Messages in this thread Patch in this message | | | Date | Sun, 27 Jan 2019 22:00:46 -0200 | From | Arnaldo Carvalho de Melo <> | Subject | Re: [PATCH] perf tools api fs: make xxx__mountpoint() more scalable |
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Em Wed, Jan 23, 2019 at 03:10:47PM -0800, Stephane Eranian escreveu: > The xxx_mountpoint() interface provided by fs.c finds > mount points for common pseudo filesystems. The first > time xxx_mountpoint() is invoked, it scans the mount > table (/proc/mounts) looking for a match. If found, it > is cached. The price to scan /proc/mounts is paid once > if the mount is found. > > When the mount point is not found, subsequent calls to > xxx_mountpoint() scan /proc/mounts over and over again. > There is no cacheing. > > This causes a scaling issue in perf record with hugeltbfs__mountpoint(). > The function is called for each process found in synthesize__mmap_events(). > If the machine has thousands of processes and if the /proc/mounts has many > entries this could cause major overhead in perf record. We have observed > multi-second slowdowns on some configurations. > > As an example on a laptop: > > Before: > $ sudo umount /dev/hugepages > $ strace -etrace=open -o /tmp/tt perf record -a ls > $ fgrep mounts /tmp/tt > 285 > > After: > $ sudo umount /dev/hugepages > $ strace -etrace=open -o /tmp/tt perf record -a ls > $ fgrep mounts /tmp/tt > 1 > > One could argue that the non-cacheing in case the moint point is not found > is intentional. That way subsequent calls may discover a moint point if > the syadmin mounts the filesystem. But the same argument could be made > against cacheing the moint point. It could be unmounted causing errors. > It all depends on the intent of the interface. This patch assumes it > is expected to scan /proc/mounts once. The patch documents the cacheing > behavior in the fs.h header file. > > An alternative would be to just fix perf record. But it would solve > the problem with hugetlbs__mountpoint() but there could be similar > issues (possibly down the line) with other xxx_mountpoint() calls > in perf or other tools. > > Signed-off-by: Stephane Eranian <eranian@google.com> > --- > tools/lib/api/fs/fs.c | 19 +++++++++++++++++++ > tools/lib/api/fs/fs.h | 11 +++++++++++ > 2 files changed, 30 insertions(+) > > diff --git a/tools/lib/api/fs/fs.c b/tools/lib/api/fs/fs.c > index 7aba8243a0e7..6934da54c96b 100644 > --- a/tools/lib/api/fs/fs.c > +++ b/tools/lib/api/fs/fs.c > @@ -90,6 +90,7 @@ struct fs { > const char * const *mounts; > char path[PATH_MAX]; > bool found; > + bool checked; > long magic; > }; > > @@ -111,31 +112,37 @@ static struct fs fs__entries[] = { > .name = "sysfs", > .mounts = sysfs__fs_known_mountpoints, > .magic = SYSFS_MAGIC, > + .checked= false,
No need for these initializations, 0 == false, and since we initialize some of the other fields, the ones that haven't are set to zero.
> }, > [FS__PROCFS] = { > .name = "proc", > .mounts = procfs__known_mountpoints, > .magic = PROC_SUPER_MAGIC, > + .checked= false, > }, > [FS__DEBUGFS] = { > .name = "debugfs", > .mounts = debugfs__known_mountpoints, > .magic = DEBUGFS_MAGIC, > + .checked= false, > }, > [FS__TRACEFS] = { > .name = "tracefs", > .mounts = tracefs__known_mountpoints, > .magic = TRACEFS_MAGIC, > + .checked= false, > }, > [FS__HUGETLBFS] = { > .name = "hugetlbfs", > .mounts = hugetlbfs__known_mountpoints, > .magic = HUGETLBFS_MAGIC, > + .checked= false, > }, > [FS__BPF_FS] = { > .name = "bpf", > .mounts = bpf_fs__known_mountpoints, > .magic = BPF_FS_MAGIC, > + .checked= false, > }, > }; > > @@ -158,6 +165,7 @@ static bool fs__read_mounts(struct fs *fs) > } > > fclose(fp); > + fs->checked = true; > return fs->found = found; > } > > @@ -219,6 +227,7 @@ static bool fs__env_override(struct fs *fs) > return false; > > fs->found = true; > + fs->checked = true; > strncpy(fs->path, override_path, sizeof(fs->path)); > return true; > } > @@ -244,6 +253,16 @@ static const char *fs__mountpoint(int idx) > if (fs->found) > return (const char *)fs->path; > > + /* the mount point was already checked for the mount point
Nit, we start with:
/* * The mount point was already checked for the mount point > + * but and did not exist, so return NULL to avoid scanning again. > + * This makes the found and not found paths cost equivalent > + * in case of multiple calls. This was not the case before > + * and could cause significant scaling issues with callers. > + * in case /proc/mounts need to be checked many times . > + */ > + if (fs->checked) > + return NULL; > + > return fs__get_mountpoint(fs); > } > > diff --git a/tools/lib/api/fs/fs.h b/tools/lib/api/fs/fs.h > index 92d03b8396b1..00a5127b00e8 100644 > --- a/tools/lib/api/fs/fs.h > +++ b/tools/lib/api/fs/fs.h > @@ -18,6 +18,17 @@ > const char *name##__mount(void); \ > bool name##__configured(void); \ > > +/* > + * The xxxx__mountpoint() entry points find the first match mount point for each
Nicely written :-)
> + * filesystems listed below, where xxxx is the filesystem type. > + * > + * The interface is as follows: > + * - If a mount point is found on first call, it is cached and used for all subsequent > + * calls. > + * > + * - If a mount point is not found, NULL is returned on first call and all > + * subsequent calls. Ditto. > + */ > FS(sysfs) > FS(procfs) > FS(debugfs) > -- > 2.7.4
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