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

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Date:   Thu, 19 Oct 2023 12:52:31 +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:

> +#define ARCH_LBR_EVENT_LOG_WIDTH	2
> +#define ARCH_LBR_EVENT_LOG_MASK		0x3

event log ?


> +static __always_inline void intel_pmu_update_lbr_event(u64 *lbr_events, int idx, int pos)
> +{
> +	u64 logs = *lbr_events >> (LBR_INFO_EVENTS_OFFSET +
> +				   idx * ARCH_LBR_EVENT_LOG_WIDTH);
> +
> +	logs &= ARCH_LBR_EVENT_LOG_MASK;
> +	*lbr_events |= logs << (pos * ARCH_LBR_EVENT_LOG_WIDTH);
> +}
> +
> +/*
> + * The enabled order may be different from the counter order.
> + * Update the lbr_events with the enabled order.
> + */
> +static void intel_pmu_lbr_event_reorder(struct cpu_hw_events *cpuc,
> +					struct perf_event *event)
> +{
> +	int i, j, pos = 0, enabled[X86_PMC_IDX_MAX];
> +	struct perf_event *leader, *sibling;
> +
> +	leader = event->group_leader;
> +	if (branch_sample_counters(leader))
> +		enabled[pos++] = leader->hw.idx;
> +
> +	for_each_sibling_event(sibling, leader) {
> +		if (!branch_sample_counters(sibling))
> +			continue;
> +		enabled[pos++] = sibling->hw.idx;
> +	}

Ok, so far so good: enabled[x] = y, is a mapping of hardware index (y)
to group order (x).

Although I would perhaps name that order[] instead of enabled[].

> +
> +	if (!pos)
> +		return;

How would we ever get here if this is the case?

> +
> +	for (i = 0; i < cpuc->lbr_stack.nr; i++) {
> +		for (j = 0; j < pos; j++)
> +			intel_pmu_update_lbr_event(&cpuc->lbr_events[i], enabled[j], j);

But this confuses me... per that function it:

 - extracts counter value for enabled[j] and,
 - or's it into the same variable at j

But what if j is already taken by something else?

That is, suppose enabled[] = {3,2,1,0}, and lbr_events = 11 10 01 00

Then: for (j) intel_pmu_update_lbt_event(&lbr_event, enabled[j], j);

0: 3->0, 11 10 01 00 -> 11 10 01 11
1: 2->1, 11 10 01 11 -> 11 10 11 11
2: 1->2, 11 10 11 11 -> 11 11 11 11



> +
> +		/* Clear the original counter order */
> +		cpuc->lbr_events[i] &= ~LBR_INFO_EVENTS;
> +	}
> +}

Would not something like:

	src = lbr_events[i];
	dst = 0;
	for (j = 0; j < pos; j++) {
		cnt = (src >> enabled[j]*2) & 3;
		dst |= cnt << j*2
	}
	lbr_events[i] = dst;

be *FAR* clearer, and actually work?