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From: Shakeel Butt To: Andrew Morton Cc: Tejun Heo , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Vlastimil Babka , Alexei Starovoitov , Sebastian Andrzej Siewior , Michal =?utf-8?Q?Koutn=C3=BD?= , Harry Yoo , Yosry Ahmed , bpf@vger.kernel.org, linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Meta kernel team Subject: Re: [PATCH 0/3] cgroup: nmi safe css_rstat_updated Message-ID: References: <20250609225611.3967338-1-shakeel.butt@linux.dev> <20250609164410.568fd70e6a1deb6556e25af7@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250609164410.568fd70e6a1deb6556e25af7@linux-foundation.org> X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: F273F40009 X-Stat-Signature: 7j6q9tchygnubxdarjzneaksmtdkt4sj X-Rspam-User: X-HE-Tag: 1749513128-514796 X-HE-Meta: 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 WmBB7O88 6X/bltm+eR6KyGxKSe+r8PxSeBN8flLKm99FNA09uUSLlNewlS3ltjiDsNF4Nj7e6oUVvNAA+ncZ6CImqZJFD0CXsFiZdIInVdHEtEvus+SW6r5f/TLYWEeZ4GOj1G7pUfWQ3e/esRKIdUkoiIYtVq39vM0faUjEYFFGGRwGKANcws7es4AG7YIZUSYKx13tna3YLvfjuVgrHDOuEvUaFkpf7/b6gCZjSzNUKm/lRTtUnop6MzVf0CPkvoznEyx7F6RhNEhZs52fa4KI= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Jun 09, 2025 at 04:44:10PM -0700, Andrew Morton wrote: > On Mon, 9 Jun 2025 15:56:08 -0700 Shakeel Butt 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. > > > > This series made css_rstat_updated by using per-cpu lockless lists whose > > node in embedded in individual struct cgroup_subsys_state and the > > per-cpu head is placed in struct cgroup_subsys. For rstat users without > > cgroup_subsys, a global per-cpu lockless list head is created. The main > > challenge to use lockless in this scenario was the potential multiple > > inserters using the same lockless node of a cgroup_subsys_state which is > > different from traditional users of lockless lists. > > > > The multiple inserters using potentially same lockless node was resolved > > by making one of them succeed on reset the lockless node and the winner > > gets to insert the lockless node in the corresponding lockless list. > > And what happens with the losers? Losers can continue their normal work without worrying about this specific insertion. Basically we need one successful insertion. In addition this is a contention between process context, softirq, hardirq and nmi on the same cpu for the same cgroup which should be very unlikely.