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Message-ID: <234ea0c7-601c-4262-9381-a26129d6b457@bytedance.com>
Date: Wed, 27 Aug 2025 12:57:04 +0800
From: Julian Sun <sunjunchao@...edance.com>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>, linux-ext4@...r.kernel.org
Cc: tytso@....edu, jack@...e.com, yi.zhang@...wei.com
Subject: Re: [External] Re: [PATCH] jbd2: Increase IO priority of checkpoint.
在 2025/8/27 04:55, Ritesh Harjani (IBM) 写道:
> Julian Sun <sunjunchao@...edance.com> writes:
>
>> In commit 6a3afb6ac6df ("jbd2: increase the journal IO's priority"),
>> the priority of IOs initiated by jbd2 has been raised, exempting them
>> from WBT throttling.
>> Checkpoint is also a crucial operation of jbd2. While no serious issues
>> have been observed so far, it should still be reasonable to exempt
>> checkpoint from WBT throttling.
>>
>
> Interesting.. I was wondering whether we were able to observe any
> throttling for jbd2 log writes or for jbd2 checkpoint?
> Maybe It would have been nice, if we had some kind of data for this.
Good idea. But AFAICS wbt lacks of such a obversation mechanism now..>
> BTW - does it make sense for fastcommit path too maybe for non-tail
> fc write requests? I think it uses ext4_fc_submit_bh().
Yeah, I think so.
After a rough check of the code, the following code paths may result in
high latency or even task hangs:
1. fastcommit io is throttled by wbt or other block layer qos policies.
2. jbd2_fc_wait_bufs() might wait for a long time while
JBD2_FAST_COMMIT_ONGOING is set in journal->flags, and then
jbd2_journal_commit_transaction() waits for the
JBD2_FAST_COMMIT_ONGOING bit for a long time while holding the write
lock of j_state_lock.
3. start_this_handle() waits for read lock of j_state_lock which
results in high latency or task hang.
Hi, Jan, please correct me if I'm missing anything.
>
> -ritesh
>
>
>> Signed-off-by: Julian Sun <sunjunchao@...edance.com>
>> ---
>> fs/jbd2/checkpoint.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
>> index 38861ca04899..2d0719bf6d87 100644
>> --- a/fs/jbd2/checkpoint.c
>> +++ b/fs/jbd2/checkpoint.c
>> @@ -131,7 +131,7 @@ __flush_batch(journal_t *journal, int *batch_count)
>>
>> blk_start_plug(&plug);
>> for (i = 0; i < *batch_count; i++)
>> - write_dirty_buffer(journal->j_chkpt_bhs[i], REQ_SYNC);
>> + write_dirty_buffer(journal->j_chkpt_bhs[i], JBD2_JOURNAL_REQ_FLAGS);
>> blk_finish_plug(&plug);
>>
>> for (i = 0; i < *batch_count; i++) {
>> --
>> 2.20.1
Thanks,
--
Julian Sun <sunjunchao@...edance.com>
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