Re: Linux 2.6.26-rc2

[Linus Torvalds]

On Mon, 12 May 2008, Bart Van Assche wrote:
> 
> Sorry if it's my fault that I do not understand the above message
> completely. But from the above it's not completely clear to me which
> kernel versions (2.6.2?.? releases) are affected and which are not
> affected by the performance and correctness issues due to the
> interaction between the semaphore implementation and the preemptable
> BKL.

No released kernels are affected. It's purely a matter that has happened 
after 2.6.25. The semaphore simplifcation in -rc1 caused a huge 
performance regression on some benchmarks, and the fix to that in turn 
caused a semaphore correctness issue, so I just rolled back to the 
original BKL code that doesn't have any of those interactions.

In a historical context, the issues involved would only have happened with 
CONFIG_PREEMPT_BKL. That config option was made the only one in January, 
and as a result of these issues, we effectively switched it off.

So you can *think* of the effect of the changes as having gone from 
CONFIG_PREEMPT_BKL=y to CONFIG_PREEMPT_BKL=n, even though technically we 
had removed the actual config option to let people choose (so the config 
option has basically become a static code change).

We may end up having to re-instate the config option due to this. 
Personally, I hope not. It would be nicer if we could just avoid 
PREEMPT_BKL entirely. 

(To make things somewhat more confusing, some non-PREEMPT_BKL code has 
then bitrotted since, so if can actually see latency issues, you might 
want to try the patch here at the end of this email to see if it fixes 
the worst of them. "cond_resched()" has regressed since the PREEMPT_BKL 
config option went away).

			Linus
---
 include/linux/sched.h |    7 -------
 1 files changed, 0 insertions(+), 7 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 5a63f2d..75c284f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2038,17 +2038,10 @@ static inline int need_resched(void)
  * cond_resched_softirq() will enable bhs before scheduling.
  */
 extern int _cond_resched(void);
-#ifdef CONFIG_PREEMPT
-static inline int cond_resched(void)
-{
-	return 0;
-}
-#else
 static inline int cond_resched(void)
 {
 	return _cond_resched();
 }
-#endif
 extern int cond_resched_lock(spinlock_t * lock);
 extern int cond_resched_softirq(void);
 static inline int cond_resched_bkl(void)
--
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