Re: [patch v11 00/13] extensible prctl task isolation interface and vmstat sync

[Marcelo Tosatti <mtosatti@redhat.com>]

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=).