linux-kernel - [tip: perf/core] perf: Remove too early and redundant CPU hotplug handling

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Message-ID: <174670046846.406.16724744697892248650.tip-bot2@tip-bot2>
Date: Thu, 08 May 2025 10:34:28 -0000
From: "tip-bot2 for Frederic Weisbecker" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Frederic Weisbecker <frederic@...nel.org>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject:
 [tip: perf/core] perf: Remove too early and redundant CPU hotplug handling

The following commit has been merged into the perf/core branch of tip:

Commit-ID:     9d4ea6fdba71e56c7cf0231a0b0c44723f64c3e7
Gitweb:        https://git.kernel.org/tip/9d4ea6fdba71e56c7cf0231a0b0c44723f64c3e7
Author:        Frederic Weisbecker <frederic@...nel.org>
AuthorDate:    Thu, 24 Apr 2025 18:11:27 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Fri, 02 May 2025 12:40:41 +02:00

perf: Remove too early and redundant CPU hotplug handling

The CPU hotplug handlers are called twice: at prepare and online stage.

Their role is to:

1) Enable/disable a CPU context. This is irrelevant and even buggy at
   the prepare stage because the CPU is still offline. On early
   secondary CPU up, creating an event attached to that CPU might
   silently fail because the CPU context is observed as online but the
   context installation's IPI failure is ignored.

2) Update the scope cpumasks and re-migrate the events accordingly in
   the CPU down case. This is irrelevant at the prepare stage.

3) Remove the events attached to the context of the offlining CPU. It
   even uses an (unnecessary) IPI for it. This is also irrelevant at the
   prepare stage.

Also none of the *_PREPARE and *_STARTING architecture perf related CPU
hotplug callbacks rely on CPUHP_PERF_PREPARE.

CPUHP_AP_PERF_ONLINE is enough and the right place to perform the work.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20250424161128.29176-4-frederic@kernel.org
---
 include/linux/cpuhotplug.h | 1 -
 kernel/cpu.c               | 5 -----
 2 files changed, 6 deletions(-)

diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index 1987400..df366ee 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -60,7 +60,6 @@ enum cpuhp_state {
 	/* PREPARE section invoked on a control CPU */
 	CPUHP_OFFLINE = 0,
 	CPUHP_CREATE_THREADS,
-	CPUHP_PERF_PREPARE,
 	CPUHP_PERF_X86_PREPARE,
 	CPUHP_PERF_X86_AMD_UNCORE_PREP,
 	CPUHP_PERF_POWER,
diff --git a/kernel/cpu.c b/kernel/cpu.c
index b08bb34..a59e009 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2069,11 +2069,6 @@ static struct cpuhp_step cpuhp_hp_states[] = {
 		.teardown.single	= NULL,
 		.cant_stop		= true,
 	},
-	[CPUHP_PERF_PREPARE] = {
-		.name			= "perf:prepare",
-		.startup.single		= perf_event_init_cpu,
-		.teardown.single	= perf_event_exit_cpu,
-	},
 	[CPUHP_RANDOM_PREPARE] = {
 		.name			= "random:prepare",
 		.startup.single		= random_prepare_cpu,