Re: [PATCH 4/4] btrfs: Be smarter when sleeping in transaction_kthread

[David Sterba <dsterba@suse.cz>]

On Thu, Oct 08, 2020 at 03:24:30PM +0300, Nikolay Borisov wrote:
> If transaction_kthread is woken up before
> btrfs_fs_info::commit_interval seconds have elapsed it will sleep for a
> fixed period of 5 seconds. This is not a problem per-se but is not
> accuaret, instead the code should sleep for an interval which guarantees
> on next wakeup commit_interval would have passed. Since time tracking is
> not accurate add 1 second to ensure next wake up would be after
> commit_interval.
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
>  fs/btrfs/disk-io.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index c5d3e7f75066..a1fe99cf0831 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -1735,7 +1735,7 @@ static int transaction_kthread(void *arg)
>  		if (cur->state < TRANS_STATE_COMMIT_START &&
>  		    delta < fs_info->commit_interval) {
>  			spin_unlock(&fs_info->trans_lock);
> -			delay = msecs_to_jiffies(5000);
> +			delay = msecs_to_jiffies((1+delta) * 1000);

This does not seem right. Delta is number of seconds since the
transaction started, so we need to sleep for the remaining time. Which
is commit_interval - delta.