Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value

[Mel Gorman <mgorman@suse.de>]

On Thu, May 28, 2020 at 06:11:12PM +0200, Peter Zijlstra wrote:
> > FWIW, I think you're referring to Mel's notice in OSPM regarding the overhead.
> > Trying to see what goes on in there.
> 
> Indeed, that one. The fact that regular distros cannot enable this
> feature due to performance overhead is unfortunate. It means there is a
> lot less potential for this stuff.

During that talk, I was a vague about the cost, admitted I had not looked
too closely at mainline performance and had since deleted the data given
that the problem was first spotted in early April. If I heard someone
else making statements like I did at the talk, I would consider it a bit
vague, potentially FUD, possibly wrong and worth rechecking myself. In
terms of distributions "cannot enable this", we could but I was unwilling
to pay the cost for a feature no one has asked for yet. If they had, I
would endevour to put it behind static branches and disable it by default
(like what happened for PSI). I was contacted offlist about my comments
at OSPM and gathered new data to respond properly. For the record, here
is an editted version of my response;

--8<--

(Some context deleted that is not relevant)

> Does it need any special admin configuration for system
> services, cgroups, scripts, etc?

Nothing special -- out of box configuration. Tests were executed via
mmtests.

> Which mmtests config file did you use?
> 

I used network-netperf-unbound and network-netperf-cstate.
network-netperf-unbound is usually the default but for some issues, I
use the cstate configuration to limit C-states.

For a perf profile, I used network-netperf-cstate-small and
network-netperf-unbound-small to limit the amount of profile data that
was collected. Just collecting data for 64 byte buffers was enough.

> The server that I am going to configure is x86_64 numa, not arm64.

That's fine, I didn't actually test arm64 at all.

> I have a 2 socket 24 CPUs X86 server (4 NUMA nodes, AMD Opteron 6174,
> L2 512KB/cpu, L3 6MB/node, RAM 40GB/node).
> Which machine did you run it on?
> 

It was a 2-socket Haswell machine (E5-2670 v3) with 2 NUMA nodes. I used
5.7-rc7 with the openSUSE Leap 15.1 kernel configuration as a baseline.
I compared with and without uclamp enabled.

For network-netperf-unbound I see

netperf-udp
                                  5.7.0-rc7              5.7.0-rc7
                                 with-clamp          without-clamp
Hmean     send-64         238.52 (   0.00%)      257.28 *   7.87%*
Hmean     send-128        477.10 (   0.00%)      511.57 *   7.23%*
Hmean     send-256        945.53 (   0.00%)      982.50 *   3.91%*
Hmean     send-1024      3655.74 (   0.00%)     3846.98 *   5.23%*
Hmean     send-2048      6926.84 (   0.00%)     7247.04 *   4.62%*
Hmean     send-3312     10767.47 (   0.00%)    10976.73 (   1.94%)
Hmean     send-4096     12821.77 (   0.00%)    13506.03 *   5.34%*
Hmean     send-8192     22037.72 (   0.00%)    22275.29 (   1.08%)
Hmean     send-16384    35935.31 (   0.00%)    34737.63 *  -3.33%*
Hmean     recv-64         238.52 (   0.00%)      257.28 *   7.87%*
Hmean     recv-128        477.10 (   0.00%)      511.57 *   7.23%*
Hmean     recv-256        945.45 (   0.00%)      982.50 *   3.92%*
Hmean     recv-1024      3655.74 (   0.00%)     3846.98 *   5.23%*
Hmean     recv-2048      6926.84 (   0.00%)     7246.51 *   4.62%*
Hmean     recv-3312     10767.47 (   0.00%)    10975.93 (   1.94%)
Hmean     recv-4096     12821.76 (   0.00%)    13506.02 *   5.34%*
Hmean     recv-8192     22037.71 (   0.00%)    22274.55 (   1.07%)
Hmean     recv-16384    35934.82 (   0.00%)    34737.50 *  -3.33%*

