linux-kernel - Re: [PATCH v3 04/19] mm: slub: implement SLUB version of obj_to

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Date:   Tue, 28 Apr 2020 13:45:26 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
Cc:     Christopher Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        kernel-team@...com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 04/19] mm: slub: implement SLUB version of
 obj_to_index()

On Mon, Apr 27, 2020 at 09:46:38AM -0700, Roman Gushchin wrote:
> On Mon, Apr 27, 2020 at 04:21:01PM +0000, Christoph Lameter wrote:
> > On Fri, 24 Apr 2020, Roman Gushchin wrote:
> > 
> > > > The patch seems to only use it for setup and debugging? It is used for
> > > > every "accounted" allocation???? Where? And what is an "accounted"
> > > > allocation?
> > > >
> > > >
> > >
> > > Please, take a look at the whole series:
> > > https://lore.kernel.org/linux-mm/20200422204708.2176080-1-guro@fb.com/T/#t
> > >
> > > I'm sorry, I had to cc you directly for the whole thing. Your feedback
> > > will be highly appreciated.
> > >
> > > It's used to calculate the offset of the memcg pointer for every slab
> > > object which is charged to a memory cgroup. So it must be quite hot.
> > 
> > 
> > Ahh... Thanks. I just looked at it.
> > 
> > You need this because you have a separate structure attached to a page
> > that tracks membership of the slab object to the cgroup. This is used to
> > calculate the offset into that array....
> > 
> > Why do you need this? Just slap a pointer to the cgroup as additional
> > metadata onto the slab object. Is that not much simpler, safer and faster?
> > 
> 
> So, the problem is that not all slab objects are accounted, and sometimes
> we don't know if advance if they are accounted or not (with the current semantics
> of __GFP_ACCOUNT and SLAB_ACCOUNT flags). So we either have to increase
> the size of ALL slab objects, either create a pair of slab caches for each size.

Both options seem completely disproportionate in their memory cost,
and the latter one in terms of code complexity, to avoid the offset
calculation. As a share of the total object accounting cost, I'd
expect this to be minimal.

Does the mult stand out in an annotated perf profile?

Is it enough to bring back 500+ lines of code, an additional branch on
accounted allocations, and the memory fragmentation of split caches?

I highly doubt it.