Re: [PATCH 3/4] perf: Remove too early and redundant CPU hotplug handling

[Peter Zijlstra]

On Thu, Apr 24, 2025 at 06:11:27PM +0200, Frederic Weisbecker wrote:
> 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.

Oh hey, that's curious indeed.

> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> ---
>  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 1987400000b4..df366ee15456 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 b08bb34b1718..a59e009e0be4 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,
> -- 
> 2.48.1
>