linux-kernel - Re: [PATCH v2] perf sched timehist: Add pre-migration wait time option

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Message-ID: <eb218ff7-b3cc-4c51-999f-59641c92c139@linux.ibm.com>
Date: Wed, 2 Oct 2024 20:19:47 +0530
From: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Athira Rajeev
 <atrajeev@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>, Adrian Hunter <adrian.hunter@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>, acme@...hat.com,
        linux-perf-users <linux-perf-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
Subject: Re: [PATCH v2] perf sched timehist: Add pre-migration wait time
 option

Hi Namhyung,

On 02/10/24 06:12, Namhyung Kim wrote:
> On Tue, Oct 01, 2024 at 04:36:20PM +0530, Madadi Vineeth Reddy wrote:
>> pre-migration wait time is the time that a task unnecessarily spends
>> on the runqueue of a CPU but doesn't get switched-in there. In terms
>> of tracepoints, it is the time between sched:sched_wakeup and
>> sched:sched_migrate_task.

[snip]

>>  static bool is_idle_sample(struct perf_sample *sample,
>> @@ -2598,13 +2611,19 @@ static int timehist_sched_wakeup_event(const struct perf_tool *tool,
>>  	if (tr == NULL)
>>  		return -1;
>>  
>> -	if (tr->ready_to_run == 0)
>> -		tr->ready_to_run = sample->time;
>> +	if (!strcmp(evsel__name(evsel), "sched:sched_waking")) {
> 
> I guess it won't work when there's no sched_waking event.  Can you
> simply handle pre-migration in sched_waking?
> 

I believe it should still work even without the sched_waking event because
I've updated the condition to ensure that timehist_sched_wakeup_ignore is
not selected when the pre-migration option is enabled.

> Thanks,
> Namhyung
> 
> 
>> +		if (tr->ready_to_run == 0)
>> +			tr->ready_to_run = sample->time;

[snip]

>>  	/* prefer sched_waking if it is captured */
>> -	if (evlist__find_tracepoint_by_name(session->evlist, "sched:sched_waking"))
>> +	if (!sched->pre_migrations &&
>> +		evlist__find_tracepoint_by_name(session->evlist, "sched:sched_waking"))
>>  		handlers[1].handler = timehist_sched_wakeup_ignore;

In this case, it checks if pre-migration is enabled. If so, the handler will still
use timehist_sched_wakeup_event for the sched:sched_wakeup event.

The reason I initially chose the sched:sched_wakeup tracepoint instead of
sched:sched_waking is that there could be instances where the CPU chosen
during sched_waking may not match the actual CPU where the task ends up running,
as shown in the example below:

      wdavdaemon   14789 [006] 31357.614692: sched:sched_waking: comm=wdavdaemon pid=14778 prio=120 target_cpu=005
	[snip]
         swapper       0 [002] 31357.614695: sched:sched_wakeup: wdavdaemon:14778 [120] CPU:002

However, since we are already accounting for the sched_migrate_task event occurring
between sched_waking and sched_switch and don't need to check target_cpu, switching
to sched_waking should work just as well, with only a very minor time difference.

I'll go ahead and send a v3 with sched_waking. Thanks again for the feedback.

Thanks,
Madadi Vineeth Reddy

>>  
>>  	/* setup per-evsel handlers */
>> @@ -3280,8 +3309,14 @@ static int perf_sched__timehist(struct perf_sched *sched)
>>  		goto out;
>>  	}
>>  
>> -	if (sched->show_migrations &&
>> -	    perf_session__set_tracepoints_handlers(session, migrate_handlers))
>> +	if (sched->pre_migrations && !evlist__find_tracepoint_by_name(session->evlist,
>> +									"sched:sched_wakeup")) {
>> +		pr_err("No sched_wakeup events found. sched_wakeup tracepoint is mandatory for -P option\n");
>> +		goto out;
>> +	}
>> +
>> +	if ((sched->show_migrations || sched->pre_migrations) &&
>> +		perf_session__set_tracepoints_handlers(session, migrate_handlers))
>>  		goto out;
>>  
>>  	/* pre-allocate struct for per-CPU idle stats */
>> @@ -3823,6 +3858,7 @@ int cmd_sched(int argc, const char **argv)
>>  	OPT_BOOLEAN(0, "show-prio", &sched.show_prio, "Show task priority"),
>>  	OPT_STRING(0, "prio", &sched.prio_str, "prio",
>>  		   "analyze events only for given task priority(ies)"),
>> +	OPT_BOOLEAN('P', "pre-migrations", &sched.pre_migrations, "Show pre-migration wait time"),
>>  	OPT_PARENT(sched_options)
>>  	};
>>  
>> -- 
>> 2.43.2
>>