On Wed, Oct 30, 2024 at 1:22 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Wed, 30 Oct 2024 00:16:48 +0000,
> Raghavendra Rao Ananta <rananta@xxxxxxxxxx> wrote:
> >
> > On Tue, Oct 29, 2024 at 11:47 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > >
> > > On Tue, 29 Oct 2024 17:06:09 +0000,
> > > Raghavendra Rao Ananta <rananta@xxxxxxxxxx> wrote:
> > > >
> > > > On Tue, Oct 29, 2024 at 9:27 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Mon, 28 Oct 2024 23:45:33 +0000,
> > > > > Raghavendra Rao Ananta <rananta@xxxxxxxxxx> wrote:
> > > > > >
> > > > > Did you have a chance to check whether this had any negative impact on
> > > > > actual workloads? Since the entry/exit code is a bit of a hot spot,
> > > > > I'd like to make sure we're not penalising the common case (I only
> > > > > wrote this patch while waiting in an airport, and didn't test it at
> > > > > all).
> > > > >
> > > > I ran the kvm selftests, kvm-unit-tests and booted a linux guest to
> > > > test the change and noticed no failures.
> > > > Any specific test you want to try out?
> > >
> > > My question is not about failures (I didn't expect any), but
> > > specifically about *performance*, and whether checking the flag
> > > without a static key can lead to any performance drop on the hot path.
> > >
> > > Can you please run an exit-heavy workload (such as hackbench, for
> > > example), and report any significant delta you could measure?
> >
> > Oh, I see. I ran hackbench and micro-bench from kvm-unit-tests (which
> > also causes a lot of entry/exits), on Ampere Altra with kernel at
> > v6.12-rc1, and see no significant difference in perf.
>
> Thanks for running this stuff.
>
> > timer_10ms 231040.0 902.0
> > timer_10ms 234120.0 914.0
>
> This seems to be the only case were we are adversely affected by this
> change.
Hmm, I'm not sure how much we want to trust this comparison. For
instance, I just ran micro-bench again a few more times and here are
the outcomes of timer_10ms for each try with the patch:
Tries total ns
avg ns
-----------------------------------------------------------------------------------
1_timer_10ms 231840.0 905.0
2_timer_10ms 234560.0 916.0
3_timer_10ms 227440.0 888.0
4_timer_10ms 236640.0 924.0
5_timer_10ms 231200.0 903.0
Here's a few on the baseline:
Tries total ns
avg ns
-----------------------------------------------------------------------------------
1_timer_10ms 231080.0 902.0
2_timer_10ms 238040.0 929.0
3_timer_10ms 231680.0 905.0
4_timer_10ms 229280.0 895.0
5_timer_10ms 228520.0 892.0
> In the grand scheme of thins, that's noise. But this gives us
> a clear line of sight for the removal of the in-kernel interrupts back
> to userspace.
Sorry, I didn't follow you completely on this part.
Thank you.
Raghavendra
