On Thu, 23 Jun 2022 18:50:07 -0400 Xin Long wrote:

From the perf data, we can see __sk_mem_reduce_allocated() is the one
using CPU the most more than before, and mem_cgroup APIs are also
called in this function. It means the mem cgroup must be enabled in
the test env, which may explain why I couldn't reproduce it.

The Commit 4890b686f4 ("net: keep sk->sk_forward_alloc as small as
possible") uses sk_mem_reclaim(checking reclaimable >= PAGE_SIZE) to
reclaim the memory, which is *more frequent* to call
__sk_mem_reduce_allocated() than before (checking reclaimable >=
SK_RECLAIM_THRESHOLD). It might be cheap when
mem_cgroup_sockets_enabled is false, but I'm not sure if it's still
cheap when mem_cgroup_sockets_enabled is true.

I think SCTP netperf could trigger this, as the CPU is the bottleneck
for SCTP netperf testing, which is more sensitive to the extra
function calls than TCP.

Can we re-run this testing without mem cgroup enabled?

FWIW I defer to Eric, thanks a lot for double checking the report 
and digging in!