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Commit e22834f0 authored by Dmitry Monakhov's avatar Dmitry Monakhov Committed by Theodore Ts'o
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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: default avatarDmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent 6ae4c5a6

fs/ext4/super.c

+31 −10
Original line number Diff line number Diff line
@@ -2749,7 +2749,6 @@ static int ext4_run_li_request(struct ext4_li_request *elr) 
	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 @@ static int ext4_run_li_request(struct ext4_li_request *elr) 
		elr->lr_next_sched = jiffies + elr->lr_timeout;

		elr->lr_next_group = group + 1;

	}

	sb_end_write(sb);



	return ret;

}
@@ -2842,19 +2839,43 @@ static int ext4_lazyinit_thread(void *arg) 
			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) {

			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;

		}