linux-kernel - Re: [PATCH] perf record: Synthesize cgroup events only if needed

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Date:   Fri, 27 Nov 2020 16:45:57 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Ian Rogers <irogers@...gle.com>
Subject: Re: [PATCH] perf record: Synthesize cgroup events only if needed

On Fri, Nov 27, 2020 at 02:43:56PM +0900, Namhyung Kim wrote:
> It didn't check the tool->cgroup_events bit which is set when
> the --all-cgroups option is given.  Without it, samples will not have
> cgroup info so no reason to synthesize.
> 
> We can check the PERF_RECORD_CGROUP records after running perf record
> *WITHOUT* the --all-cgroups option:
> 
> Before:
>   $ perf report -D | grep CGROUP
>   0 0 0x8430 [0x38]: PERF_RECORD_CGROUP cgroup: 1 /
>           CGROUP events:          1
>           CGROUP events:          0
>           CGROUP events:          0
> 
> After:
>   $ perf report -D | grep CGROUP
>           CGROUP events:          0
>           CGROUP events:          0
>           CGROUP events:          0
> 
> Fixes: 8fb4b67939e16 ("perf record: Add --all-cgroups option")
> Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> ---
>  tools/perf/util/synthetic-events.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/tools/perf/util/synthetic-events.c b/tools/perf/util/synthetic-events.c
> index 8a23391558cf..d9c624377da7 100644
> --- a/tools/perf/util/synthetic-events.c
> +++ b/tools/perf/util/synthetic-events.c
> @@ -563,6 +563,9 @@ int perf_event__synthesize_cgroups(struct perf_tool *tool,
>  	char cgrp_root[PATH_MAX];
>  	size_t mount_len;  /* length of mount point in the path */
>  
> +	if (!tool || !tool->cgroup_events)
> +		return 0;

can !tool actually happen here? or it's just being extra cautious

Acked-by: Jiri Olsa <jolsa@...hat.com>

thanks,
jirka

> +
>  	if (cgroupfs_find_mountpoint(cgrp_root, PATH_MAX, "perf_event") < 0) {
>  		pr_debug("cannot find cgroup mount point\n");
>  		return -1;
> -- 
> 2.29.2.454.gaff20da3a2-goog
>