linux-kernel - Re: [PATCH v2] perf bench: Use condition variables in numa.

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Date:   Wed, 14 Oct 2020 13:45:00 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Ian Rogers <irogers@...gle.com>
Cc:     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>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Peng Fan <fanpeng@...ngson.cn>, linux-kernel@...r.kernel.org,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v2] perf bench: Use condition variables in numa.

On Mon, Oct 12, 2020 at 09:16:11AM -0700, Ian Rogers wrote:

SNIP

> @@ -483,6 +484,18 @@ static void init_global_mutex(pthread_mutex_t *mutex)
>  	pthread_mutex_init(mutex, &attr);
>  }
>  
> +/*
> + * Return a process-shared (global) condition variable:
> + */
> +static void init_global_cond(pthread_cond_t *cond)
> +{
> +	pthread_condattr_t attr;
> +
> +	pthread_condattr_init(&attr);
> +	pthread_condattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
> +	pthread_cond_init(cond, &attr);
> +}
> +
>  static int parse_cpu_list(const char *arg)
>  {
>  	p0.cpu_list_str = strdup(arg);
> @@ -1136,15 +1149,18 @@ static void *worker_thread(void *__tdata)
>  	if (g->p.serialize_startup) {
>  		pthread_mutex_lock(&g->startup_mutex);
>  		g->nr_tasks_started++;
> +		/* The last thread wakes the main process. */
> +		if (g->nr_tasks_started == g->p.nr_tasks)
> +			pthread_cond_signal(&g->startup_cond);

should you remove the condition? it's not necessary
and making this racy, no?

just single pthread_cond_signal should be enough,
because the wait code is checking the number of tasks

jirka