Re: [net] 4890b686f4: netperf.Throughput_Mbps -69.4% regression

[Eric Dumazet]

On Fri, Jun 24, 2022 at 7:14 AM Feng Tang <feng.tang@xxxxxxxxx> wrote:
>
> Hi Eric,
>
> On Fri, Jun 24, 2022 at 06:13:51AM +0200, Eric Dumazet wrote:
> > On Fri, Jun 24, 2022 at 3:57 AM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
> > >
> > > 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!
> >
> > I did tests with TCP + memcg and noticed a very small additional cost
> > in memcg functions,
> > because of suboptimal layout:
> >
> > Extract of an internal Google bug, update from June 9th:
> >
> > --------------------------------
> > I have noticed a minor false sharing to fetch (struct
> > mem_cgroup)->css.parent, at offset 0xc0,
> > because it shares the cache line containing struct mem_cgroup.memory,
> > at offset 0xd0
> >
> > Ideally, memcg->socket_pressure and memcg->parent should sit in a read
> > mostly cache line.
> > -----------------------
> >
> > But nothing that could explain a "-69.4% regression"
>
> We can double check that.
>
> > memcg has a very similar strategy of per-cpu reserves, with
> > MEMCG_CHARGE_BATCH being 32 pages per cpu.
>
> We have proposed patch to increase the batch numer for stats
> update, which was not accepted as it hurts the accuracy and
> the data is used by many tools.
>
> > It is not clear why SCTP with 10K writes would overflow this reserve constantly.
> >
> > Presumably memcg experts will have to rework structure alignments to
> > make sure they can cope better
> > with more charge/uncharge operations, because we are not going back to
> > gigantic per-socket reserves,
> > this simply does not scale.
>
> Yes, the memcg statitics and charge/unchage update is very sensitive
> with the data alignemnt layout, and can easily trigger peformance
> changes, as we've seen quite some similar cases in the past several
> years.
>
> One pattern we've seen is, even if a memcg stats updating or charge
> function only takes about 2%~3% of the CPU cycles in perf-profile data,
> once it got affected, the peformance change could be amplified to up to
> 60% or more.
>

Reorganizing "struct mem_cgroup" to put "struct page_counter memory"
in a separate cache line would be beneficial.

Many low hanging fruits, assuming nobody will use __randomize_layout on it ;)

Also some fields are written even if their value is not changed.

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index abec50f31fe64100f4be5b029c7161b3a6077a74..53d9c1e581e78303ef73942e2b34338567987b74
100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -7037,10 +7037,12 @@ bool mem_cgroup_charge_skmem(struct mem_cgroup
*memcg, unsigned int nr_pages,
                struct page_counter *fail;

                if (page_counter_try_charge(&memcg->tcpmem, nr_pages, &fail)) {
-                       memcg->tcpmem_pressure = 0;
+                       if (READ_ONCE(memcg->tcpmem_pressure))
+                               WRITE_ONCE(memcg->tcpmem_pressure, 0);
                        return true;
                }
-               memcg->tcpmem_pressure = 1;
+               if (!READ_ONCE(memcg->tcpmem_pressure))
+                       WRITE_ONCE(memcg->tcpmem_pressure, 1);
                if (gfp_mask & __GFP_NOFAIL) {
                        page_counter_charge(&memcg->tcpmem, nr_pages);
                        return true;