linux-kernel - Re: [PATCH 2/4] perf cs-etm: Use previous thread for branch sample source IP

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Date:   Thu, 8 Jun 2023 18:32:10 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     James Clark <james.clark@....com>
Cc:     coresight@...ts.linaro.org, denik@...omium.org,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mike Leach <mike.leach@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        John Garry <john.g.garry@...cle.com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] perf cs-etm: Use previous thread for branch sample
 source IP

On Thu, Jun 08, 2023 at 06:25:55PM +0800, Leo Yan wrote:

[...]


> Seems to me, this is a synchronization issue between the field
> 'tidq->prev_thread' and 'tidq->prev_packet'.
> 
> It's still hard for me to understand "two adjacent packets on the same
> thread will say they branched from the previous thread that ran", IIUC,
> even we move thread swapping into cs_etm__set_thread(), if the two
> adjacent packets are in the same thread context, we can skip to update
> fields 'tidq->prev_thread' and 'tidq->prev_packet'.

Sorry for typo, here should be:

... skip to update fields 'tidq->prev_thread' and 'tidq->thread'.

> So I am curious if below cs_etm__set_thread() works or not?
> 
> static void cs_etm__set_thread(struct cs_etm_auxtrace *etm,
> 			       struct cs_etm_traceid_queue *tidq, pid_t tid)
> {
> 	struct machine *machine = &etm->session->machines.host;
> 
> 	/* No context switching, bail out */
> 	if ((tidq->thread->tid != tid)
> 		return;
> 
> 	/* If tid is -1, we simply use idle thread context */
> 	if (tid == -1)
> 		goto find_idle_thread;
> 
> 	/*
> 	 * The new incoming tid is different from current thread,
> 	 * so it's to switch to the next thread context.
> 	 */
> 
> 	/* Swap thread contexts */
> 	thread__put(tidq->prev_thread);
> 	tidq->prev_thread = thread__get(tidq->thread);
> 
> 	/* Find thread context for new tid */
> 	thread__zput(tidq->thread);
> 	tidq->thread = machine__find_thread(machine, -1, tid);
> 
> find_idle_thread:
> 	/* Couldn't find a known thread */
> 	if (!tidq->thread)
> 		tidq->thread = machine__idle_thread(machine);
> }
> 
> Thanks,
> Leo