Re: [PATCH 21/24] sched/eevdf: Allow shorter slices to wakeup-preempt

Chen Yu
2024-08-08T12:32:16Z

On 2024-08-08 at 12:22:07 +0200, Peter Zijlstra wrote:
> On Thu, Aug 08, 2024 at 06:15:50PM +0800, Chen Yu wrote:
> > Hi Peter,
> >
> > On 2024-07-27 at 12:27:53 +0200, Peter Zijlstra wrote:
> > > Part of the reason to have shorter slices is to improve
> > > responsiveness. Allow shorter slices to preempt longer slices on
> > > wakeup.
> > >
> > > Task | Runtime ms | Switches | Avg delay ms | Max delay ms | Sum delay ms |
> > >
> > > 100ms massive_intr 500us cyclictest NO_PREEMPT_SHORT
> > >
> > > 1 massive_intr:(5) | 846018.956 ms | 779188 | avg: 0.273 ms | max: 58.337 ms | sum:212545.245 ms |
> > > 2 massive_intr:(5) | 853450.693 ms | 792269 | avg: 0.275 ms | max: 71.193 ms | sum:218263.588 ms |
> > > 3 massive_intr:(5) | 843888.920 ms | 771456 | avg: 0.277 ms | max: 92.405 ms | sum:213353.221 ms |
> > > 1 chromium-browse:(8) | 53015.889 ms | 131766 | avg: 0.463 ms | max: 36.341 ms | sum:60959.230 ms |
> > > 2 chromium-browse:(8) | 53864.088 ms | 136962 | avg: 0.480 ms | max: 27.091 ms | sum:65687.681 ms |
> > > 3 chromium-browse:(9) | 53637.904 ms | 132637 | avg: 0.481 ms | max: 24.756 ms | sum:63781.673 ms |
> > > 1 cyclictest:(5) | 12615.604 ms | 639689 | avg: 0.471 ms | max: 32.272 ms | sum:301351.094 ms |
> > > 2 cyclictest:(5) | 12511.583 ms | 642578 | avg: 0.448 ms | max: 44.243 ms | sum:287632.830 ms |
> > > 3 cyclictest:(5) | 12545.867 ms | 635953 | avg: 0.475 ms | max: 25.530 ms | sum:302374.658 ms |
> > >
> > > 100ms massive_intr 500us cyclictest PREEMPT_SHORT
> > >
> > > 1 massive_intr:(5) | 839843.919 ms | 837384 | avg: 0.264 ms | max: 74.366 ms | sum:221476.885 ms |
> > > 2 massive_intr:(5) | 852449.913 ms | 845086 | avg: 0.252 ms | max: 68.162 ms | sum:212595.968 ms |
> > > 3 massive_intr:(5) | 839180.725 ms | 836883 | avg: 0.266 ms | max: 69.742 ms | sum:222812.038 ms |
> > > 1 chromium-browse:(11) | 54591.481 ms | 138388 | avg: 0.458 ms | max: 35.427 ms | sum:63401.508 ms |
> > > 2 chromium-browse:(8) | 52034.541 ms | 132276 | avg: 0.436 ms | max: 31.826 ms | sum:57732.958 ms |
> > > 3 chromium-browse:(8) | 55231.771 ms | 141892 | avg: 0.469 ms | max: 27.607 ms | sum:66538.697 ms |
> > > 1 cyclictest:(5) | 13156.391 ms | 667412 | avg: 0.373 ms | max: 38.247 ms | sum:249174.502 ms |
> > > 2 cyclictest:(5) | 12688.939 ms | 665144 | avg: 0.374 ms | max: 33.548 ms | sum:248509.392 ms |
> > > 3 cyclictest:(5) | 13475.623 ms | 669110 | avg: 0.370 ms | max: 37.819 ms | sum:247673.390 ms |
> > >
> > > As per the numbers the, this makes cyclictest (short slice) it's
> > > max-delay more consistent and consistency drops the sum-delay. The
> > > trade-off is that the massive_intr (long slice) gets more context
> > > switches and a slight increase in sum-delay.
> > >
> > > [mike: numbers]
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > > Tested-by: Mike Galbraith <umgwanakikbuti@gmail.com>
> >
> > Besides this short preemption, it seems that an important patch is missing from
> > this patch set, that was originally from Prateek and you refined it to fix the
> > current task's false negative eligibility:
> > https://lore.kernel.org/lkml/20240424150721.GQ30852@noisy.programming.kicks-ass.net/
> >
> > The RESPECT_SLICE is introduced to honor the current task's slice during wakeup preemption.
> > Without it we got reported that over-preemption and performance downgrading are observed
> > when running SPECjbb on servers.
>
> So I *think* that running as SCHED_BATCH gets you exactly that
> behaviour, no?

SCHED_BATCH should work as it avoids the wakeup preemption as much as possible.
Except that RESPECT_SLICE considers the cgroup hierarchical to check if the current
sched_entity has used up its slice, which seems to be less aggressive.

thanks,
Chenyu