From 5522a27e59c62b3fb60448eb6c3640cf307247bb Mon Sep 17 00:00:00 2001 From: Filipe Manana Date: Tue, 11 Aug 2020 12:43:37 +0100 Subject: [PATCH] btrfs: do not take the log_mutex of the subvolume when pinning the log During a rename we pin the log to make sure no one commits a log that reflects an ongoing rename operation, as it might result in a committed log where it recorded the unlink of the old name without having recorded the new name. However we are taking the subvolume's log_mutex before incrementing the log_writers counter, which is not necessary since that counter is atomic and we only remove the old name from the log and add the new name to the log after we have incremented log_writers, ensuring that no one can commit the log after we have removed the old name from the log and before we added the new name to the log. By taking the log_mutex lock we are just adding unnecessary contention on the lock, which can become visible for workloads that mix renames with fsyncs, writes for files opened with O_SYNC and unlink operations (if the inode or its parent were fsynced before in the current transaction). So just remove the lock and unlock of the subvolume's log_mutex at btrfs_pin_log_trans(). Using dbench, which mixes different types of operations that end up taking that mutex (fsyncs, renames, unlinks and writes into files opened with O_SYNC) revealed some small gains. The following script that calls dbench was used: #!/bin/bash DEV=/dev/nvme0n1 MNT=/mnt/btrfs MOUNT_OPTIONS="-o ssd -o space_cache=v2" MKFS_OPTIONS="-m single -d single" THREADS=32 echo "performance" | tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor mkfs.btrfs -f $MKFS_OPTIONS $DEV mount $MOUNT_OPTIONS $DEV $MNT dbench -s -t 600 -D $MNT $THREADS umount $MNT The test was run on bare metal, no virtualization, on a box with 12 cores (Intel i7-8700), 64Gb of RAM and using a NVMe device, with a kernel configuration that is the default of typical distributions (debian in this case), without debug options enabled (kasan, kmemleak, slub debug, debug of page allocations, lock debugging, etc). Results before this patch: Operation Count AvgLat MaxLat ---------------------------------------- NTCreateX 4410848 0.017 738.640 Close 3240222 0.001 0.834 Rename 186850 7.478 1272.476 Unlink 890875 0.128 785.018 Deltree 128 2.846 12.081 Mkdir 64 0.002 0.003 Qpathinfo 3997659 0.009 11.171 Qfileinfo 701307 0.001 0.478 Qfsinfo 733494 0.002 1.103 Sfileinfo 359362 0.004 3.266 Find 1546226 0.041 4.128 WriteX 2202803 7.905 1376.989 ReadX 6917775 0.003 3.887 LockX 14392 0.002 0.043 UnlockX 14392 0.001 0.085 Flush 309225 0.128 1033.936 Throughput 231.555 MB/sec (sync open) 32 clients 32 procs max_latency=1376.993 ms Results after this patch: Operation Count AvgLat MaxLat ---------------------------------------- NTCreateX 4603244 0.017 232.776 Close 3381299 0.001 1.041 Rename 194871 7.251 1073.165 Unlink 929730 0.133 119.233 Deltree 128 2.871 10.199 Mkdir 64 0.002 0.004 Qpathinfo 4171343 0.009 11.317 Qfileinfo 731227 0.001 1.635 Qfsinfo 765079 0.002 3.568 Sfileinfo 374881 0.004 1.220 Find 1612964 0.041 4.675 WriteX 2296720 7.569 1178.204 ReadX 7213633 0.003 3.075 LockX 14976 0.002 0.076 UnlockX 14976 0.001 0.061 Flush 322635 0.102 579.505 Throughput 241.4 MB/sec (sync open) 32 clients 32 procs max_latency=1178.207 ms (+4.3% throughput, -14.4% max latency) Reviewed-by: Josef Bacik Signed-off-by: Filipe Manana Signed-off-by: David Sterba --- fs/btrfs/tree-log.c | 2 -- 1 file changed, 2 deletions(-) diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c index a6e69378b706..4c3565c282e2 100644 --- a/fs/btrfs/tree-log.c +++ b/fs/btrfs/tree-log.c @@ -215,9 +215,7 @@ static int join_running_log_trans(struct btrfs_root *root) */ void btrfs_pin_log_trans(struct btrfs_root *root) { - mutex_lock(&root->log_mutex); atomic_inc(&root->log_writers); - mutex_unlock(&root->log_mutex); } /*