From: Stephane Eranian eranian@google.com
[ Upstream commit c6fddb28bad26e5472cb7acf7b04cd5126f1a4ab ]
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.
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 Cc: Alexander Shishkin alexander.shishkin@linux.intel.com Cc: Andrey Zhizhikin andrey.z@gmail.com Cc: Kan Liang kan.liang@linux.intel.com Cc: Kefeng Wang wangkefeng.wang@huawei.com Cc: Mark Rutland mark.rutland@arm.com Cc: Namhyung Kim namhyung@kernel.org Cc: Peter Zijlstra peterz@infradead.org Cc: Petr Mladek pmladek@suse.com Cc: Thomas Gleixner tglx@linutronix.de Link: http://lore.kernel.org/lkml/20200402154357.107873-3-irogers@google.com Signed-off-by: Ian Rogers irogers@google.com Signed-off-by: Arnaldo Carvalho de Melo acme@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org Signed-off-by: Yang Yingliang yangyingliang@huawei.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 bd021a0eeef8..4cc69675c2a9 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 92d03b8396b1..3b70003e7cfb 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)