linux-kernel - Re: [PATCH v2] perf: Rewrite core context handling

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Message-ID: <Y0V3kOWInrvCvVtk@hirez.programming.kicks-ass.net>
Date:   Tue, 11 Oct 2022 16:02:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ravi Bangoria <ravi.bangoria@....com>
Cc:     acme@...nel.org, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org, songliubraving@...com,
        eranian@...gle.com, ak@...ux.intel.com, mark.rutland@....com,
        frederic@...nel.org, maddy@...ux.ibm.com, irogers@...gle.com,
        will@...nel.org, robh@...nel.org, mingo@...hat.com,
        catalin.marinas@....com, ndesaulniers@...gle.com,
        srw@...dewatkins.net, linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        sandipan.das@....com, ananth.narayan@....com, kim.phillips@....com,
        santosh.shukla@....com
Subject: Re: [PATCH v2] perf: Rewrite core context handling

On Tue, Oct 11, 2022 at 06:49:55PM +0530, Ravi Bangoria wrote:
> On 11-Oct-22 4:59 PM, Peter Zijlstra wrote:
> > On Sat, Oct 08, 2022 at 11:54:24AM +0530, Ravi Bangoria wrote:
> > 
> >> +static void perf_event_swap_task_ctx_data(struct perf_event_context *prev_ctx,
> >> +					  struct perf_event_context *next_ctx)
> >> +{
> >> +	struct perf_event_pmu_context *prev_epc, *next_epc;
> >> +
> >> +	if (!prev_ctx->nr_task_data)
> >> +		return;
> >> +
> >> +	prev_epc = list_first_entry(&prev_ctx->pmu_ctx_list,
> >> +				    struct perf_event_pmu_context,
> >> +				    pmu_ctx_entry);
> >> +	next_epc = list_first_entry(&next_ctx->pmu_ctx_list,
> >> +				    struct perf_event_pmu_context,
> >> +				    pmu_ctx_entry);
> >> +
> >> +	while (&prev_epc->pmu_ctx_entry != &prev_ctx->pmu_ctx_list &&
> >> +	       &next_epc->pmu_ctx_entry != &next_ctx->pmu_ctx_list) {
> >> +
> >> +		WARN_ON_ONCE(prev_epc->pmu != next_epc->pmu);
> >> +
> >> +		/*
> >> +		 * PMU specific parts of task perf context can require
> >> +		 * additional synchronization. As an example of such
> >> +		 * synchronization see implementation details of Intel
> >> +		 * LBR call stack data profiling;
> >> +		 */
> >> +		if (prev_epc->pmu->swap_task_ctx)
> >> +			prev_epc->pmu->swap_task_ctx(prev_epc, next_epc);
> >> +		else
> >> +			swap(prev_epc->task_ctx_data, next_epc->task_ctx_data);
> > 
> > Did I forget to advance the iterators here?
> 
> Yeah. Seems so. I overlooked it too.

OK; so I'm not slowly going crazy staring at this code ;-) Let me go add
it now then. :-)

But first I gotta taxi the kids around for a bit, bbl.