Re: [REGRESSION] 5-10% increase in IO latencies with nohz balance patch

[Thorsten Leemhuis]

Hi, this is your Linux kernel regression tracker speaking.

Top-posting for once, to make this easy accessible to everyone.

Adding the regression mailing list to the list of recipients, as it
should be in the loop for all regressions, as explained here:
https://www.kernel.org/doc/html/latest/admin-guide/reporting-issues.html

To be sure this issue doesn't fall through the cracks unnoticed, I'm
adding it to regzbot, my Linux kernel regression tracking bot:

#regzbot ^introduced 7fd7a9e0caba
#regzbot ignore-activity

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Sending this to everyone that got the initial report, to make all aware
of the tracking. I also hope that messages like this motivate people to
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regzbot involved when dealing with regressions, as messages like this
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Don't worry, I'll send further messages wrt to this regression just to
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long as they are intended just for regzbot. With a bit of luck no such
messages will be needed anyway.

Ciao, Thorsten, your Linux kernel regression tracker.

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On 29.11.21 18:03, Josef Bacik wrote:
> 
> Our nightly performance testing found a performance regression when we rebased
> our devel tree onto v5.16-rc.  This took me a few days to bisect down, but this
> patch
> 
> 7fd7a9e0caba ("sched/fair: Trigger nohz.next_balance updates when a CPU goes NOHZ-idle")
> 
> is the one that introduces the regression.  My performance testing box is a 2
> socket, with a model name "Intel(R) Xeon(R) Bronze 3204 CPU @ 1.90GHz", for a
> total of 12 cpu's reported in cpuinfo.  It has 128gib of RAM, and these perf
> tests are being run against a SSD and spinning rust device, but the regression
> is consistent across both configurations.  You can see the historical graph of
> the completion latencies for this specific run
> 
> http://toxicpanda.com/performance/emptyfiles500k_write_clat_ns_p99.png
> 
> Or for something a little more braindead (untar firefox) you can see a increase
> in the runtime
> 
> http://toxicpanda.com/performance/untarfirefox_elapsed.png
> 
> These two tests are single threaded, the regression doesn't appear to affect
> multi-threaded tests.  For a simple reproducer you can simply download a tarball
> of the firefox sources and untar it onto a clean btrfs file system.  The time
> before and after this commit goes up ~1-2 seconds on my machine.  For a less
> simple test you can create a clean btrfs file system and run
> 
> fio --name emptyfiles500k --create_on_open=1 --nrfiles=31250 --readwrite=write \
> 	--readwrite=write --ioengine=filecreate --fallocate=none --filesize=4k \
> 	--openfiles=1 --alloc-size 98304 --allrandrepeat=1 --randseed=12345 \
> 	--directory <mount point>
> 
> And you are looking for the "Write clat ns p99" metric.  You'll see a 5-10%
> increase in the latency time.  If you want to run our tests directly it's
> relatively easy to setup, you can clone the fsperf repo
> 
> https://github.com/josefbacik/fsperf
> 
> Then in the fsperf directory edit the local.cfg and add
> 
> [main]
> directory=/mnt/test
> 
> [btrfs]
> device=/dev/sdc
> iosched=none
> mkfs=mkfs.btrfs -f
> mount=mount -o noatime
> 
> And then run the following on the baseline kernel
> 
> ./fsperf -p regression -c btrfs -n 10 emptyfiles500k
> 
> This will run the test 10 times and save the results to the database.  Then you
> can boot into your changed kernel and runn
> 
> ./fsperf -p regrssion -c btrfs -n 10 -t emptyfiles500k
> 
> This will run the test 10 times and take the average and compare it to the
> baseline and print out the values, you'll see the increase latency values there.
> 
> I can reproduce this at will, if you want to just throw patches at me I'm happy
> to run it and let you know what happens.  I'm attaching my .config as well in
> case that is needed, but the HZ and PREEMPT settings are
> 
> CONFIG_NO_HZ_COMMON=y
> CONFIG_NO_HZ_FULL=y
> CONFIG_NO_HZ=y
> CONFIG_HZ_1000=y
> CONFIG_PREEMPT=y
> CONFIG_PREEMPT_COUNT=y
> CONFIG_PREEMPTION=y
> CONFIG_PREEMPT_DYNAMIC=y
> CONFIG_PREEMPT_RCU=y
> CONFIG_HAVE_PREEMPT_DYNAMIC=y
> CONFIG_PREEMPT_NOTIFIERS=y
> CONFIG_DEBUG_PREEMPT=y
> 
> Thanks,
> 
> Josef
>