Messages in this thread | | | Subject | Re: yield() in i2c non-happy paths hits BUG under -rt patch | From | Peter Zijlstra <> | Date | Thu, 19 Nov 2009 15:15:54 +0100 |
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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.
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