linux-kernel - Re: [PATCH V2 1/2] perf/x86/intel: enable CPU ref

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Date:   Mon, 12 Jun 2017 12:36:31 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     kan.liang@...el.com
Cc:     peterz@...radead.org, mingo@...hat.com, eranian@...gle.com,
        linux-kernel@...r.kernel.org, alexander.shishkin@...ux.intel.com,
        acme@...hat.com, torvalds@...ux-foundation.org, tglx@...utronix.de,
        vincent.weaver@...ne.edu, ak@...ux.intel.com
Subject: Re: [PATCH V2 1/2] perf/x86/intel: enable CPU ref_cycles for GP
 counter

On Fri, Jun 09, 2017 at 10:28:02AM -0700, kan.liang@...el.com wrote:

SNIP

> +	/*
> +	 * Correct the count number if applying ref_cycles replacement.
> +	 * There is a constant ratio between ref_cycles (event A) and
> +	 * ref_cycles replacement (event B). The delta result is from event B.
> +	 * To get accurate event A count, the real delta result must be
> +	 * multiplied with the constant ratio.
> +	 *
> +	 * It is handled differently for sampling and counting.
> +	 * - Fixed period Sampling: The period is already adjusted in
> +	 *   scheduling for event B. The period_left is the remaining period
> +	 *   for event B. Don't need to modify period_left.
> +	 *   It's enough to only adjust event->count here.
> +	 *
> +	 * - Fixed freq Sampling: The adaptive frequency algorithm needs
> +	 *   the last_period and event counts for event A to estimate the next
> +	 *   period for event A. So the period_left is the remaining period
> +	 *   for event A. It needs to multiply the result with the ratio.
> +	 *   However, the period_left is also used to reload the event counter
> +	 *   for event B in x86_perf_event_set_period. It has to be adjusted to
> +	 *   event B's remaining period there.
> +	 *
> +	 * - Counting: It's enough to only adjust event->count for perf stat.
> +	 *
> +	 * - RDPMC: User can also read the counter directly by rdpmc/mmap.
> +	 *   Users have to handle the adjustment themselves.
> +	 *   For kernel, it only needs to guarantee that the offset is correct.
> +	 *   In x86's arch_perf_update_userpage, the offset will be corrected
> +	 *   if event B is used.
> +	 */
> +	adjust_delta = delta;
> +	if (hwc->flags & PERF_X86_EVENT_REF_CYCLES_REP) {
> +		adjust_delta = delta * x86_pmu.ref_cycles_factor;
> +
> +		if (is_sampling_event(event) && event->attr.freq)
> +			local64_sub(adjust_delta, &hwc->period_left);
> +		else
> +			local64_sub(delta, &hwc->period_left);

shouldn't you use adjust_delta in here also ^^^ ?

jirka