Re: [RFC PATCH v7 11/23] sched/fair: core wide cfs task priority comparison

[chris hyser]

On 9/16/20 10:24 AM, chris hyser wrote:
> On 9/16/20 8:57 AM, Li, Aubrey wrote:
> > > Here are the uperf results of the various patchsets. Note, that disabling smt is better for these tests and that that 
> > > presumably reflects the overall overhead of core scheduling which went from bad to really bad. The primary focus in 
> > > this email is to start to understand what happened within core sched itself.
> > >
> > > patchset          smt=on/cs=off  smt=off    smt=on/cs=on
> > > --------------------------------------------------------
> > > v5-v5.6.y      :    1.78Gb/s     1.57Gb/s     1.07Gb/s
> > > pre-v6-v5.6.y  :    1.75Gb/s     1.55Gb/s    822.16Mb/s
> > > v6-5.7         :    1.87Gs/s     1.56Gb/s    561.6Mb/s
> > > v6-5.7-hotplug :    1.75Gb/s     1.58Gb/s    438.21Mb/s
> > > v7             :    1.80Gb/s     1.61Gb/s    440.44Mb/s
> >
> > I haven't had a chance to play with v7, but I got something different.
> >
> >    branch        smt=on/cs=on
> > coresched/v5-v5.6.y    1.09Gb/s
> > coresched/v6-v5.7.y    1.05Gb/s
> >
> > I attached my kernel config in case you want to make a comparison, or you
> > can send yours, I'll try to see I can replicate your result.
>
> I will give this config a try. One of the reports forwarded to me about the drop in uperf perf was an email from you I 
> believe mentioning a 50% perf drop between v5 and v6?? I was actually setting out to duplicate your results. :-)

The first thing I did was to verify I built and tested the right bits. Presumably as I get same numbers. I'm still 
trying to tweak your config to get a root disk in my setup. Oh, one thing I missed in reading your first response, I had 
24 cores/48 cpus. I think you had half that, though my guess is that that should have actually made the numbers even 
worse. :-)

The following was forwarded to me originally sent on Aug 3, by you I believe:

> We found uperf(in cgroup) throughput drops by ~50% with corescheduling.
>
> The problem is, uperf triggered a lot of softirq and offloaded softirq
> service to *ksoftirqd* thread.
>
> - default, ksoftirqd thread can run with uperf on the same core, we saw
>   100% CPU utilization.
> - coresched enabled, ksoftirqd's core cookie is different from uperf, so
>   they can't run concurrently on the same core, we saw ~15% forced idle.
>
> I guess this kind of performance drop can be replicated by other similar
> (a lot of softirq activities) workloads.
>
> Currently core scheduler picks cookie-match tasks for all SMT siblings, does
> it make sense we add a policy to allow cookie-compatible task running together?
> For example, if a task is trusted(set by admin), it can work with kernel thread.
> The difference from corescheduling disabled is that we still have user to user
> isolation.
>
> Thanks,
> -Aubrey

Would you please elaborate on what this test was? In trying to duplicate this, I just kept adding uperf threads to my 
setup until I started to see performance losses similar to what is reported above (and a second report about v7). Also, 
I wasn't looking for absolute numbers per-se, just significant enough differences to try to track where the performance 
went.

-chrish