linux-kernel - Re: [PATCH] perf tools: Do not rely on errno values for precise

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Date:   Wed, 3 Jul 2019 11:52:18 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Konstantin Kharlamov <Hi-Angel@...dex.ru>,
        Quentin Monnet <quentin.monnet@...ronome.com>,
        Kim Phillips <kim.phillips@....com>,
        Andi Kleen <ak@...ux.intel.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] perf tools: Do not rely on errno values for precise_ip
 fallback

Em Wed, Jul 03, 2019 at 10:09:49AM +0200, Jiri Olsa escreveu:
> Konstantin reported problem with default perf record command,
> which fails on some AMD servers, because of the default maximum
> precise config.
> 
> The current fallback mechanism counts on getting ENOTSUP errno for
> precise_ip fails, but that's not the case on some AMD servers.
> 
> We can fix this by removing the errno check completely, because the
> precise_ip fallback is separated. We can just try  (if requested by
> evsel->precise_max) all possible precise_ip, and if one succeeds we
> win, if not, we continue with standard fallback.

Thanks, applied.

Simple test shows that behaviour continues the same on x86_64.

- Arnaldo
 
> Reported-by: Konstantin Kharlamov <Hi-Angel@...dex.ru>
> Cc: Quentin Monnet <quentin.monnet@...ronome.com>
> Cc: Kim Phillips <kim.phillips@....com>
> Cc: Andi Kleen <ak@...ux.intel.com>
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  tools/perf/util/evsel.c | 10 ++--------
>  1 file changed, 2 insertions(+), 8 deletions(-)
> 
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 5ab31a4a658d..7fb4ae82f34c 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -1800,14 +1800,8 @@ static int perf_event_open(struct perf_evsel *evsel,
>  		if (fd >= 0)
>  			break;
>  
> -		/*
> -		 * Do quick precise_ip fallback if:
> -		 *  - there is precise_ip set in perf_event_attr
> -		 *  - maximum precise is requested
> -		 *  - sys_perf_event_open failed with ENOTSUP error,
> -		 *    which is associated with wrong precise_ip
> -		 */
> -		if (!precise_ip || !evsel->precise_max || (errno != ENOTSUP))
> +		/* Do not try less precise if not requested. */
> +		if (!evsel->precise_max)
>  			break;
>  
>  		/*
> -- 
> 2.21.0

-- 

- Arnaldo