linux-kernel - Re: [PATCH V2 04/23] perf/x86/intel: Support adaptive PEBSv4

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Message-ID: <20190321212059.GD7905@worktop.programming.kicks-ass.net>
Date:   Thu, 21 Mar 2019 22:20:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
Cc:     acme@...nel.org, mingo@...hat.com, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, jolsa@...nel.org, eranian@...gle.com,
        alexander.shishkin@...ux.intel.com, ak@...ux.intel.com
Subject: Re: [PATCH V2 04/23] perf/x86/intel: Support adaptive PEBSv4

On Thu, Mar 21, 2019 at 01:56:44PM -0700, kan.liang@...ux.intel.com wrote:
> @@ -933,6 +1001,34 @@ pebs_update_state(bool needed_cb, struct cpu_hw_events *cpuc, struct pmu *pmu)
>  		update = true;
>  	}
>  
> +	/*
> +	 * The PEBS record doesn't shrink on the del. Because to get
> +	 * an accurate config needs to go through all the existing pebs events.
> +	 * It's not necessary.
> +	 * There is no harmful for a bigger PEBS record, except little
> +	 * performance impacts.
> +	 * Also, for most cases, the same pebs config is applied for all
> +	 * pebs events.
> +	 */
> +	if (x86_pmu.intel_cap.pebs_baseline && add) {
> +		u64 pebs_data_cfg;
> +
> +		/* Clear pebs_data_cfg and pebs_record_size for first PEBS. */
> +		if (cpuc->n_pebs == 1) {
> +			cpuc->pebs_data_cfg = 0;
> +			cpuc->pebs_record_size = sizeof(struct pebs_basic);
> +		}

Argh, no. This is daft. The previous site was fine, it was just the
pebs_record_size assignment I'm confused about.

Note how by setting ->pebs_data_cfs to 0, you force the below branch to
true and call adaptive_pebs_record_size_update() ? So _why_ do you have
to set pebs_record_size()?

> +
> +		pebs_data_cfg = pebs_update_adaptive_cfg(event);
> +
> +		/* Update pebs_record_size if new event requires more data. */
> +		if (pebs_data_cfg & ~cpuc->pebs_data_cfg) {
> +			cpuc->pebs_data_cfg |= pebs_data_cfg;
> +			adaptive_pebs_record_size_update();
> +			update = true;
> +		}
> +	}
> +
>  	if (update)
>  		pebs_update_threshold(cpuc);
>  }