Messages in this thread |  | | From | Arnaldo Carvalho de Melo <> | Date | Mon, 6 Apr 2020 11:07:21 -0300 | Subject | Re: [PATCH v2 2/5] tools api fs: make xxx__mountpoint() more scalable |
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Em Thu, Apr 02, 2020 at 08:43:54AM -0700, Ian Rogers escreveu: > From: Stephane Eranian <eranian@google.com> > > 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 caching.
Thanks, applied, lets see if someone screams...
- Arnaldo
> 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 -e trace=openat -o /tmp/tt perf record -a ls > $ fgrep mounts /tmp/tt > 285 > > After: > $ sudo umount /dev/hugepages > $ strace -e trace=openat -o /tmp/tt perf record -a ls > $ fgrep mounts /tmp/tt > 1 > > One could argue that the non-caching in case the moint point is not found > is intentional. That way subsequent calls may discover a moint point if > the sysadmin mounts the filesystem. But the same argument could be made > against caching the mount 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 caching > 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> > Reviewed-by: Ian Rogers <irogers@google.com> > Acked-by: Jiri Olsa <jolsa@redhat.com> > Signed-off-by: Ian Rogers <irogers@google.com> > --- > tools/lib/api/fs/fs.c | 17 +++++++++++++++++ > tools/lib/api/fs/fs.h | 12 ++++++++++++ > 2 files changed, 29 insertions(+) > > diff --git a/tools/lib/api/fs/fs.c b/tools/lib/api/fs/fs.c > index 027b18f7ed8c..82f53d81a7a7 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, > }, > [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; > } > > @@ -220,6 +228,7 @@ static bool fs__env_override(struct fs *fs) > return false; > > fs->found = true; > + fs->checked = true; > strncpy(fs->path, override_path, sizeof(fs->path) - 1); > fs->path[sizeof(fs->path) - 1] = '\0'; > return true; > @@ -246,6 +255,14 @@ 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 > + * 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. > + */ > + 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 936edb95e1f3..aa222ca30311 100644 > --- a/tools/lib/api/fs/fs.h > +++ b/tools/lib/api/fs/fs.h > @@ -18,6 +18,18 @@ > const char *name##__mount(void); \ > bool name##__configured(void); \ > > +/* > + * The xxxx__mountpoint() entry points find the first match mount point for each > + * 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. > + */ > FS(sysfs) > FS(procfs) > FS(debugfs) > -- > 2.26.0.rc2.310.g2932bb562d-goog >
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- Arnaldo
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