linux-kernel - Re: [ANNOUNCE] v5.10-rc2-rt4

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Message-ID: <20201106161413.7c65uxenamy474uh@beryllium.lan>
Date:   Fri, 6 Nov 2020 17:14:13 +0100
From:   Daniel Wagner <wagi@...om.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [ANNOUNCE] v5.10-rc2-rt4

On Fri, Nov 06, 2020 at 11:54:47AM +0100, Sebastian Andrzej Siewior wrote:
> > rpi3    signaltest      5.9.0-rc8-rt12
> >   813   0_signaltest         t0-max-latency      : fail     214.00
> > rpi3    signaltest      5.9.0-rc8-rt12
> >   874   0_signaltest         t0-max-latency      : fail     217.00
> > rpi3    signaltest      5.9.0-rt16
> >   963   0_signaltest         t0-max-latency      : fail     321.00
> 
> Here, rt 13,14,15 would be interesting so we could narrow down the
> ~100us.
> v5.9-rc8-rt14 got new migrate-disable but I wouldn't expect it to cause
> it. The other changes look also harmless (like the rtmutex redo which
> should be a 0 change but then it mighe behave differently in regard to
> workqueue in some corner cases).

rpi3    signaltest      5.9.0-rc8-rt13
  1196  0_signaltest         t0-max-latency      : fail     207.00
  1196  0_signaltest         t0-avg-latency      : pass      46.00
  1196  0_signaltest         t0-min-latency      : pass      22.00
rpi3    signaltest      5.9.0-rc8-rt14
  1197  0_signaltest         t0-max-latency      : fail     301.00
  1197  0_signaltest         t0-avg-latency      : pass      47.00
  1197  0_signaltest         t0-min-latency      : pass      20.00
rpi3    signaltest      5.9.0-rt15
  1198  0_signaltest         t0-max-latency      : fail     323.00
  1198  0_signaltest         t0-avg-latency      : pass      47.00
  1198  0_signaltest         t0-min-latency      : pass      21.00

> > rpi3    signaltest      5.9.1-rt19
> >   1038  0_signaltest         t0-max-latency      : fail     341.00
> > rpi3    signaltest      5.9.1-rt20
> >   1079  0_signaltest         t0-max-latency      : fail     318.00
>
> So I have nothing to explain 20us improvement.

I think 20us is in the range of the standard deviation for this test. So
I don't think you should be concerned too much about it as long I don't
have proper statistical numbers.

One thing I also see is that the average was pretty constant at 47us for
5.9-rt and for 5.10-rt series it's around 55us. So something makes the
whole operation slightly more expensive.

> > rpi3    signaltest      5.10.0-rc1-rt1
> >   1118  0_signaltest         t0-max-latency      : fail     415.00
> > rpi3    signaltest      5.10.0-rc2-rt4
> >   1163  0_signaltest         t0-max-latency      : fail     340.00
> 
> -rt2 gained new kmap code.
> -rt3 received an update of the above

rpi3    signaltest      5.10.0-rc1-rt2
  1199  0_signaltest         t0-max-latency      : fail     399.00
  1199  0_signaltest         t0-avg-latency      : pass      55.00
  1199  0_signaltest         t0-min-latency      : pass      25.00
rpi3    signaltest      5.10.0-rc2-rt3
  1200  0_signaltest         t0-max-latency      : fail     420.00
  1200  0_signaltest         t0-avg-latency      : pass      55.00
  1200  0_signaltest         t0-min-latency      : pass      25.00

> But all this is only signal right?

Correct. I've observed this only for signaltest and sigwaittest.

> Nothing on the cyclictest front?

Correct, cyclictest doesn't show any regression.

> If lazy-preempt broke in a way then it should be only noticed by
> cyclictest. You can however disable lazy-preempt just to be sure.

Sure, will do a full run on Monday.