nfs: avoid i_lock contention in nfs_clear_invalid_mapping

Multi-threaded buffered reads to the same file exposed significant
inode spinlock contention in nfs_clear_invalid_mapping().

Eliminate this spinlock contention by checking flags without locking,
instead using smp_rmb and smp_load_acquire accordingly, but then take
spinlock and double-check these inode flags.

Also refactor nfs_set_cache_invalid() slightly to use
smp_store_release() to pair with nfs_clear_invalid_mapping()'s
smp_load_acquire().

While this fix is beneficial for all multi-threaded buffered reads
issued by an NFS client, this issue was identified in the context of
surprisingly low LOCALIO performance with 4K multi-threaded buffered
read IO.  This fix dramatically speeds up LOCALIO performance:

before: read: IOPS=1583k, BW=6182MiB/s (6482MB/s)(121GiB/20002msec)
after:  read: IOPS=3046k, BW=11.6GiB/s (12.5GB/s)(232GiB/20001msec)

Fixes: 17dfeb911339 ("NFS: Fix races in nfs_revalidate_mapping")
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
index 9e884e1ce5d8ef5180ec81680da3cd36830faf29..596f3517013728a3db818798f3f313b8cc842ebb 100644 (file)
--- a/fs/nfs/inode.c
+++ b/fs/nfs/inode.c
@@ -205,12 +205,15 @@ void nfs_set_cache_invalid(struct inode *inode, unsigned long flags)
                nfs_fscache_invalidate(inode, 0);
        flags &= ~NFS_INO_REVAL_FORCED;
 
-       nfsi->cache_validity |= flags;
+       flags |= nfsi->cache_validity;
+       if (inode->i_mapping->nrpages == 0)
+               flags &= ~NFS_INO_INVALID_DATA;
 
-       if (inode->i_mapping->nrpages == 0) {
-               nfsi->cache_validity &= ~NFS_INO_INVALID_DATA;
-               nfs_ooo_clear(nfsi);
-       } else if (nfsi->cache_validity & NFS_INO_INVALID_DATA) {
+       /* pairs with nfs_clear_invalid_mapping()'s smp_load_acquire() */
+       smp_store_release(&nfsi->cache_validity, flags);
+
+       if (inode->i_mapping->nrpages == 0 ||
+           nfsi->cache_validity & NFS_INO_INVALID_DATA) {
                nfs_ooo_clear(nfsi);
        }
        trace_nfs_set_cache_invalid(inode, 0);
@@ -1421,6 +1424,13 @@ int nfs_clear_invalid_mapping(struct address_space *mapping)
                                         TASK_KILLABLE|TASK_FREEZABLE_UNSAFE);
                if (ret)
                        goto out;
+               smp_rmb(); /* pairs with smp_wmb() below */
+               if (test_bit(NFS_INO_INVALIDATING, bitlock))
+                       continue;
+               /* pairs with nfs_set_cache_invalid()'s smp_store_release() */
+               if (!(smp_load_acquire(&nfsi->cache_validity) & NFS_INO_INVALID_DATA))
+                       goto out;
+               /* Slow-path that double-checks with spinlock held */
                spin_lock(&inode->i_lock);
                if (test_bit(NFS_INO_INVALIDATING, bitlock)) {
                        spin_unlock(&inode->i_lock);