Re: [PATCH v2] futex: lower the lock contention on the HB lock during wake up

Re: [PATCH v2] futex: lower the lock contention on the HB lock during wake up

[Kevin Hilman]

On Wed, Jun 17, 2015 at 1:33 AM, Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
> wake_futex_pi() wakes the task before releasing the hash bucket lock
> (HB). The first thing the woken up task usually does is to acquire the
> lock which requires the HB lock. On SMP Systems this leads to blocking
> on the HB lock which is released by the owner shortly after.
> This patch rearranges the unlock path by first releasing the HB lock and
> then waking up the task.
>
> [bigeasy: redo ontop of lockless wake-queues]
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
> * Davidlohr Bueso | 2015-06-16 12:50:26 [-0700]:
>
>>I prefer having two separate patches, thus keeping their own changelog
>>for the change justification.
>
> okay, here it is on top of #1.

A handful of boot test failures on ARM/OMAP were found by kernelci.org
in next-20150619[1] and were bisected down to this patch, which hit
next-20150619 in the form of commit 881bd58d6e9e (futex: Lower the
lock contention on the HB lock during wake up).  I confirmed that
reverting that patch on top of next-20150619 gets things booting again
for the affected platforms.

I haven't debugged this any further, but full boot logs are available
for the boot failures[2][3] and the linux-omap list and maintainer are
Cc'd here to help investigate further if needed.

Kevin

[1] http://kernelci.org/boot/all/job/next/kernel/next-20150619/
[2] http://storage.kernelci.org/next/next-20150619/arm-multi_v7_defconfig/lab-khilman/boot-omap5-uevm.html
[3] http://storage.kernelci.org/next/next-20150619/arm-omap2plus_defconfig/lab-tbaker/boot-omap3-beagle-xm.html

Re: [PATCH v2] futex: lower the lock contention on the HB lock during wake up

[Thomas Gleixner]

On Fri, 19 Jun 2015, Kevin Hilman wrote:
> On Wed, Jun 17, 2015 at 1:33 AM, Sebastian Andrzej Siewior
> A handful of boot test failures on ARM/OMAP were found by kernelci.org
> in next-20150619[1] and were bisected down to this patch, which hit
> next-20150619 in the form of commit 881bd58d6e9e (futex: Lower the
> lock contention on the HB lock during wake up).  I confirmed that
> reverting that patch on top of next-20150619 gets things booting again
> for the affected platforms.
> 
> I haven't debugged this any further, but full boot logs are available
> for the boot failures[2][3] and the linux-omap list and maintainer are
> Cc'd here to help investigate further if needed.

Found it. Dunno, how I missed that one. Fix below.

Thanks,

	tglx
---

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 10dbeb6fe96f..5674b073473c 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1365,9 +1365,14 @@ rt_mutex_fastunlock(struct rt_mutex *lock,
 	if (likely(rt_mutex_cmpxchg(lock, current, NULL))) {
 		rt_mutex_deadlock_account_unlock(current);
 
-	} else if (slowfn(lock, &wake_q)) {
+	} else {
+		bool deboost = slowfn(lock, &wake_q);
+
+		wake_up_q(&wake_q);
+
 		/* Undo pi boosting if necessary: */
-		rt_mutex_adjust_prio(current);
+		if (deboost)
+			rt_mutex_adjust_prio(current);
 	}
 }
 

Re: [PATCH v2] futex: lower the lock contention on the HB lock during wake up

[Kevin Hilman]

Thomas Gleixner <tglx@linutronix.de> writes:

> On Fri, 19 Jun 2015, Kevin Hilman wrote:
>> On Wed, Jun 17, 2015 at 1:33 AM, Sebastian Andrzej Siewior
>> A handful of boot test failures on ARM/OMAP were found by kernelci.org
>> in next-20150619[1] and were bisected down to this patch, which hit
>> next-20150619 in the form of commit 881bd58d6e9e (futex: Lower the
>> lock contention on the HB lock during wake up).  I confirmed that
>> reverting that patch on top of next-20150619 gets things booting again
>> for the affected platforms.
>> 
>> I haven't debugged this any further, but full boot logs are available
>> for the boot failures[2][3] and the linux-omap list and maintainer are
>> Cc'd here to help investigate further if needed.
>
> Found it. Dunno, how I missed that one. Fix below.
>

Yup, that fix on top of next-20150619 gets the two OMAP platforms
booting again.

Tested-by: Kevin Hilman <khilman@linaro.org>

Kevin