linux-kernel - Re: [Patch v4 07/13] perf/x86/intel: Allocate arch-PEBS buffer and initialize PEBS

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Message-ID: <20250621092001.GT1613376@noisy.programming.kicks-ass.net>
Date: Sat, 21 Jun 2025 11:20:01 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Dapeng Mi <dapeng1.mi@...ux.intel.com>
Cc: 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>,
	Andi Kleen <ak@...ux.intel.com>,
	Eranian Stephane <eranian@...gle.com>, linux-kernel@...r.kernel.org,
	linux-perf-users@...r.kernel.org, Dapeng Mi <dapeng1.mi@...el.com>
Subject: Re: [Patch v4 07/13] perf/x86/intel: Allocate arch-PEBS buffer and
 initialize PEBS_BASE MSR

On Fri, Jun 20, 2025 at 10:39:03AM +0000, Dapeng Mi wrote:

> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
> index b6eface4dccd..72b925b8c482 100644
> --- a/arch/x86/events/intel/ds.c
> +++ b/arch/x86/events/intel/ds.c
> @@ -625,13 +625,22 @@ static int alloc_pebs_buffer(int cpu)
>  	int max, node = cpu_to_node(cpu);
>  	void *buffer, *insn_buff, *cea;
>  
> -	if (!x86_pmu.ds_pebs)
> +	if (!intel_pmu_has_pebs())
>  		return 0;
>  
> -	buffer = dsalloc_pages(bsiz, GFP_KERNEL, cpu);
> +	/*
> +	 * alloc_pebs_buffer() could be called by init_arch_pebs_buf_on_cpu()
> +	 * which is in atomic context.
> +	 */
> +	buffer = dsalloc_pages(bsiz, preemptible() ? GFP_KERNEL : GFP_ATOMIC, cpu);
>  	if (unlikely(!buffer))
>  		return -ENOMEM;

Here we go again.. that is CPU_STARTING context, that has IRQs disabled
and as such no allocation is allowed. Not even GFP_ATOMIC -- this will
break PREEMPT_RT.