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Message-ID: <20230608103210.GC123723@leoy-huanghe>
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
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