Commit 867da60d authored by Mike Snitzer's avatar Mike Snitzer Committed by Anna Schumaker
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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: 17dfeb91 ("NFS: Fix races in nfs_revalidate_mapping")
Signed-off-by: default avatarMike Snitzer <snitzer@kernel.org>
Reviewed-by: default avatarJeff Layton <jlayton@kernel.org>
Signed-off-by: default avatarAnna Schumaker <anna.schumaker@oracle.com>
parent bc294086

fs/nfs/inode.c

+15 −5
Original line number Diff line number Diff line
@@ -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);