linux-kernel - Re: [PATCH v3 11/19] unwind: Add deferred user space unwinding API

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Message-ID: <20241029181137.GH14555@noisy.programming.kicks-ass.net>
Date: Tue, 29 Oct 2024 19:11:37 +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.kerne.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>
Subject: Re: [PATCH v3 11/19] unwind: Add deferred user space unwinding API

On Tue, Oct 29, 2024 at 10:05:26AM -0700, Josh Poimboeuf wrote:
> On Tue, Oct 29, 2024 at 02:49:18PM +0100, Peter Zijlstra wrote:
> > On Mon, Oct 28, 2024 at 02:47:38PM -0700, Josh Poimboeuf wrote:
> > 
> > > +static void unwind_user_task_work(struct callback_head *head)
> > > +{
> > ...
> > > +	guard(rwsem_read)(&callbacks_rwsem);
> > > +
> > > +	for_each_set_bit(i, &pending, UNWIND_MAX_CALLBACKS) {
> > > +		if (callbacks[i])
> > > +			callbacks[i]->func(&trace, cookie, privs[i]);
> > > +	}
> > 
> > I'm fairly sure people will come with pitchforks for that read-lock
> > there. They scale like shit on big systems. Please use SRCU or somesuch.
> 
> I'd expect that unwind_user_{register,unregister}() would only be called
> once per tracing component during boot so there wouldn't be any
> contention.

The read-lock does an atomic op on the lock word, try and do that with
200+ CPUs and things get really sad.

> But I think I can make SRCU work here.

Thanks!