NFSD: Remove be32_to_cpu() from DRC hash function

chucklever · chucklever · commit 7578b2f628db · 2021-12-13T13:42:59.000-05:00
Commit 7142b98 ("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
@@ -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 *

Original file line number	Diff line number	Diff line change
`@@ -87,7 +87,7 @@ nfsd_hashsize(unsigned int limit)`
`87`	`87`	`static u32`
`88`	`88`	`nfsd_cache_hash(__be32 xid, struct nfsd_net *nn)`
`89`	`89`	`{`
`90`		`- return hash_32(be32_to_cpu(xid), nn->maskbits);`
	`90`	`+ return hash_32((__force u32)xid, nn->maskbits);`
`91`	`91`	`}`
`92`	`92`
`93`	`93`	`static struct svc_cacherep *`