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Date: Tue, 13 May 2008 01:22:27 +0200 From: Kasper Sandberg <lkml@...anurb.dk> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Bart Van Assche <bart.vanassche@...il.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: Linux 2.6.26-rc2 On Mon, 2008-05-12 at 12:55 -0700, Linus Torvalds wrote: > > 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). uhm.. but .25 doesent have PREEMPT_BKL either.. does that mean its on or off? > > 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. you mean avoid preempting the bkl, or avoid having the option to choose to preempt it, and just always do it? > > (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) > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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