linux-kernel - Re: [PATCH V4 4/7] perf/x86/intel: Support LBR event logging

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Message-ID: <20231019111202.GJ36211@noisy.programming.kicks-ass.net>
Date:   Thu, 19 Oct 2023 13:12:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
Cc:     mingo@...hat.com, acme@...nel.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...nel.org, namhyung@...nel.org, irogers@...gle.com,
        adrian.hunter@...el.com, ak@...ux.intel.com, eranian@...gle.com,
        alexey.v.bayduraev@...ux.intel.com, tinghao.zhang@...el.com
Subject: Re: [PATCH V4 4/7] perf/x86/intel: Support LBR event logging

On Wed, Oct 04, 2023 at 11:40:41AM -0700, kan.liang@...ux.intel.com wrote:
> +static __always_inline void get_lbr_events(struct cpu_hw_events *cpuc,
> +					   int i, u64 info)
> +{
> +	/*
> +	 * The later code will decide what content can be disclosed
> +	 * to the perf tool. It's no harmful to unconditionally update
> +	 * the cpuc->lbr_events.
> +	 * Pleae see intel_pmu_lbr_event_reorder()
> +	 */
> +	cpuc->lbr_events[i] = info & LBR_INFO_EVENTS;
> +}

You could be forcing an extra cachemiss here. A long time ago I had
hacks to profile perf with perf, but perhaps PT can be abused for that
now?