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Date:   Thu, 8 Sep 2022 11:16:11 +0530
From:   "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
To:     Alexey Lyashkov <alexey.lyashkov@gmail.com>
Cc:     linux-ext4@vger.kernel.org, Theodore Ts'o <tytso@mit.edu>,
        Andreas Dilger <adilger@dilger.ca>,
        Artem Blagodarenko <artem.blagodarenko@gmail.com>,
        Andrew Perepechko <anserper@ya.ru>
Subject: Re: [PATCH] jbd2: wake up journal waiters in FIFO order, not  LIFO
Message-ID: <20220908054611.vjcb27wmq4dggqmv@riteshh-domain>
References: <20220907165959.1137482-1-alexey.lyashkov@gmail.com>
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In-Reply-To: <20220907165959.1137482-1-alexey.lyashkov@gmail.com>
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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
>