linux-kernel - Re: [tip:sched/core] sched/preempt: Optimize preemption operations on _

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Message-ID: <20150721110700.GN19282@twins.programming.kicks-ass.net>
Date:	Tue, 21 Jul 2015 13:07:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, fweisbec@...il.com,
	tglx@...utronix.de, hpa@...or.com, torvalds@...ux-foundation.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [tip:sched/core] sched/preempt: Optimize preemption operations
 on __schedule() callers

On Mon, Jul 20, 2015 at 06:14:27PM -0400, Sasha Levin wrote:
> On 05/19/2015 03:17 AM, tip-bot for Frederic Weisbecker wrote:
> > Commit-ID:  b30f0e3ffedfa52b1d67a302ae5860c49998e5e2

> > sched/preempt: Optimize preemption operations on __schedule() callers
> > 

> I was seeing RCU stall warnings that appeared to have different backtrace
> each time, but would reliably trigger when fuzzing and looked something like this:

> [4394886.426627] preempt_schedule_irq (./arch/x86/include/asm/paravirt.h:807 kernel/sched/core.c:3218)

The only 'obvious' difference there seems to be something like the
below. Prior to the patch preempt_schedule_irq() used
__preempt_count_{add,sub}() which are the !tracing versions.

Does this make it go again? If not, can you provide a .config?

---
 include/linux/preempt.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index 84991f185173..235a9f2d76fe 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -139,14 +139,14 @@ extern void preempt_count_sub(int val);
 
 #define preempt_active_enter() \
 do { \
-	preempt_count_add(PREEMPT_ACTIVE + PREEMPT_DISABLE_OFFSET); \
+	__preempt_count_add(PREEMPT_ACTIVE + PREEMPT_DISABLE_OFFSET); \
 	barrier(); \
 } while (0)
 
 #define preempt_active_exit() \
 do { \
 	barrier(); \
-	preempt_count_sub(PREEMPT_ACTIVE + PREEMPT_DISABLE_OFFSET); \
+	__preempt_count_sub(PREEMPT_ACTIVE + PREEMPT_DISABLE_OFFSET); \
 } while (0)
 
 #ifdef CONFIG_PREEMPT_COUNT

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