Re: [PATCH] btrfs: Remove unnecessary check from join_running_log_trans

[Filipe Manana <fdmanana@gmail.com>]

On Thu, May 23, 2019 at 12:53 PM Nikolay Borisov <nborisov@suse.com> wrote:
>
> join_running_log_trans checks btrfs_root::log_root outside of
> btrfs_root::log_mutex to avoid contention on the mutex. Turns out this
> check is not necessary because the two callers of join_running_log_trans
> (both of which deal with removing entries from the tree-log during
> unlink) explicitly check whether the respective inode has been logged in
> the current transaction. If it hasn't then it won't have any items in
> the tree-log and call path will return before calling
> join_running_log_trans. If the check passes, however, then it's
> guaranteed that btrfs_root::log_root is set because the inode is logged.
>
> Those guarantees allows us to remove the speculative as well as the
> implicity and tricky memory barrier. No functionl changes.
>
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>

Reviewed-by: Filipe Manana <fdmanana@suse.com>

Looks good, thanks.

> ---
>
> I have evaluated if there is any performance impact (there isn't one). Here's
> the script used:
>
> for i in {1..10}; do
>         echo "Testun run : $i"
>         time ./ltp/fsstress -d /media/scratch/ -p5 -n 100000 -z -fcreat=100 -f write=100 -f fsync=70 -f unlink=80
>         rm -rf /media/scratch/*
>         echo "Executions of join_running_trans : $(trace-cmd show | wc -l)"
>         trace-cmd clear
> done
>
> And the result :
>
>                         Unpatched (Sys)  Unpatched (Real)  Unpatched (JRT exec)    Patched (Sys)  Patched(Real)  Patched (JRT exec)
>                         161              387               153215                  183                   393             149153
>                         165              392               158490                  159                   404             158118
>                         140              381               147707                  145                   373             145676
>                         143              394               147129                  148                   383             131029
>                         206              410               157987                  152                   383             136134
>                         152              376               157771                  143                   387             131048
>                         140              371               153929                  146                   376             149885
>                         149              376               152723                  207                   407             147477
>                         164              396               157385                  160                   393             155272
>                         146              373               147937                  148                   384             152828
>
> stddev     19.75             12.44             4525                    20.5                  11.06           9722
> mean       156.6             385.6             153427.3                159.1                 388.3           145662
> median     150.5             384               153572                  150                   385.5           148315
>
>
> JRT exec means executions of join_running_transaction during that iteration of
> the test case.
>
>  fs/btrfs/tree-log.c | 4 ----
>  1 file changed, 4 deletions(-)
>
> diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
> index 6c47f6ed3e94..6c8aff105b0c 100644
> --- a/fs/btrfs/tree-log.c
> +++ b/fs/btrfs/tree-log.c
> @@ -188,10 +188,6 @@ static int join_running_log_trans(struct btrfs_root *root)
>  {
>         int ret = -ENOENT;
>
> -       smp_mb();
> -       if (!root->log_root)
> -               return -ENOENT;
> -
>         mutex_lock(&root->log_mutex);
>         if (root->log_root) {
>                 ret = 0;
> --
> 2.17.1
>


-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”