Hi Josef,
On 29/11/21 12:03, Josef Bacik wrote:
> Hello,
>
> 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 for the report!
That patch you bisected does add more NOHZ kicks that aren't time-gated
like nohz.next_blocked / nohz.next_balance, so I'm thinking that a
pathological scenario would be a low-period bursty task which keeps
flicking a CPU idle/!idle. SCHED_SOFTIRQ running the NOHZ work on the
task's previous CPU would then repeatedly delay / force the task to be
placed on another CPU.
Would you happen to have execution traces by any chance? If not I should be
able to get one out of that fsperf thingie.
Now, races between nohz_balance_enter_idle() and nohz_idle_balance() make
it tricky to compare rq.next_balance vs nohz.next_balance to figure out
whether to do an update or not so I did the "stupid" thing of delegating
that to the ILB. Let me scratch my head some more and see if I can end up
with something less trigger-happy wrt NOHZ kicks.
Thanks,
Valentin
On Mon, Nov 29, 2021 at 06:03:24PM +0000, Valentin Schneider wrote:
>
> Hi Josef,
>
> On 29/11/21 12:03, Josef Bacik wrote:
> > Hello,
> >
> > 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 for the report!
>
> That patch you bisected does add more NOHZ kicks that aren't time-gated
> like nohz.next_blocked / nohz.next_balance, so I'm thinking that a
> pathological scenario would be a low-period bursty task which keeps
> flicking a CPU idle/!idle. SCHED_SOFTIRQ running the NOHZ work on the
> task's previous CPU would then repeatedly delay / force the task to be
> placed on another CPU.
>
> 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,
Josef
