linux-ext4 - jbd2_clear_buffer_revoked_flags() takes a long time

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Date:   Wed, 10 Oct 2018 16:43:27 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Andreas Dilger <adilger.kernel@...ger.ca>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        Theodore Ts'o <tytso@....edu>
Subject: jbd2_clear_buffer_revoked_flags() takes a long time

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)?

Or do you have any other suggestions?

Regards
Adrian