netperf-tcp
                             5.7.0-rc7              5.7.0-rc7
                            with-clamp          without-clamp
Min       64        2004.71 (   0.00%)     2033.23 (   1.42%)
Min       128       3657.58 (   0.00%)     3733.35 (   2.07%)
Min       256       6063.25 (   0.00%)     6105.67 (   0.70%)
Min       1024     18152.50 (   0.00%)    18487.00 (   1.84%)
Min       2048     28544.54 (   0.00%)    29218.11 (   2.36%)
Min       3312     33962.06 (   0.00%)    36094.97 (   6.28%)
Min       4096     36234.82 (   0.00%)    38223.60 (   5.49%)
Min       8192     42324.06 (   0.00%)    43328.72 (   2.37%)
Min       16384    44323.33 (   0.00%)    45315.21 (   2.24%)
Hmean     64        2018.36 (   0.00%)     2038.53 *   1.00%*
Hmean     128       3700.12 (   0.00%)     3758.20 *   1.57%*
Hmean     256       6236.14 (   0.00%)     6212.77 (  -0.37%)
Hmean     1024     18214.97 (   0.00%)    18601.01 *   2.12%*
Hmean     2048     28749.56 (   0.00%)    29728.26 *   3.40%*
Hmean     3312     34585.50 (   0.00%)    36345.09 *   5.09%*
Hmean     4096     36777.62 (   0.00%)    38576.17 *   4.89%*
Hmean     8192     43149.08 (   0.00%)    43903.77 *   1.75%*
Hmean     16384    45478.27 (   0.00%)    46372.93 (   1.97%)

The cstate-limited config had similar results for UDP_STREAM but was
mostly indifferent for TCP_STREAM.

So for UDP_STREAM,. there is a fairly sizable difference for uclamp. There
are caveats, netperf is not 100% stable from a performance perspective on
NUMA machines. That's improved quite a bit with 5.7 but it still should
be treated with care.

When I first saw a problem, I was using ftrace looking for latencies and
uclamp appeared to crop up. As I didn't actually need uclamp and there was
no user request to support it, I simply dropped it from the master config
so it would get propogated to any distro we release with a 5.x kernel.

From a perf profile, it's not particularly obvious that uclamp is
involved so it could be in error but I doubt it. A diff of without vs
with looks like

# Event 'cycles:ppp'
#
# Baseline  Delta Abs  Shared Object             Symbol
# ........  .........  ........................  ..............................................
#
     9.59%     -2.87%  [kernel.vmlinux]          [k] poll_idle
     0.19%     +1.85%  [kernel.vmlinux]          [k] activate_task
               +1.17%  [kernel.vmlinux]          [k] dequeue_task
               +0.89%  [kernel.vmlinux]          [k] update_rq_clock.part.73
     3.88%     +0.73%  [kernel.vmlinux]          [k] try_to_wake_up
     3.17%     +0.68%  [kernel.vmlinux]          [k] __schedule
     1.16%     -0.60%  [kernel.vmlinux]          [k] __update_load_avg_cfs_rq
     2.20%     -0.54%  [kernel.vmlinux]          [k] resched_curr
     2.08%     -0.29%  [kernel.vmlinux]          [k] _raw_spin_lock_irqsave
     0.44%     -0.29%  [kernel.vmlinux]          [k] cpus_share_cache
     1.13%     +0.23%  [kernel.vmlinux]          [k] _raw_spin_lock_bh

A lot of the uclamp functions appear to be inlined so it is not be
particularly obvious from a raw profile but it shows up in the annotated
profile in activate_task and dequeue_task for example. In the case of
dequeue_task, uclamp_rq_dec_id() is extremely expensive according to the
annotated profile.

I'm afraid I did not dig into this deeply once I knew I could just disable
it even within the distribution.

-- 
Mel Gorman
SUSE Labs