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

[Usama Arif]

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.

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;
 
 	vmpr = memcg_to_vmpressure(memcg);
-- 
2.52.0