linux-kernel - [PATCH 1/2] perf/x86: Remove perf_events_lapic_init() calling from x86_pmu

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Date: Tue, 30 Apr 2024 11:56:52 +0800
From: Dapeng Mi <dapeng1.mi@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Kan Liang <kan.liang@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org,
	Dapeng Mi <dapeng1.mi@...el.com>,
	Dapeng Mi <dapeng1.mi@...ux.intel.com>
Subject: [PATCH 1/2] perf/x86: Remove perf_events_lapic_init() calling from x86_pmu_enable()

perf_events_lapic_init() helper is called to configure PMI to NMI vector
and clear MASK bit simultaneously by writing APIC_LVTPC MSR. It's called
firstly to initialize APIC_LVTPC MSR by init_hw_perf_events(), and the
PMI handler would always to clear the MASK bit in APIC_LVTPC MSR by
writing APIC_LVTPC MSR directly.

So it becomes unnecessary to call perf_events_lapic_init() again in
x86_pmu_enable(), and worse x86_pmu_enable() could be called very
frequently in some scenarios with very high context-switches. This would
cause performance overhead which can't be ignored especially in KVM guest
environment since frequent APIC_LVTPC writing would cause huge number
of VM-Exits.

For example, in guest environment Geekbench score (running multiplxing
perf-stat command in background) increases 1% and perf-sched benchmark
increases 7% after removing perf_events_lapic_init() calling from
x86_pmu_enable().

Signed-off-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>
---
 arch/x86/events/core.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 5b0dd07b1ef1..580923443813 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -1347,7 +1347,6 @@ static void x86_pmu_enable(struct pmu *pmu)
 			x86_pmu_start(event, PERF_EF_RELOAD);
 		}
 		cpuc->n_added = 0;
-		perf_events_lapic_init();
 	}

 	cpuc->enabled = 1;

base-commit: 854dd99b5ddc9d90e31e5f112462a5994dd31810
-- 
2.40.1