netdev - Re: [net] 4890b686f4: netperf.Throughput

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Date:   Thu, 23 Jun 2022 18:57:30 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Xin Long <lucien.xin@...il.com>
Cc:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        kernel test robot <oliver.sang@...el.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        network dev <netdev@...r.kernel.org>,
        linux-s390@...r.kernel.org, mptcp@...ts.linux.dev,
        "linux-sctp @ vger . kernel . org" <linux-sctp@...r.kernel.org>,
        lkp@...ts.01.org, kbuild test robot <lkp@...el.com>,
        Huang Ying <ying.huang@...el.com>, feng.tang@...el.com,
        zhengjun.xing@...ux.intel.com, fengwei.yin@...el.com,
        Ying Xu <yinxu@...hat.com>
Subject: Re: [net] 4890b686f4: netperf.Throughput_Mbps -69.4% regression

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!