Re: bpf_get_branch_snapshot on qemu-kvm
flat view
From: Song Liu <hidden>
Date: 2021-09-29 20:05:11
Also in:
bpf
Hi Kan,
On Sep 29, 2021, at 9:35 AM, Liang, Kan [off-list ref] wrote:quoted
quoted
quoted
- get confirmation that clearing GLOBAL_CTRL is suffient to supress PEBS, in which case we can simply remove the PEBS_ENABLE clear.How should we confirm this? Can we run some tests for this? Or do we need hardware experts' input for this?I'll put it on the list to ask the hardware people when I talk to them next. But maybe Kan or Andi know without asking.If the GLOBAL_CTRL is explicitly disabled, the counters do not count anymore. It doesn't matter if PEBS is enabled or not. See 6c1c07b33eb0 ("perf/x86/intel: Avoid unnecessary PEBS_ENABLE MSR access in PMI "). We optimized the PMU handler base on it.
Thanks for these information! IIUC, all we need is the following on top of bpf-next/master:
diff --git i/arch/x86/events/intel/core.c w/arch/x86/events/intel/core.c
index 1248fc1937f82..d0d357e7d6f21 100644
--- i/arch/x86/events/intel/core.c
+++ w/arch/x86/events/intel/core.c@@ -2209,7 +2209,6 @@ intel_pmu_snapshot_branch_stack(struct perf_branch_entry *entries, unsigned int /* must not have branches... */ local_irq_save(flags); __intel_pmu_disable_all(false); /* we don't care about BTS */ - __intel_pmu_pebs_disable_all(); __intel_pmu_lbr_disable(); /* ... until here */ return __intel_pmu_snapshot_branch_stack(entries, cnt, flags);
@@ -2223,7 +2222,6 @@ intel_pmu_snapshot_arch_branch_stack(struct perf_branch_entry *entries, unsigned /* must not have branches... */ local_irq_save(flags); __intel_pmu_disable_all(false); /* we don't care about BTS */ - __intel_pmu_pebs_disable_all(); __intel_pmu_arch_lbr_disable(); /* ... until here */ return __intel_pmu_snapshot_branch_stack(entries, cnt, flags);
In the test, this does eliminate the warning. Thanks, Song