On Thu, May 23, 2019 at 12:53 PM Nikolay Borisov <nborisov@xxxxxxxx> 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@xxxxxxxx>
Reviewed-by: Filipe Manana <fdmanana@xxxxxxxx>
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.”
