linux-kernel - Re: [PATCH v5 12/17] perf: Skip user unwind if !current->mm

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Message-ID: <20250424163718.GF18306@noisy.programming.kicks-ass.net>
Date: Thu, 24 Apr 2025 18:37:18 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Indu Bhagat <indu.bhagat@...cle.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	linux-toolchains@...r.kernel.org, Jordan Rome <jordalgo@...a.com>,
	Sam James <sam@...too.org>,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Jens Remus <jremus@...ux.ibm.com>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>, Weinan Liu <wnliu@...gle.com>,
	Blake Jones <blakejones@...gle.com>,
	Beau Belgrave <beaub@...ux.microsoft.com>,
	"Jose E. Marchesi" <jemarch@....org>
Subject: Re: [PATCH v5 12/17] perf: Skip user unwind if !current->mm

On Thu, Apr 24, 2025 at 12:25:41PM -0400, Steven Rostedt wrote:
> From: Josh Poimboeuf <jpoimboe@...nel.org>
> 
> If the task doesn't have any memory, there's no stack to unwind.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
> Note from v4: https://lore.kernel.org/all/24cde1381495cd637f0d5663ab7629572bc0eca6.1737511963.git.jpoimboe@kernel.org/
> 
> - From the previous version, Peter Zijlstra brought up concern about
>   io_uring helpers, but this was never resolved. No update was made.

What is there to resolve? If you want to check for kthreads, don't check
for ->mm

> 
>  kernel/events/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 009f34bd9e63..40b98b05fb7c 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -8101,7 +8101,7 @@ struct perf_callchain_entry *
>  perf_callchain(struct perf_event *event, struct pt_regs *regs)
>  {
>  	bool kernel = !event->attr.exclude_callchain_kernel;
> -	bool user   = !event->attr.exclude_callchain_user;
> +	bool user   = !event->attr.exclude_callchain_user && current->mm;
>  	/* Disallow cross-task user callchains. */
>  	bool crosstask = event->ctx->task && event->ctx->task != current;
>  	const u32 max_stack = event->attr.sample_max_stack;
> -- 
> 2.47.2
> 
>