On Thu, 2009-11-19 at 15:00 +0100, Jean Delvare wrote: > > cond_resched(); > > Are you saying that most calls to yield() should be replaced with calls > to cond_resched()? No, depends on the reason yield() is used. Some cases can be replaced by locking constructs, such as a condition variable. > I admit I a little skeptical. While the description of cond_resched() > ("latency reduction via explicit rescheduling in places that are safe") > sounds promising, following the calls leads me to: > > static inline int need_resched(void) > { > return unlikely(test_thread_flag(TIF_NEED_RESCHED)); > } > > So apparently the condition for need_resched() to do anything is > considered unlikely... suggesting that cond_resched() is a no-op in > most cases? I don't quite get the point of moving away from sched() > because it is a no-op, if we end up with a no-op under a different name. TIF_NEED_RESCHED gets set by the scheduler whenever it decides current needs to get preempted, its unlikely() because that reduces the code impact of cond_resched() and similar in the case we don't schedule, if we do schedule() a mis-predicted branch isn't going to be noticed on the overhead of scheduling. So there's a few cases, 1) PREEMPT=n 2) Voluntary preempt 3) PREEMPT=y 1) non of this has any effect, if the scheduler wants to reschedule a task that's in the kernel, it'll have to wait until it gets back to user-space. 2) uses cond_resched() and similar to have explicit preemption points, so we don't need to wait as long as 1). 3) preempts directly when !preempt_count(), when IRQs are disabled, the IPI that will accompany TIF_NEED_RESCHED will be delayed and local_irq_enable()/restore() will effect a reschedule due to the pending IPI. If preemption was disabled while the IPI hit nothing will happen, but preempt_enable() will do the reschedule once preempt_count reaches 0. -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html