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

[Oscar Shiang]

On Feb 24, 2022, at 1:31 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> 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

Thanks,
Oscar