linux-kernel - Re: [PATCH 2/9] perf/core: Add PERF_SAMPLE

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Message-ID: <20191220152324.GG2914998@devbig004.ftw2.facebook.com>
Date:   Fri, 20 Dec 2019 07:23:24 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>,
        Stephane Eranian <eranian@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-perf-users@...r.kernel.org, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 2/9] perf/core: Add PERF_SAMPLE_CGROUP feature

On Fri, Dec 20, 2019 at 01:32:46PM +0900, Namhyung Kim wrote:
> The PERF_SAMPLE_CGROUP bit is to save (perf_event) cgroup information
> in the sample.  It will add a 64-bit id to identify current cgroup and
> it's the file handle in the cgroup file system.  Userspace should use
> this information with PERF_RECORD_CGROUP event to match which cgroup
> it belongs.

You don't need PERF_RECORD_CGROUP for that.  Something like the
following should work.

	struct {
		struct file_handle fh;
		char stor[MAX_HANDLE_SZ];
	} fh_store;
	struct file_handle *fh = &fh_store;

	fh->handle_type = 0xfe; // FILEID_KERNFS
	fh->handle_bytes = sizeof(u64);
	*(u64 *)fh->f_handle = cgrp_id;

	mnt_fd = open('/sys/fs/cgroup', O_RDONLY);
	fd = open_by_handle_at(mnt_fd, fh, O_RDONLY);

	snprintf(proc_path, PATH_MAX, "/proc/self/fd/%d", fd);
	readlink(proc_path, cgrp_path, PATH_MAX);

-- 
tejun