Re: [PATCH v3 1/2] mm/vmpressure: skip tree=true accounting on cgroup v2

[Johannes Weiner]

On Tue, Jun 30, 2026 at 04:23:32AM -0700, Usama Arif wrote:
> vmpressure() has two outputs gated by the @tree argument:
> 
>   @tree=false drives in-kernel socket pressure (mem_cgroup_set_
>               socket_pressure), consumed by TCP/SCTP. This only
>               applies on cgroup v2; on v1 socket memory is charged
>               separately via tcpmem and the consumer reads
>               memcg->tcpmem_pressure instead.
> 
>   @tree=true  drives userspace eventfd notifications via the v1
>               memory.pressure_level / cgroup.event_control interface.
>               v2 has no equivalent: userspace gets reclaim signals
>               through memory.pressure (PSI), which does not touch
>               vmpressure.
> 
> The existing early return covered v1 + @tree=false. The symmetric
> v2 + @tree=true case was falling through and doing the full lock /
> accumulate / schedule_work / parent-walk dance for an events list
> that can never be populated. bpftrace on a 176-core production host
> (cgroup v2, CONFIG_MEMCG_V1=n, 285 memcgs, sustained reclaim) showed
> ~16,200 @tree=true vmpressure() calls per minute. Add an early return
> that skips cgroup v2 + tree = true which avoids us doing all this work.
> On a v2-only host this also eliminates a lock contention path that can
> serialise reclaimers on a single global sr_lock.
> 
> Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
> ---
>  mm/vmpressure.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/vmpressure.c b/mm/vmpressure.c
> index f053554e5826..c82cee1ab43b 100644
> --- a/mm/vmpressure.c
> +++ b/mm/vmpressure.c
> @@ -246,11 +246,13 @@ void vmpressure(gfp_t gfp, int order, struct mem_cgroup *memcg, bool tree,
>  		return;
>  
>  	/*
> -	 * The in-kernel users only care about the reclaim efficiency
> -	 * for this @memcg rather than the whole subtree, and there
> -	 * isn't and won't be any in-kernel user in a legacy cgroup.
> +	 * Only two combinations have a consumer:
> +	 *   cgroup v2 + tree=false -> in-kernel socket pressure
> +	 *   cgroup v1 + tree=true  -> userspace eventfds (memory.pressure_level)
> +	 * Skip the other two: nothing consumes the result.
>  	 */
> -	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && !tree)
> +	if ((!cgroup_subsys_on_dfl(memory_cgrp_subsys) && !tree) ||
> +	    (cgroup_subsys_on_dfl(memory_cgrp_subsys) && tree))
>  		return;

I had already acked this one, with a half serious suggestion to make
this

	if (cgroup_subsys_on_dfl(memory_cgrp_subsys) == tree)
		return;

Anyway, no strong feelings. If nobody agrees,

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>