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Date: Wed, 10 Oct 2018 13:49:34 -0400
From: "Theodore Y. Ts'o" <tytso@....edu>
To: Adrian Hunter <adrian.hunter@...el.com>
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 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?
Cheers,
- Ted
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