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

Feng Tang
2022-06-27T14:48:31Z

On Mon, Jun 27, 2022 at 04:07:55PM +0200, Eric Dumazet wrote:
> On Mon, Jun 27, 2022 at 2:34 PM Feng Tang <feng.tang@intel.com> wrote:
> >
> > On Mon, Jun 27, 2022 at 10:46:21AM +0200, Eric Dumazet wrote:
> > > On Mon, Jun 27, 2022 at 4:38 AM Feng Tang <feng.tang@intel.com> wrote:
> > [snip]
> > > > > >
> > > > > > Thanks Feng. Can you check the value of memory.kmem.tcp.max_usage_in_bytes
> > > > > > in /sys/fs/cgroup/memory/system.slice/lkp-bootstrap.service after making
> > > > > > sure that the netperf test has already run?
> > > > >
> > > > > memory.kmem.tcp.max_usage_in_bytes:0
> > > >
> > > > Sorry, I made a mistake that in the original report from Oliver, it
> > > > was 'cgroup v2' with a 'debian-11.1' rootfs.
> > > >
> > > > When you asked about cgroup info, I tried the job on another tbox, and
> > > > the original 'job.yaml' didn't work, so I kept the 'netperf' test
> > > > parameters and started a new job which somehow run with a 'debian-10.4'
> > > > rootfs and acutally run with cgroup v1.
> > > >
> > > > And as you mentioned cgroup version does make a big difference, that
> > > > with v1, the regression is reduced to 1% ~ 5% on different generations
> > > > of test platforms. Eric mentioned they also got regression report,
> > > > but much smaller one, maybe it's due to the cgroup version?
> > >
> > > This was using the current net-next tree.
> > > Used recipe was something like:
> > >
> > > Make sure cgroup2 is mounted or mount it by mount -t cgroup2 none $MOUNT_POINT.
> > > Enable memory controller by echo +memory > $MOUNT_POINT/cgroup.subtree_control.
> > > Create a cgroup by mkdir $MOUNT_POINT/job.
> > > Jump into that cgroup by echo $$ > $MOUNT_POINT/job/cgroup.procs.
> > >
> > > <Launch tests>
> > >
> > > The regression was smaller than 1%, so considered noise compared to
> > > the benefits of the bug fix.
> >
> > Yes, 1% is just around noise level for a microbenchmark.
> >
> > I went check the original test data of Oliver's report, the tests was
> > run 6 rounds and the performance data is pretty stable (0Day's report
> > will show any std deviation bigger than 2%)
> >
> > The test platform is a 4 sockets 72C/144T machine, and I run the
> > same job (nr_tasks = 25% * nr_cpus) on one CascadeLake AP (4 nodes)
> > and one Icelake 2 sockets platform, and saw 75% and 53% regresson on
> > them.
> >
> > In the first email, there is a file named 'reproduce', it shows the
> > basic test process:
> >
> > "
> > use 'performane' cpufre governor for all CPUs
> >
> > netserver -4 -D
> > modprobe sctp
> > netperf -4 -H 127.0.0.1 -t SCTP_STREAM_MANY -c -C -l 300 -- -m 10K &
> > netperf -4 -H 127.0.0.1 -t SCTP_STREAM_MANY -c -C -l 300 -- -m 10K &
> > netperf -4 -H 127.0.0.1 -t SCTP_STREAM_MANY -c -C -l 300 -- -m 10K &
> > (repeat 36 times in total)
> > ...
> >
> > "
> >
> > Which starts 36 (25% of nr_cpus) netperf clients. And the clients number
> > also matters, I tried to increase the client number from 36 to 72(50%),
> > and the regression is changed from 69.4% to 73.7%"
> >
>
> This seems like a lot of opportunities for memcg folks :)
>
> struct page_counter has poor field placement [1], and no per-cpu cache.
>
> [1] "atomic_long_t usage" is sharing cache line with read mostly fields.
>
> (struct mem_cgroup also has poor field placement, mainly because of
> struct page_counter)
>
> 28.69% [kernel] [k] copy_user_enhanced_fast_string
> 16.13% [kernel] [k] intel_idle_irq
> 6.46% [kernel] [k] page_counter_try_charge
> 6.20% [kernel] [k] __sk_mem_reduce_allocated
> 5.68% [kernel] [k] try_charge_memcg
> 5.16% [kernel] [k] page_counter_cancel

Yes, I also analyzed the perf-profile data, and made some layout changes
which could recover the changes from 69% to 40%.

7c80b038d23e1f4c 4890b686f4088c90432149bd6de 332b589c49656a45881bca4ecc0
---------------- --------------------------- ---------------------------
15722 -69.5% 4792 -40.8% 9300 netperf.Throughput_Mbps

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 1bfcfb1af352..aa37bd39116c 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -179,14 +179,13 @@ struct cgroup_subsys_state {
atomic_t online_cnt;

/* percpu_ref killing and RCU release */
- struct work_struct destroy_work;
struct rcu_work destroy_rwork;
-
+ struct cgroup_subsys_state *parent;
+ struct work_struct destroy_work;
/*
* PI: the parent css. Placed here for cache proximity to following
* fields of the containing structure.
*/
- struct cgroup_subsys_state *parent;
};

/*
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 9ecead1042b9..963b88ab9930 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -239,9 +239,6 @@ struct mem_cgroup {
/* Private memcg ID. Used to ID objects that outlive the cgroup */
struct mem_cgroup_id id;

- /* Accounted resources */
- struct page_counter memory; /* Both v1 & v2 */
-
union {
struct page_counter swap; /* v2 only */
struct page_counter memsw; /* v1 only */
@@ -251,6 +248,9 @@ struct mem_cgroup {
struct page_counter kmem; /* v1 only */
struct page_counter tcpmem; /* v1 only */

+ /* Accounted resources */
+ struct page_counter memory; /* Both v1 & v2 */
+
/* Range enforcement for interrupt charges */
struct work_struct high_work;

@@ -313,7 +313,6 @@ struct mem_cgroup {
atomic_long_t memory_events[MEMCG_NR_MEMORY_EVENTS];
atomic_long_t memory_events_local[MEMCG_NR_MEMORY_EVENTS];

- unsigned long socket_pressure;

/* Legacy tcp memory accounting */
bool tcpmem_active;
@@ -349,6 +348,7 @@ struct mem_cgroup {
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
struct deferred_split deferred_split_queue;
#endif
+ unsigned long socket_pressure;

struct mem_cgroup_per_node *nodeinfo[];
};

And some of these are specific for network and may not be a universal
win, though I think the 'cgroup_subsys_state' could keep the
read-mostly 'parent' away from following written-mostly counters.

Btw, I tried your debug patch which compiled fail with 0Day's kbuild
system, but it did compile ok on my local machine.

Thanks,
Feng

>
> > Thanks,
> > Feng
> >
> > > >
> > > > Thanks,
> > > > Feng