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

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Message-ID: <20250123083131.GE3808@noisy.programming.kicks-ass.net>
Date: Thu, 23 Jan 2025 09:31:31 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.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 02:36:25PM -0800, Josh Poimboeuf wrote:
> 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.

Return to user runs with interrupts disabled, if an NMI hits that, it
will have to set TIF_NOTIFY_RESUME again and queue the IRQ work thing.
That self-IPI will hit the moment we do IRET (which is what re-enables
interrupts) and we're going back into the kernel.

Anyway, I suppose that is a long way of saying that you should be able
to do this on return to user.

But yes, enter-from-user should work too.