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Message-ID: <Z4bQwTL5VK6jGDZz@tassilo>
Date: Tue, 14 Jan 2025 13:01:53 -0800
From: Andi Kleen <ak@...ux.intel.com>
To: Ian Rogers <irogers@...gle.com>
Cc: 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>,
	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>,
	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 11:43:45AM -0800, Ian Rogers 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.

A much easier fix is to keep a global/heap allocate perf event
around that has these parts zeroed and only override the fields
needed and clear them afterwards.

(similar strategy as a slab constructor in the kernel)

-Andi

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