Re: [PATCH] jbd2: wake up journal waiters in FIFO order, not LIFO

Ritesh Harjani (IBM)
2022-09-08T05:46:21Z

On 22/09/07 07:59PM, Alexey Lyashkov wrote:
> From: Andrew Perepechko <anserper@ya.ru>
>
> LIFO wakeup order is unfair and sometimes leads to a journal
> user not being able to get a journal handle for hundreds of
> transactions in a row.
>
> FIFO wakeup can make things more fair.

prepare_to_wait() will always add the task to the head of the list.
While prepare_to_wait_exclusive() will add the task to the tail since all of the
exclusive tasks are added to the tail.
wake_up() function will wake up all non-exclusive and single exclusive task
v/s
wake_up_all() function will wake up all tasks irrespective.

So your change does makes the ordering to FIFO, in which the task which came in
first will be woken up first.

Although I was wondering about 2 things -
1. In what scenario this was observed to become a problem/bottleneck for you?
Could you kindly give more details of your problem?

2. What about start_this_handle() function where we call wait_event()
for j_barrier_count to be 0? I guess that doesn't happen often.

-ritesh

>
> Signed-off-by: Alexey Lyashkov <alexey.lyashkov@gmail.com>
> ---
> fs/jbd2/commit.c | 2 +-
> fs/jbd2/transaction.c | 6 +++---
> 2 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
> index b2b2bc9b88d9..ec2b55879e3a 100644
> --- a/fs/jbd2/commit.c
> +++ b/fs/jbd2/commit.c
> @@ -570,7 +570,7 @@ void jbd2_journal_commit_transaction(journal_t *journal)
> journal->j_running_transaction = NULL;
> start_time = ktime_get();
> commit_transaction->t_log_start = journal->j_head;
> - wake_up(&journal->j_wait_transaction_locked);
> + wake_up_all(&journal->j_wait_transaction_locked);
> write_unlock(&journal->j_state_lock);
>
> jbd2_debug(3, "JBD2: commit phase 2a\n");
> diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> index e1be93ccd81c..6a404ac1c178 100644
> --- a/fs/jbd2/transaction.c
> +++ b/fs/jbd2/transaction.c
> @@ -168,7 +168,7 @@ static void wait_transaction_locked(journal_t *journal)
> int need_to_start;
> tid_t tid = journal->j_running_transaction->t_tid;
>
> - prepare_to_wait(&journal->j_wait_transaction_locked, &wait,
> + prepare_to_wait_exclusive(&journal->j_wait_transaction_locked, &wait,
> TASK_UNINTERRUPTIBLE);
> need_to_start = !tid_geq(journal->j_commit_request, tid);
> read_unlock(&journal->j_state_lock);
> @@ -194,7 +194,7 @@ static void wait_transaction_switching(journal_t *journal)
> read_unlock(&journal->j_state_lock);
> return;
> }
> - prepare_to_wait(&journal->j_wait_transaction_locked, &wait,
> + prepare_to_wait_exclusive(&journal->j_wait_transaction_locked, &wait,
> TASK_UNINTERRUPTIBLE);
> read_unlock(&journal->j_state_lock);
> /*
> @@ -920,7 +920,7 @@ void jbd2_journal_unlock_updates (journal_t *journal)
> write_lock(&journal->j_state_lock);
> --journal->j_barrier_count;
> write_unlock(&journal->j_state_lock);
> - wake_up(&journal->j_wait_transaction_locked);
> + wake_up_all(&journal->j_wait_transaction_locked);
> }
>
> static void warn_dirty_buffer(struct buffer_head *bh)
> --
> 2.31.1
>