Re: [PATCH] perf/core: fix the bug in the event multiplexing

[Shijie Huang]

Hi Mark,

在 2023/8/9 17:22, Mark Rutland 写道:
> On Wed, Aug 09, 2023 at 08:25:07AM +0000, Oliver Upton wrote:
> > Hi Huang,
> >
> > On Wed, Aug 09, 2023 at 09:39:53AM +0800, Huang Shijie wrote:
> > > 2.) Root cause.
> > > 	There is only 7 counters in my arm64 platform:
> > > 	  (one cycle counter) + (6 normal counters)
> > >
> > > 	In 1.3 above, we will use 10 event counters.
> > > 	Since we only have 7 counters, the perf core will trigger
> > >         	event multiplexing in hrtimer:
> > > 	     merge_sched_in() -->perf_mux_hrtimer_restart() -->
> > > 	     perf_rotate_context().
> > >
> > >         In the perf_rotate_context(), it does not restore some PMU registers
> > >         as context_switch() does.  In context_switch():
> > >               kvm_sched_in()  --> kvm_vcpu_pmu_restore_guest()
> > >               kvm_sched_out() --> kvm_vcpu_pmu_restore_host()
> > >
> > >         So we got wrong result.
> > This is a rather vague description of the problem. AFAICT, the
> > issue here is on VHE systems we wind up getting the EL0 count
> > enable/disable bits backwards when entering the guest, which is
> > corroborated by the data you have below.
> Yep; IIUC the issue here is that when we take an IRQ from a guest and reprogram
> the PMU in the IRQ handler, the IRQ handler will program the PMU with
> appropriate host/guest/user/etc filters for a *host* context, and then we'll
> return back into the guest without reconfigurign the event filtering for a
> *guest* context.
Yes.
> That can happen for perf_rotate_context(), or when we install an event into a
> running context, as that'll happen via an IPI.
>
> > > +void arch_perf_rotate_pmu_set(void)
> > > +{
> > > +	if (is_guest())
> > > +		kvm_vcpu_pmu_restore_guest(NULL);
> > > +	else
> > > +		kvm_vcpu_pmu_restore_host(NULL);
> > > +}
> > > +
> > This sort of hook is rather nasty, and I'd strongly prefer a solution
> > that's confined to KVM. I don't think the !is_guest() branch is
> > necessary at all. Regardless of how the pmu context is changed, we need
> > to go through vcpu_put() before getting back out to userspace.
> >
> > We can check for a running vCPU (ick) from kvm_set_pmu_events() and either
> > do the EL0 bit flip there or make a request on the vCPU to call
> > kvm_vcpu_pmu_restore_guest() immediately before reentering the guest.
> > I'm slightly leaning towards the latter, unless anyone has a better idea
> > here.
> The latter sounds reasonable to me.

okay. I prefer the latter one now. :)


Thanks

Huang Shijie

> I suspect we need to take special care here to make sure we leave *all* events
> in a good state when re-entering the guest or if we get to kvm_sched_out()
> after *removing* an event via an IPI -- it'd be easy to mess either case up and
> leave some events in a bad state.
>
> Thanks,
> Mark.