Re: [PATCH 3/3] tools/lib/fs: Cache cgroupfs mount point

From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Namhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>, Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Stephane Eranian <eranian@google.com>,
	Ian Rogers <irogers@google.com>, Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH 3/3] tools/lib/fs: Cache cgroupfs mount point
Date: Tue, 29 Dec 2020 08:51:58 -0300	[thread overview]
Message-ID: <20201229115158.GH521329@kernel.org> (raw)
In-Reply-To: <20201216090556.813996-3-namhyung@kernel.org>

Em Wed, Dec 16, 2020 at 06:05:56PM +0900, Namhyung Kim escreveu:
> Currently it parses the /proc file everytime it opens a file in the
> cgroupfs.  Save the last result to avoid it (assuming it won't be
> changed between the accesses).

Which is the most likely case, but can't we use something like inotify
to detect that and bail out or warn the user?

- Arnaldo

> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
>  tools/lib/api/fs/cgroup.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/tools/lib/api/fs/cgroup.c b/tools/lib/api/fs/cgroup.c
> index 262a4229e293..1573dae4259d 100644
> --- a/tools/lib/api/fs/cgroup.c
> +++ b/tools/lib/api/fs/cgroup.c
> @@ -8,6 +8,14 @@
>  #include <string.h>
>  #include "fs.h"
>  
> +struct cgroupfs_cache_entry {
> +	char	subsys[32];
> +	char	mountpoint[PATH_MAX];
> +};
> +
> +/* just cache last used one */
> +static struct cgroupfs_cache_entry cached;
> +
>  int cgroupfs_find_mountpoint(char *buf, size_t maxlen, const char *subsys)
>  {
>  	FILE *fp;
> @@ -16,6 +24,14 @@ int cgroupfs_find_mountpoint(char *buf, size_t maxlen, const char *subsys)
>  	char *p, *path;
>  	char mountpoint[PATH_MAX];
>  
> +	if (!strcmp(cached.subsys, subsys)) {
> +		if (strlen(cached.mountpoint) < maxlen) {
> +			strcpy(buf, cached.mountpoint);
> +			return 0;
> +		}
> +		return -1;
> +	}
> +
>  	fp = fopen("/proc/mounts", "r");
>  	if (!fp)
>  		return -1;
> @@ -75,6 +91,9 @@ int cgroupfs_find_mountpoint(char *buf, size_t maxlen, const char *subsys)
>  	free(line);
>  	fclose(fp);
>  
> +	strncpy(cached.subsys, subsys, sizeof(cached.subsys) - 1);
> +	strcpy(cached.mountpoint, mountpoint);
> +
>  	if (mountpoint[0] && strlen(mountpoint) < maxlen) {
>  		strcpy(buf, mountpoint);
>  		return 0;
> -- 
> 2.29.2.684.gfbc64c5ab5-goog
> 

-- 

- Arnaldo