NFS: Don't run wake_up_bit() when nobody is waiting...

"perf lock" shows fairly heavy contention for the bit waitqueue locks
when doing an I/O heavy workload.
Use a bit to tell whether or not there has been contention for a lock
so that we can optimise away the bit waitqueue options in those cases.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

2 files changed, 18 insertions, 1 deletions

diff --git a/fs/nfs/pagelist.c b/fs/nfs/pagelist.c
index 8a23e2b40b04..de9066a92c0d 100644
--- a/fs/nfs/pagelist.c
+++ b/fs/nfs/pagelist.c
@@ -155,9 +155,12 @@ nfs_page_group_lock(struct nfs_page *req, bool nonblock)
         if (!test_and_set_bit(PG_HEADLOCK, &head->wb_flags))
                 return 0;
 
-        if (!nonblock)
+        if (!nonblock) {
+                set_bit(PG_CONTENDED1, &head->wb_flags);
+                smp_mb__after_atomic();
                 return wait_on_bit_lock(&head->wb_flags, PG_HEADLOCK,
                                 TASK_UNINTERRUPTIBLE);
+        }
 
         return -EAGAIN;
 }
@@ -175,6 +178,10 @@ nfs_page_group_lock_wait(struct nfs_page *req)
 
         WARN_ON_ONCE(head != head->wb_head);
 
+        if (!test_bit(PG_HEADLOCK, &head->wb_flags))
+                return;
+        set_bit(PG_CONTENDED1, &head->wb_flags);
+        smp_mb__after_atomic();
         wait_on_bit(&head->wb_flags, PG_HEADLOCK,
                 TASK_UNINTERRUPTIBLE);
 }
@@ -193,6 +200,8 @@ nfs_page_group_unlock(struct nfs_page *req)
         smp_mb__before_atomic();
         clear_bit(PG_HEADLOCK, &head->wb_flags);
         smp_mb__after_atomic();
+        if (!test_bit(PG_CONTENDED1, &head->wb_flags))
+                return;
         wake_up_bit(&head->wb_flags, PG_HEADLOCK);
 }
 
@@ -383,6 +392,8 @@ void nfs_unlock_request(struct nfs_page *req)
         smp_mb__before_atomic();
         clear_bit(PG_BUSY, &req->wb_flags);
         smp_mb__after_atomic();
+        if (!test_bit(PG_CONTENDED2, &req->wb_flags))
+                return;
         wake_up_bit(&req->wb_flags, PG_BUSY);
 }
 
@@ -465,6 +476,10 @@ void nfs_release_request(struct nfs_page *req)
 int
 nfs_wait_on_request(struct nfs_page *req)
 {
+        if (!test_bit(PG_BUSY, &req->wb_flags))
+                return 0;
+        set_bit(PG_CONTENDED2, &req->wb_flags);
+        smp_mb__after_atomic();
         return wait_on_bit_io(&req->wb_flags, PG_BUSY,
                               TASK_UNINTERRUPTIBLE);
 }
diff --git a/include/linux/nfs_page.h b/include/linux/nfs_page.h
index abbee2d15dce..d67b67ae6c8b 100644
--- a/include/linux/nfs_page.h
+++ b/include/linux/nfs_page.h
@@ -33,6 +33,8 @@ enum {
         PG_UPTODATE,            /* page group sync bit in read path */
         PG_WB_END,              /* page group sync bit in write path */
         PG_REMOVE,              /* page group sync bit in write path */
+        PG_CONTENDED1,          /* Is someone waiting for a lock? */
+        PG_CONTENDED2,          /* Is someone waiting for a lock? */
 };
 
 struct nfs_inode;