linux-kernel - Re: [PATCH v5 11/17] perf: Simplify get_perf

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Message-ID: <20250425071339.GJ18306@noisy.programming.kicks-ass.net>
Date: Fri, 25 Apr 2025 09:13:39 +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 11/17] perf: Simplify get_perf_callchain() user logic

On Thu, Apr 24, 2025 at 01:28:00PM -0400, Steven Rostedt wrote:
> On Thu, 24 Apr 2025 18:36:07 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > > +++ b/kernel/events/callchain.c
> > > @@ -246,22 +246,20 @@ get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
> > >  
> > >  	if (user) {
> > >  		if (!user_mode(regs)) {
> > > -			if  (current->mm)
> > > -				regs = task_pt_regs(current);
> > > -			else
> > > -				regs = NULL;
> > > +			if (!current->mm)
> > > +				goto exit_put;
> > > +			regs = task_pt_regs(current);  
> > 
> > I'm thinking this might be one of those is-kthread test written as
> > has-mm, and they're broken.
> > 
> > Notably things like the io-uring kthreads do have mm.
> 
> Would there ever be a case where:
> 
> 	current->mm == NULL && !(current->flags & PF_KTHREAD)
> 
> ?
> 
> That is, do we still need to check for current->mm if it's not a kernel
> thread, or can we assume it exists?

IIRC just checking PF_KTHREAD should be sufficient.