linux-kernel - Re: [PATCH v2 8/8] mm: Centralize & improve oom reporting in show

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Date:   Fri, 22 Apr 2022 08:09:48 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Kent Overstreet <kent.overstreet@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, hch@....de, hannes@...xchg.org,
        akpm@...ux-foundation.org, linux-clk@...r.kernel.org,
        linux-tegra@...r.kernel.org, linux-input@...r.kernel.org
Subject: Re: [PATCH v2 8/8] mm: Centralize & improve oom reporting in
 show_mem.c

On Fri, Apr 22, 2022 at 02:58:19PM +0200, Michal Hocko wrote:
> On Thu 21-04-22 19:48:37, Kent Overstreet wrote:
> > This patch:
> >  - Changes show_mem() to always report on slab usage
> >  - Instead of reporting on all slabs, we only report on top 10 slabs,
> >    and in sorted order
> 
> As I've already pointed out in the email thread for the previous
> version, this would be better in its own patch explaining why we want to
> make this unconditional and why to limit the number caches to print.
> Why the trashold shouldn't be absolute size based?
> 
> >  - Also reports on shrinkers, with the new shrinkers_to_text().
> >    Shrinkers need to be included in OOM/allocation failure reporting
> >    because they're responsible for memory reclaim - if a shrinker isn't
> >    giving up its memory, we need to know which one and why.
>
> Again, I do agree that information about shrinkers can be useful but
> there are two main things to consider. Do we want to dump that
> information unconditionaly? E.g. does it make sense to print for all
> allocation requests (even high order, GFP_NOWAIT...)? Should there be
> any explicit trigger when to dump this data (like too many shrinkers
> failing etc)?

To add a concern: largest shrinkers are usually memcg-aware. Scanning
over the whole cgroup tree (with potentially hundreds or thousands of cgroups)
and over all shrinkers from the oom context sounds like a bad idea to me.

IMO it's more appropriate to do from userspace by oomd or a similar daemon,
well before the in-kernel OOM kicks in.

> 
> Last but not least let me echo the concern from the other reply. Memory
> allocations are not really reasonable to be done from the oom context so
> the pr_buf doesn't sound like a good tool here.

+1