Date:   Fri, 3 Dec 2021 14:00:24 -0500
From:   Josef Bacik <josef@toxicpanda.com>
To:     Valentin Schneider <valentin.schneider@arm.com>
Cc:     peterz@infradead.org, vincent.guittot@linaro.org,
        torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-btrfs@vger.kernel.org
Subject: Re: [REGRESSION] 5-10% increase in IO latencies with nohz balance
 patch
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On Fri, Dec 03, 2021 at 12:03:27PM +0000, Valentin Schneider wrote:
> On 30/11/21 00:26, Valentin Schneider wrote:
> > On 29/11/21 14:49, Josef Bacik wrote:
> >> On Mon, Nov 29, 2021 at 06:31:17PM +0000, Valentin Schneider wrote:
> >>> On 29/11/21 13:15, Josef Bacik wrote:
> >>> > On Mon, Nov 29, 2021 at 06:03:24PM +0000, Valentin Schneider wrote:
> >>> >> Would you happen to have execution traces by any chance? If not I should be
> >>> >> able to get one out of that fsperf thingie.
> >>> >>
> >>> >
> >>> > I don't, if you want to tell me how I can do it right now.  I've disabled
> >>> > everything on this box for now so it's literally just sitting there waiting to
> >>> > have things done to it.  Thanks,
> >>> >
> >>>
> >>> I see you have Ftrace enabled in your config, so that ought to do it:
> >>>
> >>>   trace-cmd record -e 'sched:*' -e 'cpu_idle' $your_test_cmd
> >>>
> >>
> >> http://toxicpanda.com/performance/trace.dat
> >>
> >> it's like 16mib.  Enjoy,
> >>
> >
> > Neat, thanks!
> >
> > Runqueue depth seems to be very rarely greater than 1, tasks with ~1ms
> > runtime and lots of sleeping (also bursty kworker activity with activations
> > of tens of ?s), and some cores (Internet tells me that Xeon Bronze 3204
> > doesn't have SMT) spend most of their time idling. Not the most apocalyptic
> > task placement vs ILB selection, but the task activation patterns roughly
> > look like what I was thinking of - there might be hope for me yet.
> >
> > I'll continue the headscratching after tomorrow's round of thinking juice.
> >
> 
> Could you give the 4 top patches, i.e. those above
> 8c92606ab810 ("sched/cpuacct: Make user/system times in cpuacct.stat more precise")
> a try?
> 
> https://git.gitlab.arm.com/linux-arm/linux-vs.git -b mainline/sched/nohz-next-update-regression
> 
> I gave that a quick test on the platform that caused me to write the patch
> you bisected and looks like it didn't break the original fix. If the above
> counter-measures aren't sufficient, I'll have to go poke at your
> reproducers...
> 

It's better but still around 6% regression.  If I compare these patches to the
average of the last few days worth of runs you're 5% better than before, so
progress but not completely erased.

     metric         baseline   current      stdev           diff      
======================================================================
write_io_kbytes       125000     125000            0    0.00%
read_clat_ns_p99           0          0            0    0.00%
write_bw_bytes      1.73e+08   1.74e+08   5370366.50    0.69%
read_iops                  0          0            0    0.00%
write_clat_ns_p50   18265.60   18150.40       345.21   -0.63%
read_io_kbytes             0          0            0    0.00%
read_io_bytes              0          0            0    0.00%
write_clat_ns_p99   84684.80   90316.80      6607.94    6.65%
read_bw_bytes              0          0            0    0.00%
elapsed                    1          1            0    0.00%
write_lat_ns_min           0          0            0    0.00%
sys_cpu                91.22      91.00         1.40   -0.24%
write_lat_ns_max           0          0            0    0.00%
read_lat_ns_min            0          0            0    0.00%
write_iops          42308.54   42601.71      1311.12    0.69%
read_lat_ns_max            0          0            0    0.00%
read_clat_ns_p50           0          0            0    0.00%

Thanks,

Josef