ext4: improve ext4lazyinit scalability
ext4lazyinit is a global thread. This thread performs itable
initalization under li_list_mtx mutex.
It basically does the following:
ext4_lazyinit_thread
->mutex_lock(&eli->li_list_mtx);
->ext4_run_li_request(elr)
->ext4_init_inode_table-> Do a lot of IO if the list is large
And when new mount/umount arrive they have to block on ->li_list_mtx
because lazy_thread holds it during full walk procedure.
ext4_fill_super
->ext4_register_li_request
->mutex_lock(&ext4_li_info->li_list_mtx);
->list_add(&elr->lr_request, &ext4_li_info >li_request_list);
In my case mount takes 40minutes on server with 36 * 4Tb HDD.
Common user may face this in case of very slow dev ( /dev/mmcblkXXX)
Even more. If one of filesystems was frozen lazyinit_thread will simply
block on sb_start_write() so other mount/umount will be stuck forever.
This patch changes logic like follows:
- grab ->s_umount read sem before processing new li_request.
After that it is safe to drop li_list_mtx because all callers of
li_remove_request are holding ->s_umount for write.
- li_thread skips frozen SB's
Locking order:
Mh KOrder is asserted by umount path like follows: s_umount ->li_list_mtx so
the only way to to grab ->s_mount inside li_thread is via down_read_trylock
xfstests:ext4/023
#PSBM-49658
Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 5819b0e..50912cc 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2749,7 +2749,6 @@
sb = elr->lr_super;
ngroups = EXT4_SB(sb)->s_groups_count;
- sb_start_write(sb);
for (group = elr->lr_next_group; group < ngroups; group++) {
gdp = ext4_get_group_desc(sb, group, NULL);
if (!gdp) {
@@ -2776,8 +2775,6 @@
elr->lr_next_sched = jiffies + elr->lr_timeout;
elr->lr_next_group = group + 1;
}
- sb_end_write(sb);
-
return ret;
}
@@ -2842,19 +2839,43 @@
mutex_unlock(&eli->li_list_mtx);
goto exit_thread;
}
-
list_for_each_safe(pos, n, &eli->li_request_list) {
+ int err = 0;
+ int progress = 0;
elr = list_entry(pos, struct ext4_li_request,
lr_request);
- if (time_after_eq(jiffies, elr->lr_next_sched)) {
- if (ext4_run_li_request(elr) != 0) {
- /* error, remove the lazy_init job */
- ext4_remove_li_request(elr);
- continue;
- }
+ if (time_before(jiffies, elr->lr_next_sched)) {
+ if (time_before(elr->lr_next_sched, next_wakeup))
+ next_wakeup = elr->lr_next_sched;
+ continue;
}
-
+ if (down_read_trylock(&elr->lr_super->s_umount)) {
+ if (sb_start_write_trylock(elr->lr_super)) {
+ progress = 1;
+ /*
+ * We hold sb->s_umount, sb can not
+ * be removed from the list, it is
+ * now safe to drop li_list_mtx
+ */
+ mutex_unlock(&eli->li_list_mtx);
+ err = ext4_run_li_request(elr);
+ sb_end_write(elr->lr_super);
+ mutex_lock(&eli->li_list_mtx);
+ n = pos->next;
+ }
+ up_read((&elr->lr_super->s_umount));
+ }
+ /* error, remove the lazy_init job */
+ if (err) {
+ ext4_remove_li_request(elr);
+ continue;
+ }
+ if (!progress) {
+ elr->lr_next_sched = jiffies +
+ (prandom_u32()
+ % (EXT4_DEF_LI_MAX_START_DELAY * HZ));
+ }
if (time_before(elr->lr_next_sched, next_wakeup))
next_wakeup = elr->lr_next_sched;
}