objects.

Thanks!