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From: Shakeel Butt To: Usama Arif Cc: Andrew Morton , david@kernel.org, linux-mm@kvack.org, hannes@cmpxchg.org, tj@kernel.org, mkoutny@suse.com, roman.gushchin@linux.dev, liam@infradead.org, linux-kernel@vger.kernel.org, ljs@kernel.org, mhocko@suse.com, rppt@kernel.org, surenb@google.com, vbabka@kernel.org, kernel-team@meta.com Subject: Re: [PATCH 0/2] mm/vmpressure: reduce CPU, memory and code overhead on cgroup v2 Message-ID: References: <20260606114158.3126210-1-usama.arif@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260606114158.3126210-1-usama.arif@linux.dev> X-Migadu-Flow: FLOW_OUT On Sat, Jun 06, 2026 at 04:41:32AM -0700, Usama Arif wrote: > The vmpressure subsystem has two distinct consumers, gated by the > @tree argument: > > tree=false : in-kernel socket pressure, consumed by TCP/SCTP. This > is cgroup v2 only; v1 sockets read memcg->tcpmem_pressure > instead. We should really move v2 away from vmpressure. > tree=true : cgroup v1 userspace eventfd notifications via the > memory.pressure_level / cgroup.event_control interface. > v2 has no equivalent (userspace gets reclaim signals > through memory.pressure / PSI, which doesn't touch > vmpressure). > > So of the four (hierarchy, tree) combinations, only two carry data > that anyone reads. The existing early return in vmpressure() covered > v1 + tree=false; the symmetric v2 + tree=true case was falling through > and doing the full lock / accumulate / schedule_work / parent-walk > dance, even though the events list it eventually iterates is empty > on cgroup v2 (vmpressure_register_event() is wired up only through the > v1 cftype "memory.pressure_level" and can't be reached from a v2 > memcg). > > Patch 1 extends the existing early return to also skip v2 + tree=true. > On a v2-only host this eliminates a contended path where reclaimers > can serialize on a single global sr_lock. bpftrace on a 176-core production > host (cgroup v2, 285 memcgs, sustained reclaim) showed ~16,200 such calls > per minute with tree = true. This is good. > > Patch 2 follows up with a cleanup: it splits the v1 userspace eventfd > interface (struct vmpressure_event, the events list and its mutex, the > work_struct and its handler, the parent walk, > vmpressure_register_event / unregister_event, and vmpressure_prio) > into a new mm/vmpressure-v1.c built only when CONFIG_MEMCG_V1=y, > behind small no-op stubs in the header. mm/vmpressure.c keeps the > shared bits and the tree=false socket-pressure path. The size of > vmpressure.c goes down to half and the code is much more simpler. > The only #ifdef CONFIG_MEMCG_V1 remaining in source is around the > v1-only fields inside struct vmpressure itself. Memory savings on > CONFIG_MEMCG_V1=n: > struct vmpressure : 112B -> 24B > struct mem_cgroup : 1664B -> 1536B For this, I am wondering if we should just go ahead and work towards making vmpressure memcg-v1 only unless we foresee a lot of or complex work is needed for that and only then patch 2 makes sense.