[PATCH RT 07/10] fs/dcache: resched/chill only if we make no progress

[Steven Rostedt]

4.4.21-rt31-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Sebastian Andrzej Siewior <bige...@linutronix.de>

Upstream commit 47be61845c77 ("fs/dcache.c: avoid soft-lockup in
dput()") changed the condition _when_ cpu_relax() / cond_resched() was
invoked. This change was adapted in -RT into mostly the same thing
except that if cond_resched() did nothing we had to do cpu_chill() to
force the task off CPU for a tiny little bit in case the task had RT
priority and did not want to leave the CPU.
This change resulted in a performance regression (in my testcase the
build time on /dev/shm increased from 19min to 24min). The reason is
that with this change cpu_chill() was invoked even dput() made progress
(dentry_kill() returned a different dentry) instead only if we were
trying this operation on the same dentry over and over again.

This patch brings back to the old behavior back to cond_resched() &
chill if we make no progress. A little improvement is to invoke
cpu_chill() only if we are a RT task (and avoid the sleep otherwise).
Otherwise the scheduler should remove us from the CPU if we make no
progress.

Cc: stable...@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bige...@linutronix.de>
Signed-off-by: Steven Rostedt <rost...@goodmis.org>
---
 fs/dcache.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 76f007eb28f8..3730c7f757ff 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -40,6 +40,8 @@
 #include <linux/ratelimit.h>
 #include <linux/list_lru.h>
 #include <linux/kasan.h>
+#include <linux/sched/rt.h>
+#include <linux/sched/deadline.h>
 
 #include "internal.h"
 #include "mount.h"
@@ -748,6 +750,8 @@ static inline bool fast_dput(struct dentry *dentry)
  */
 void dput(struct dentry *dentry)
 {
+       struct dentry *parent;
+
        if (unlikely(!dentry))
                return;
 
@@ -784,14 +788,17 @@ repeat:
        return;
 
 kill_it:
-       dentry = dentry_kill(dentry);
-       if (dentry) {
+       parent = dentry_kill(dentry);
+       if (parent) {
                int r;
 
-               /* the task with the highest priority won't schedule */
-               r = cond_resched();
-               if (!r)
-                       cpu_chill();
+               if (parent == dentry) {
+                       /* the task with the highest priority won't schedule */
+                       r = cond_resched();
+                       if (!r && (rt_task(current) || dl_task(current)))
+                               cpu_chill();
+               } else
+                       dentry = parent;
                goto repeat;
        }
 }
-- 
2.8.1