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

Steven Rostedt
2016-09-22T23:19:52Z

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3.18.42-rt45-rc1 stable review patch.
If anyone has any objections, please let me know.

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

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

diff --git a/fs/dcache.c b/fs/dcache.c
index 1cb13a024cf8..44a3419c7125 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -39,6 +39,8 @@
#include <linux/prefetch.h>
#include <linux/ratelimit.h>
#include <linux/list_lru.h>
+#include <linux/sched/rt.h>
+#include <linux/sched/deadline.h>
#include "internal.h"
#include "mount.h"

@@ -722,6 +724,8 @@ static inline bool fast_dput(struct dentry *dentry)
*/
void dput(struct dentry *dentry)
{
+ struct dentry *parent;
+
if (unlikely(!dentry))
return;

@@ -758,14 +762,17 @@ 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