[PATCH v4] proc: Allow pid_revalidate() during LOOKUP_RCU

[Stephen Brennan]

The pid_revalidate() function drops from RCU into REF lookup mode. When
many threads are resolving paths within /proc in parallel, this can
result in heavy spinlock contention on d_lockref as each thread tries to
grab a reference to the /proc dentry (and drop it shortly thereafter).

Investigation indicates that it is not necessary to drop RCU in
pid_revalidate(), as no RCU data is modified and the function never
sleeps. So, remove the LOOKUP_RCU check.

Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>
---
When running running ~100 parallel instances of "TZ=/etc/localtime ps -fe
>/dev/null" on a 100CPU machine, the %sys utilization reaches 90%, and perf
shows the following code path as being responsible for heavy contention on
the d_lockref spinlock:

      walk_component()
        lookup_fast()
          d_revalidate()
            pid_revalidate() // returns -ECHILD
          unlazy_child()
            lockref_get_not_dead(&nd->path.dentry->d_lockref) <-- contention

By applying this patch, %sys utilization falls to around 60% under the same
workload. Although this particular workload is a bit contrived, we have seen
some monitoring scripts which produced similarly high %sys time due to this
contention.

Changes in v4:
- Simplify by unconditionally calling pid_update_inode() from pid_revalidate,
  and removing the LOOKUP_RCU check.
Changes in v3:
- Rather than call pid_update_inode() with flags, create
  proc_inode_needs_update() to determine whether the call can be skipped.
- Restore the call to the security hook (see next patch).
Changes in v2:
- Remove get_pid_task_rcu_user() and get_proc_task_rcu(), since they were
  unnecessary.
- Remove the call to security_task_to_inode().

 fs/proc/base.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index f52217f432bc..633ef74e8dfd 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1974,19 +1974,18 @@ static int pid_revalidate(struct dentry *dentry, unsigned int flags)
 {
 	struct inode *inode;
 	struct task_struct *task;
+	int ret = 0;
 
-	if (flags & LOOKUP_RCU)
-		return -ECHILD;
-
-	inode = d_inode(dentry);
-	task = get_proc_task(inode);
+	rcu_read_lock();
+	inode = d_inode_rcu(dentry);
+	task = pid_task(proc_pid(inode), PIDTYPE_PID);
 
 	if (task) {
 		pid_update_inode(task, inode);
-		put_task_struct(task);
-		return 1;
+		ret = 1;
 	}
-	return 0;
+	rcu_read_unlock();
+	return ret;
 }
 
 static inline bool proc_inode_is_dead(struct inode *inode)
-- 
2.25.1