linux-kernel - Re: [PATCH v1] perf sample: Make user_regs and intr

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Message-ID: <CAP-5=fVoQRhd-8jyXfjc2dCuwt7_o1sU208aADT1FriwCNa9qw@mail.gmail.com>
Date: Mon, 10 Feb 2025 10:15:22 -0800
From: Ian Rogers <irogers@...gle.com>
To: Tavian Barnes <tavianator@...ianator.com>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, 
	Kan Liang <kan.liang@...ux.intel.com>, John Garry <john.g.garry@...cle.com>, 
	James Clark <james.clark@...aro.org>, Leo Yan <leo.yan@...ux.dev>, 
	Charlie Jenkins <charlie@...osinc.com>, Andi Kleen <ak@...ux.intel.com>, 
	Veronika Molnarova <vmolnaro@...hat.com>, Michael Petlan <mpetlan@...hat.com>, linux-kernel@...r.kernel.org, 
	linux-perf-users@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	coresight@...ts.linaro.org
Subject: Re: [PATCH v1] perf sample: Make user_regs and intr_regs optional

On Mon, Jan 13, 2025 at 11:43 AM Ian Rogers <irogers@...gle.com> wrote:
>
> The struct dump_regs contains 512 bytes of cache_regs, meaning the two
> values in perf_sample contribute 1088 bytes of its total 1384 bytes
> size. Initializing this much memory has a cost reported by Tavian
> Barnes <tavianator@...ianator.com> as about 2.5% when running `perf
> script --itrace=i0`:
> https://lore.kernel.org/lkml/d841b97b3ad2ca8bcab07e4293375fb7c32dfce7.1736618095.git.tavianator@tavianator.com/
>
> Adrian Hunter <adrian.hunter@...el.com> replied that the zero
> initialization was necessary and couldn't simply be removed.
>
> This patch aims to strike a middle ground of still zeroing the
> perf_sample, but removing 79% of its size by make user_regs and
> intr_regs optional pointers to zalloc-ed memory. To support the
> allocation accessors are created for user_regs and intr_regs. To
> support correct cleanup perf_sample__init and perf_sample__exit
> functions are created and added throughout the code base.

Ping. Given the memory savings and performance wins it would be nice
to see this land. Andi Kleen commented on doing a reimplementation,
which is fine but out-of-scope of what I'm doing here.

Thanks,
Ian