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

[Usama Arif <usama.arif@linux.dev>]

On 30/06/2026 17:07, Johannes Weiner wrote:
> 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@linux.dev>
>> Signed-off-by: Usama Arif <usama.arif@linux.dev>
>> ---
>>  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@cmpxchg.org>

Yeah sorry about this! I just amended my last patch to move code
from vmpressure-v1.c to memcontrol-v1.c and just sent it, without
other changes. Forgot Shakeels ack on v2 as well :(

Andrew would you mind applying the below fixlet? I can also respin
if its easier. Thanks!!

From 969c19da782bbcd77ae4b9e94d3a9e1d78c198d7 Mon Sep 17 00:00:00 2001
From: Usama Arif <usama.arif@linux.dev>
Date: Tue, 30 Jun 2026 09:25:05 -0700
Subject: [fixlet] mm/vmpressure: skip tree=true accounting on cgroup v2

Simplify the guard. Both cgroup_subsys_on_dfl() and tree are bool, so
the two combinations that have no consumer (v1 + tree=false, v2 +
tree=true) are exactly the cases where dfl == tree.

Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/vmpressure.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index 14470141bbe6..9629240d77ad 100644
--- a/mm/vmpressure.c
+++ b/mm/vmpressure.c
@@ -120,8 +120,7 @@ void vmpressure(gfp_t gfp, int order, struct mem_cgroup *memcg, bool tree,
 	 *   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) ||
-	    (cgroup_subsys_on_dfl(memory_cgrp_subsys) && tree))
+	if (cgroup_subsys_on_dfl(memory_cgrp_subsys) == tree)
 		return;
 
 	vmpr = memcg_to_vmpressure(memcg);
-- 
2.53.0-Meta