On Wed 10-10-18 13:49:34, Theodore Y. Ts'o wrote:
> On Wed, Oct 10, 2018 at 04:43:27PM +0300, Adrian Hunter wrote:
> > Hi
> >
> > I have a case on a v4.14 kernel where the EXT4 journal commit disables
> > preemption for 30ms due to jbd2_clear_buffer_revoked_flags(). That in turn
> > disables preemption on other CPUs as they come to spin waiting for the same
> > lock. The side-effect of that is that it periodically blocks high priority
> > tasks from running.
> >
> > I see jbd2_clear_buffer_revoked_flags() iterating 32768 times calling
> > __find_get_block().
> >
> > Is there any way to make jbd2_clear_buffer_revoked_flags() take less time,
> > or move its work out from under write_lock(&journal->j_state_lock)?
>
> Hmm.... I'd have to look a bit more carefully and then run some tests,
> but I *think* we can drop the j_state_lock at the beginning of JBD2
> commit phase 1, and then grab it again right before we set
> commit_transaction->t_state to T_FLUSH.
>
> That should be safe because while the transaction state is T_LOCKED,
> we can't start any new handles, so there can't be any new blocks added
> to the revoke table.
>
> Can you give that a try and see whether that solves your priority
> inversion problem?
Agreed. Something like attached patch (compile-tested only)?
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
>From 3627b5f30996504019cd84f326402fccbb9a298b Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@xxxxxxx>
Date: Thu, 11 Oct 2018 13:04:44 +0200
Subject: [PATCH] jbd2: Avoid long hold times of j_state_lock while committing
a transaction
We can hold j_state_lock for writing at the beginning of
jbd2_journal_commit_transaction() for a rather long time (reportedly for
30 ms) due cleaning revoke bits of all revoked buffers under it. The
handling of revoke tables as well as cleaning of t_reserved_list, and
checkpoint lists does not need j_state_lock for anything. Furthermore
the transaction is in T_LOCKED state and we waited for all outstanding
handles so nobody is going to be adding anything to the transaction.
Just drop the lock for unnecessary operations.
Reported-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
Suggested-by: "Theodore Y. Ts'o" <tytso@xxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
fs/jbd2/commit.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
index 150cc030b4d7..356b75fa3101 100644
--- a/fs/jbd2/commit.c
+++ b/fs/jbd2/commit.c
@@ -422,6 +422,7 @@ void jbd2_journal_commit_transaction(journal_t *journal)
stats.run.rs_locked);
stats.run.rs_running = jbd2_time_diff(commit_transaction->t_start,
stats.run.rs_locked);
+ write_unlock(&journal->j_state_lock);
spin_lock(&commit_transaction->t_handle_lock);
while (atomic_read(&commit_transaction->t_updates)) {
@@ -431,9 +432,7 @@ void jbd2_journal_commit_transaction(journal_t *journal)
TASK_UNINTERRUPTIBLE);
if (atomic_read(&commit_transaction->t_updates)) {
spin_unlock(&commit_transaction->t_handle_lock);
- write_unlock(&journal->j_state_lock);
schedule();
- write_lock(&journal->j_state_lock);
spin_lock(&commit_transaction->t_handle_lock);
}
finish_wait(&journal->j_wait_updates, &wait);
@@ -505,6 +504,7 @@ void jbd2_journal_commit_transaction(journal_t *journal)
atomic_sub(atomic_read(&journal->j_reserved_credits),
&commit_transaction->t_outstanding_credits);
+ write_lock(&journal->j_state_lock);
trace_jbd2_commit_flushing(journal, commit_transaction);
stats.run.rs_flushing = jiffies;
stats.run.rs_locked = jbd2_time_diff(stats.run.rs_locked,
--
2.16.4
