NFSD: Remove be32_to_cpu() from DRC hash function

Commit 7142b98d9fd7 ("nfsd: Clean up drc cache in preparation for
global spinlock elimination"), billed as a clean-up, added
be32_to_cpu() to the DRC hash function without explanation. That
commit removed two comments that state that byte-swapping in the
hash function is unnecessary without explaining whether there was
a need for that change.

On some Intel CPUs, the swab32 instruction is known to cause a CPU
pipeline stall. be32_to_cpu() does not add extra randomness, since
the hash multiplication is done /before/ shifting to the high-order
bits of the result.

As a micro-optimization, remove the unnecessary transform from the
DRC hash function.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

diff --git a/fs/nfsd/nfscache.c b/fs/nfsd/nfscache.c
index 6e0b6f3148dca4ad459425c03723182e52409b6f..a4a69ab6ab2806298f6041c4a0adc120e9d9c2b4 100644 (file)
--- a/fs/nfsd/nfscache.c
+++ b/fs/nfsd/nfscache.c
@@ -87,7 +87,7 @@ nfsd_hashsize(unsigned int limit)
 static u32
 nfsd_cache_hash(__be32 xid, struct nfsd_net *nn)
 {
-       return hash_32(be32_to_cpu(xid), nn->maskbits);
+       return hash_32((__force u32)xid, nn->maskbits);
 }
 
 static struct svc_cacherep *