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

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Message-ID: <20250123184535.hymskoka5e73ufwv@jpoimboe>
Date: Thu, 23 Jan 2025 10:45:35 -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 Thu, Jan 23, 2025 at 09:31:31AM +0100, Peter Zijlstra wrote:
> 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:
> > 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.

Indeed, I knew that but somehow overlooked the fact that the IRQ would
clear the cookie so the cache wouldn't be usable anyway.

-- 
Josh