linux-kernel - Re: [PATCH 0/3] cgroup: nmi safe css_rstat

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Message-ID: <7u4lmf3yd5jlit7qyudbeuhpxvvjrmfq7arfvsxpkqwahm4326@4fbxtdpikosd>
Date: Tue, 10 Jun 2025 09:24:26 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Michal Koutný <mkoutny@...e.com>
Cc: Tejun Heo <tj@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	Vlastimil Babka <vbabka@...e.cz>, Alexei Starovoitov <ast@...nel.org>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Harry Yoo <harry.yoo@...cle.com>, 
	Yosry Ahmed <yosry.ahmed@...ux.dev>, bpf@...r.kernel.org, linux-mm@...ck.org, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH 0/3] cgroup: nmi safe css_rstat_updated

On Tue, Jun 10, 2025 at 12:53:11PM +0200, Michal Koutný wrote:
> On Mon, Jun 09, 2025 at 03:56:08PM -0700, Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> > BPF programs can run in nmi context and may trigger memcg charged memory
> > allocation in such context. Recently linux added support to nmi safe
> > page allocation along with memcg charging of such allocations. However
> > the kmalloc/slab support and corresponding memcg charging is still
> > lacking,
> > 
> > To provide nmi safe support for memcg charging for kmalloc/slab
> > allocations, we need nmi safe memcg stats and for that we need nmi safe
> > css_rstat_updated() which adds the given cgroup state whose stats are
> > updated into the per-cpu per-ss update tree. This series took the aim to
> > make css_rstat_updated() nmi safe.
> 
> memcg charging relies on page counters and per-cpu stocks.
> css_rstat_updated() is "only" for statistics (which has admiteddly some
> in-kernel consumers but those are already affected by batching and
> flushing errors).
> 
> Have I missed some updates that make css_rstat_updated() calls critical
> for memcg charging? I'd find it useful to explain this aspect more in
> the cover letter.

For kernel memory, the charging and stats (MEMCG_KMEM,
NR_SLAB_RECLAIMABLE_B, NR_SLAB_UNRECLAIMABLE_B) updates happen together.
I will add a line or two in the next version.