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

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Date:   Thu, 19 Oct 2023 13:09:17 +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:

> @@ -3905,6 +3915,44 @@ static int intel_pmu_hw_config(struct perf_event *event)
>  	if (needs_branch_stack(event) && is_sampling_event(event))
>  		event->hw.flags  |= PERF_X86_EVENT_NEEDS_BRANCH_STACK;
>  
> +	if (branch_sample_counters(event)) {
> +		struct perf_event *leader, *sibling;
> +
> +		if (!(x86_pmu.flags & PMU_FL_LBR_EVENT) ||
> +		    (event->attr.config & ~INTEL_ARCH_EVENT_MASK))
> +			return -EINVAL;
> +
> +		/*
> +		 * The event logging is not supported in the call stack mode
> +		 * yet, since we cannot simply flush the LBR during e.g.,
> +		 * multiplexing. Also, there is no obvious usage with the call
> +		 * stack mode. Simply forbids it for now.
> +		 *
> +		 * If any events in the group enable the LBR event logging
> +		 * feature, the group is treated as a LBR event logging group,
> +		 * which requires the extra space to store the counters.
> +		 */
> +		leader = event->group_leader;
> +		if (branch_sample_call_stack(leader))
> +			return -EINVAL;
> +		leader->hw.flags  |= PERF_X86_EVENT_BRANCH_COUNTERS;

(superfluous whitespace before operator)

> +
> +		for_each_sibling_event(sibling, leader) {
> +			if (branch_sample_call_stack(sibling))
> +				return -EINVAL;
> +		}
> +
> +		/*
> +		 * Only applying the PERF_SAMPLE_BRANCH_COUNTERS doesn't
> +		 * require any branch stack setup.
> +		 * Clear the bit to avoid unnecessary branch stack setup.
> +		 */
> +		if (0 == (event->attr.branch_sample_type &
> +			  ~(PERF_SAMPLE_BRANCH_PLM_ALL |
> +			    PERF_SAMPLE_BRANCH_COUNTERS)))
> +			event->hw.flags  &= ~PERF_X86_EVENT_NEEDS_BRANCH_STACK;
> +	}

Does this / should this check the number of group members vs supported
number of lbr counters?