linux-kernel - Re: [PATCH v3 3/5] perf record: Enable defer

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Message-ID: <aReBBQAhnto3RZ2d@google.com>
Date: Fri, 14 Nov 2025 11:20:37 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Ian Rogers <irogers@...gle.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	James Clark <james.clark@...aro.org>, Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	linux-perf-users@...r.kernel.org,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Indu Bhagat <indu.bhagat@...cle.com>,
	Jens Remus <jremus@...ux.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH v3 3/5] perf record: Enable defer_callchain for user
 callchains

On Fri, Nov 14, 2025 at 01:30:09PM -0500, Steven Rostedt wrote:
> On Fri, 14 Nov 2025 10:09:26 -0800
> Ian Rogers <irogers@...gle.com> wrote:
> 
> > Just to be clear. I don't think the behavior of using frame pointers
> > should change. Deferral has downsides, for example:
> > 
> >   $ perf record -g -a sleep 1
> 
> The biggest advantage of the deferred callstack is that there's much less
> duplication of data in the ring buffer. Especially when you have deep
> stacks and long system calls.
> 
> Now, if we have frame pointers enabled, we could possibly add a feature to
> the deferred unwinder where it could try to do the deferred immediately and
> if it faults it then waits until going back to user space.

This would be great if it can share the callstack with later samples
before going to user space.


> This means that
> the frame pointer version should work (unless the user space stack was
> swapped out).
> 
> > 
> > Without deferral kernel stack traces will contain both kernel and user
> > traces. With deferral the user stack trace is only generated when the
> > system call returns and so there is a chance for kernel stack traces
> > to be missing their user part. An obvious behavioral change.

Right, this is one of my concerns too.  For system-wide profiling, the
chances are high it can have some tasks sleeping in the kernel and perf
finishes the profiling before they return to user space.

Thanks,
Namhyung


> > I think
> > for what you are doing here we can have an option something like:
> > 
> >   $ perf record --call-graph fp-deferred -a sleep 1
> 
> I would be OK with this but I would prefer a much shorter name. Adding 20
> characters to the command line will likely keep people from using it.
> 
> -- Steve