Messages in this thread | | | Date | Wed, 9 Aug 2023 17:17:26 +0800 | Subject | Re: [PATCH] perf/core: fix the bug in the event multiplexing | From | Shijie Huang <> |
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Hi Oliver,
在 2023/8/9 16:25, Oliver Upton 写道: > 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. > >> +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.
Thanks a lot, I will check the code about the latter one, and try to fix it again.
Thanks
Huang Shijie
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