Messages in this thread | | | Date | Tue, 8 Mar 2022 10:12:35 -0300 | From | Marcelo Tosatti <> | Subject | Re: [patch v11 00/13] extensible prctl task isolation interface and vmstat sync |
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On Tue, Mar 08, 2022 at 02:32:46PM +0800, Oscar Shiang wrote: > On Feb 24, 2022, at 1:31 AM, Marcelo Tosatti <mtosatti@redhat.com> wrote: > > Hi Oscar, > > > > On Sat, Feb 19, 2022 at 04:02:10PM +0800, Oscar Shiang wrote: > > > Hi Marcelo, > > > > > > I tried to apply your patches to kernel v5.15.18-rt28 and measured > > > the latencies through oslat [1]. > > > > > > It turns out that the peak latency (around 100us) can drop to about 90us. > > > The result is impressive since I only changed the guest's kernel > > > instead of installing the patched kernel to both host and guest. > > > > > > However, I am still curious about: > > > 1) Why did I catch a bigger maximum latency in almost each of the > > > results of applying task isolation patches? Or does it come from > > > other reasons? > > > > There are a number of things that need to be done in order to have an > > "well enough" isolated CPU so you can measure latency reliably: > > > > * Boot a kernel with isolated CPU (or better, use realtime-virtual-host profile of > > https://github.com/redhat-performance/tuned.git, which does a bunch of > > other things to avoid interruptions to isolated CPUs). > > * Apply the userspace patches at https://people.redhat.com/~mtosatti/task-isol-v6-userspace-patches/ > > to util-linux and rt-tests. > > > > Run oslat with chisol: > > > > chisol -q vmstat_sync -I conf oslat -c ... > > > > Where chisol is from patched util-linux and oslat from patched rt-tests. > > > > If you had "-f 1" (FIFO priority), on oslat, then the vmstat work would be hung. > > > > Are you doing those things? > > > > > 2) Why did we only get a 10us improvement on quiescing vmstat? > > > > If you did not have FIFO priority on oslat, then other daemons > > could be interrupting it, so better make sure the 10us improvement > > you see is due to vmstat_flush workqueue work not executing anymore. > > > > The testcase i use is: > > > > Stock kernel: > > > > terminal 1: > > # oslat -f 1 -c X ... > > > > terminal 2: > > # echo 1 > /proc/sys/vm/stat_refresh > > (hang) > > > > Patched kernel: > > > > terminal 1: > > # chisol -q vmstat_sync -I conf oslat -f 1 -c X ... > > > > terminal 2: > > # echo 1 > /proc/sys/vm/stat_refresh > > # > > Sure, I did see the terminal hung during oslat with FIFO priority. > > BTW, thanks for providing this test case. I used to run all workload stuff to just > verify the improvement of task isolation. It is a more straightr- forward way to do. > > > > [1]: The result and the test scripts I used can be found at > > > https://gist.github.com/OscarShiang/8b530a00f472fd1c39f5979ee601516d#testing-task-isolation-via-oslat > > > > OK, you seem to be doing everything necessary for chisol > > to work. Does /proc/pid/task_isolation of the oslat worker thread > > (note its not the same pid as the main oslat thread) show "vmstat" > > configured and activated for quiesce? > > The status of task_isolation seems to be set properly with "vmstat" and activated > > > However 100us is really high. You should be able to get < 10us with > > realtime-virtual-host (i see 4us on an idle system). > > > > The answer might be: because 10us is what it takes to execute > > vmstat_worker on the isolated CPU (you can verify with tracepoints). > > > > That time depends on the number of per-CPU vmstat variables that need flushing, > > i suppose... > > Considering the interferences outside of the KVM, I have redone the measurements > directly on my x86_64 computer [1]. > > As result, most of the latencies are down to 60us (and below). There are still > some latencies larger than 80us, I am working on and trying to figure out the reason. > > [1]: https://gist.github.com/OscarShiang/202eb691e649557fe3eaa5ec67a5aa82
Oscar,
Did you confirm with hwlatdetect that the BIOS does not have long running SMIs?
Also, for the software part, you could save time by using the realtime-virtual-host profile (check /usr/lib/tuned/realtime-virtual-host/ to see what its doing in addition to isolcpus=).
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