linux-kernel - Re: [PATCH v4 29/39] unwind_user/deferred: Add unwind cache

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Message-ID: <20250122223625.aibbrh43pjkzve6c@jpoimboe>
Date: Wed, 22 Jan 2025 14:36:25 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, Indu Bhagat <indu.bhagat@...cle.com>,
	Mark Rutland <mark.rutland@....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>, linux-trace-kernel@...r.kernel.org,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Jens Remus <jremus@...ux.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Weinan Liu <wnliu@...gle.com>
Subject: Re: [PATCH v4 29/39] unwind_user/deferred: Add unwind cache

On Wed, Jan 22, 2025 at 02:57:00PM +0100, Peter Zijlstra wrote:
> On Tue, Jan 21, 2025 at 06:31:21PM -0800, Josh Poimboeuf wrote:
> > Cache the results of the unwind to ensure the unwind is only performed
> > once, even when called by multiple tracers.
> > 
> > Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> > ---
> >  include/linux/unwind_deferred_types.h |  8 +++++++-
> >  kernel/unwind/deferred.c              | 26 ++++++++++++++++++++------
> >  2 files changed, 27 insertions(+), 7 deletions(-)
> > 
> > diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h
> > index 9749824aea09..6f71a06329fb 100644
> > --- a/include/linux/unwind_deferred_types.h
> > +++ b/include/linux/unwind_deferred_types.h
> > @@ -2,8 +2,14 @@
> >  #ifndef _LINUX_UNWIND_USER_DEFERRED_TYPES_H
> >  #define _LINUX_UNWIND_USER_DEFERRED_TYPES_H
> >  
> > -struct unwind_task_info {
> > +struct unwind_cache {
> >  	unsigned long		*entries;
> > +	unsigned int		nr_entries;
> > +	u64			cookie;
> > +};
> 
> If you make the return to user path clear nr_entries you don't need a
> second cookie field I think.

But if the NMI happens late in the exit-to-user path, with IRQs
disabled, right before nr_entries gets cleared, the cache won't get
used in the task work.

However I think we can clear it on entry-from-user.

-- 
Josh