linux-ext4 - Re: jbd2_clear_buffer_revoked

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Date:   Thu, 11 Oct 2018 15:38:11 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Jan Kara <jack@...e.cz>, "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Andreas Dilger <adilger.kernel@...ger.ca>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: jbd2_clear_buffer_revoked_flags() takes a long time

On 11/10/18 2:12 PM, Jan Kara wrote:
> 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)?

I have been testing a patch with the unlock/lock at slightly different
positions, and it definitely helps.  The incidence of my problem drops from
nearly every writeback, to a few an hour.  I haven't had time to find out
what is causing the remaining cases yet - it may not be related to EXT4.  I
should be able to test this patch tomorrow.

> 
> 								Honza
> 
> -- Jan Kara <jack@...e.com> SUSE Labs, CR
> 
> 
> 0001-jbd2-Avoid-long-hold-times-of-j_state_lock-while-com.patch
> 
>>>From 3627b5f30996504019cd84f326402fccbb9a298b Mon Sep 17 00:00:00 2001
> From: Jan Kara <jack@...e.cz>
> 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@...el.com>
> Suggested-by: "Theodore Y. Ts'o" <tytso@....edu>
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
>  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
>