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: 17dfeb9113 ("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>
867da60d46