linux-kernel - Re: [PATCH v4 28/39] unwind_user/deferred: Add deferred unwinding interface

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Message-ID: <20250124225011.aebigdykpmnnthfg@jpoimboe>
Date: Fri, 24 Jan 2025 14:50:11 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, x86@...nel.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>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Weinan Liu <wnliu@...gle.com>
Subject: Re: [PATCH v4 28/39] unwind_user/deferred: Add deferred unwinding
 interface

On Fri, Jan 24, 2025 at 02:46:48PM -0800, Josh Poimboeuf wrote:
> On Fri, Jan 24, 2025 at 04:58:03PM -0500, Steven Rostedt wrote:
> > Now the only thing I could think of is a flag gets set where the task comes
> > out of the scheduler and then does the stack trace. It doesn't need to do
> > the stack trace before it schedules. As it did just schedule, where ever it
> > scheduled must have been in a schedulable context.
> > 
> > That is, kind of like the task_work flag for entering user space and
> > exiting the kernel, could we have a sched_work flag to run after after being
> > scheduled back (exiting schedule()). Since the task has been picked to run,
> > it will not cause latency for other tasks. The work will be done in its
> > context. This is no different to the tasks accounting than if it does this
> > going back to user space. Heck, it would only need to do this once if it
> > didn't go back to user space, as the user space stack would be the same.
> > That is, if it gets scheduled multiple times, this would only happen on the
> > first instance until it leaves the kernel.
> > 
> > 
> > 	[ trigger stack trace - set sched_work ]
> > 
> > 	schedule() {
> > 		context_switch() -> CPU runs some other task
> > 				 <- gets scheduled back onto the CPU
> > 		[..]
> > 		/* preemption enabled ... */
> > 		if (sched_work) {
> > 			do stack trace() // can schedule here but
> > 					 // calls a schedule function that does not
> > 					 // do sched_work to prevent recursion
> > 		}
> > 	}
> > 
> > Could something like this work?
> 
> Yeah, this is basically a more fleshed out version of what I was trying
> to propose.
> 
> One additional wrinkle is that if @prev wakes up on another CPU while
> @next is unwinding it, the unwind goes haywire.  So that would maybe
> need to be prevented.

Hm, reading this again I'm wondering if you're actually proposing that
the unwind happens on @prev after it gets rescheduled sometime in the
future?  Does that actually solve the issue?  What if doesn't get
rescheduled within a reasonable amount of time?

-- 
Josh