linux-kernel - Re: [PATCH] perf record: Use an eventfd to wakeup when done

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Date:   Tue, 12 May 2020 14:12:32 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Anand K Mistry <amistry@...gle.com>
Cc:     linux-perf-users@...r.kernel.org,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf record: Use an eventfd to wakeup when done

On Tue, May 12, 2020 at 02:59:36PM +1000, Anand K Mistry wrote:

SNIP

> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index 1ab349abe90469..099ecaa66732a2 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -53,6 +53,7 @@
>  #include <unistd.h>
>  #include <sched.h>
>  #include <signal.h>
> +#include <sys/eventfd.h>
>  #include <sys/mman.h>
>  #include <sys/wait.h>
>  #include <sys/types.h>
> @@ -518,15 +519,28 @@ static int record__pushfn(struct mmap *map, void *to, void *bf, size_t size)
>  
>  static volatile int signr = -1;
>  static volatile int child_finished;
> +static int done_fd = -1;
>  
>  static void sig_handler(int sig)
>  {
> +	u64 tmp = 1;
>  	if (sig == SIGCHLD)
>  		child_finished = 1;
>  	else
>  		signr = sig;
>  
>  	done = 1;
> +
> +	/*
> +	 * It is possible for this signal handler to run after done is checked
> +	 * in the main loop, but before the perf counter fds are polled. If this
> +	 * happens, the poll() will continue to wait even though done is set,
> +	 * and will only break out if either another signal is received, or the
> +	 * counters are ready for read. To ensure the poll() doesn't sleep when
> +	 * done is set, use an eventfd (done_fd) to wake up the poll().
> +	 */
> +	if (write(done_fd, &tmp, sizeof(tmp)) < 0)
> +		pr_err("failed to signal wakeup fd\n");
>  }
>  
>  static void sigsegv_handler(int sig)
> @@ -1424,6 +1438,17 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
>  	int fd;
>  	float ratio = 0;
>  
> +	done_fd = eventfd(0, EFD_NONBLOCK);
> +	if (done_fd < 0) {
> +		pr_err("Failed to create wakeup eventfd, error: %m\n");
> +		return -1;
> +	}
> +	err = evlist__add_pollfd(rec->evlist, done_fd);
> +	if (err < 0) {
> +		pr_err("Failed to add wakeup eventfd to poll list\n");
> +		return -1;
> +	}

sorry I did not notice before, but I think we also
need to close done_fd descriptor on the exit path

also please change subject to PATCHv3 for the next version

thanks,
jirka

> +
>  	atexit(record__sig_exit);
>  	signal(SIGCHLD, sig_handler);
>  	signal(SIGINT, sig_handler);
> -- 
> 2.26.2.645.ge9eca65c58-goog
>