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From: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
To: Zhang Yi <yi.zhang@huaweicloud.com>, linux-ext4@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, tytso@mit.edu, adilger.kernel@dilger.ca, jack@suse.cz, yi.zhang@huawei.com, yi.zhang@huaweicloud.com, chengzhihao1@huawei.com, yukuai3@huawei.com
Subject: Re: [PATCH v2] jbd2: speed up jbd2_transaction_committed()
In-Reply-To: <20240520131831.2910790-1-yi.zhang@huaweicloud.com>
Date: Thu, 23 May 2024 18:00:53 +0530
Message-ID: <87ttiosoaq.fsf@gmail.com>
References: <20240520131831.2910790-1-yi.zhang@huaweicloud.com>
Precedence: bulk

Zhang Yi <yi.zhang@huaweicloud.com> writes:

> From: Zhang Yi <yi.zhang@huawei.com>
>
> jbd2_transaction_committed() is used to check whether a transaction with
> the given tid has already committed, it holds j_state_lock in read mode
> and check the tid of current running transaction and committing
> transaction, but holding the j_state_lock is expensive.
>
> We have already stored the sequence number of the most recently
> committed transaction in journal t->j_commit_sequence, we could do this
> check by comparing it with the given tid instead. If the given tid isn't
> smaller than j_commit_sequence, we can ensure that the given transaction
> has been committed. That way we could drop the expensive lock and
> achieve about 10% ~ 20% performance gains in concurrent DIOs on may
> virtual machine with 100G ramdisk.
>
> fio -filename=/mnt/foo -direct=1 -iodepth=10 -rw=$rw -ioengine=libaio \
>     -bs=4k -size=10G -numjobs=10 -runtime=60 -overwrite=1 -name=test \
>     -group_reporting
>
> Before:
>   overwrite       IOPS=88.2k, BW=344MiB/s
>   read            IOPS=95.7k, BW=374MiB/s
>   rand overwrite  IOPS=98.7k, BW=386MiB/s
>   randread        IOPS=102k, BW=397MiB/s
>
> After:
>   overwrite       IOPS=105k, BW=410MiB/s
>   read            IOPS=112k, BW=436MiB/s
>   rand overwrite  IOPS=104k, BW=404MiB/s
>   randread        IOPS=111k, BW=432MiB/s

I was surprised to see that even the read and randread performance
is improved with this patch which should theoritically only impact write
workloads given based on such checks we are just setting IOMAP_F_DIRTY
flag to report to iomap. 

But then I came across these two patches [1] [2]. It seems the change
[1] to set IOMAP_F_DIRTY was initially only done for IOMAP_WRITE path.
But patch [2] moved some logic to fs-dax core and filesystems were left
to always just reports if there is any dirty metadata, irrespective of
reads or writes.

[1]: https://lore.kernel.org/all/20171101153648.30166-17-jack@suse.cz/
[2]: https://lore.kernel.org/all/151062258598.8554.8157038002895095232.stgit@dwillia2-desk3.amr.corp.intel.com/


Ohh - could this patch be that reason of peformance regression when ext4
DIO moved to iomap? Should we CC: stable to when ext4 DIO was moved to
iomap atleast - which I believe was v5.5?

Looks good to me.
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

>
> CC: Dave Chinner <david@fromorbit.com>
> Suggested-by: Dave Chinner <david@fromorbit.com>
> Link: https://lore.kernel.org/linux-ext4/ZjILCPNZRHeazSqV@dread.disaster.area/

aah. This link is helpful too to understand the context. Thanks!

> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
> ---
> v1->v2:
>  - Add READ_ONCE and WRITE_ONCE to access ->j_commit_sequence
>    concurrently.
>  - Keep the jbd2_transaction_committed() helper.
>
>  fs/jbd2/commit.c  |  2 +-
>  fs/jbd2/journal.c | 12 +-----------
>  2 files changed, 2 insertions(+), 12 deletions(-)
>
> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
> index 5e122586e06e..8244cab17688 100644
> --- a/fs/jbd2/commit.c
> +++ b/fs/jbd2/commit.c
> @@ -1108,7 +1108,7 @@ void jbd2_journal_commit_transaction(journal_t *journal)
>  
>  	commit_transaction->t_state = T_COMMIT_CALLBACK;
>  	J_ASSERT(commit_transaction == journal->j_committing_transaction);
> -	journal->j_commit_sequence = commit_transaction->t_tid;
> +	WRITE_ONCE(journal->j_commit_sequence, commit_transaction->t_tid);
>  	journal->j_committing_transaction = NULL;
>  	commit_time = ktime_to_ns(ktime_sub(ktime_get(), start_time));
>  
> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index b6c114c11b97..cc586e3c4ee1 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -789,17 +789,7 @@ EXPORT_SYMBOL(jbd2_fc_end_commit_fallback);
>  /* Return 1 when transaction with given tid has already committed. */
>  int jbd2_transaction_committed(journal_t *journal, tid_t tid)
>  {
> -	int ret = 1;
> -
> -	read_lock(&journal->j_state_lock);
> -	if (journal->j_running_transaction &&
> -	    journal->j_running_transaction->t_tid == tid)
> -		ret = 0;
> -	if (journal->j_committing_transaction &&
> -	    journal->j_committing_transaction->t_tid == tid)
> -		ret = 0;
> -	read_unlock(&journal->j_state_lock);
> -	return ret;
> +	return tid_geq(READ_ONCE(journal->j_commit_sequence), tid);
>  }
>  EXPORT_SYMBOL(jbd2_transaction_committed);
>  
> -- 
> 2.39.2