linux-kernel - Re: [PATCH RFC 2/2] mm: kmem: add direct objcg pointer to task

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Date:   Thu, 22 Dec 2022 08:21:49 -0800
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH RFC 2/2] mm: kmem: add direct objcg pointer to task_struct

On Thu, Dec 22, 2022 at 02:50:44PM +0100, Michal Koutný wrote:
> On Tue, Dec 20, 2022 at 10:27:45AM -0800, Roman Gushchin <roman.gushchin@...ux.dev> wrote:
> > To charge a freshly allocated kernel object to a memory cgroup, the
> > kernel needs to obtain an objcg pointer. Currently it does it
> > indirectly by obtaining the memcg pointer first and then calling to
> > __get_obj_cgroup_from_memcg().
> 
> Jinx [1].
> 
> You report additional 7% improvement with this patch (focused on
> allocations only). I didn't see impressive numbers (different benchmark
> in [1]), so it looked as a microoptimization without big benefit to me.

Hi Michal!

Thank you for taking a look.
Do you have any numbers to share?

In general, I agree that it's a micro-optimization, but:
1) some people periodically complain that accounted allocations are slow
   in comparison to non-accounted and slower than they were with page-based
   accounting,
2) I don't see any particular hot point or obviously non-optimal place on the
   allocation path.

so if we want to make it faster, we have to micro-optimize it here and there,
no other way. It's basically the question how many cache lines we touch.

Btw, I'm working on a patch 3 for this series, which in early tests brings
additional ~25% improvement in my benchmark, hopefully will post it soon as
a part of v1.

Thanks!