Re: The purpose and implementation of cond_resched()

From: Dong Feng
Date: Wed Feb 21 2007 - 12:08:46 EST

I re-checked the code. And this time, I think cond_resched() is useful
while a kernel is compiled with no full preemption function but only
voluntary kernel preemption is enabled (i.e. CONFIG_PREEMPT_VOLUNTARY
is set but CONFIG_PREEMPT is not set). In this case, kernel performs
scheduling at explicit voluntary preemption points only, and those
points are determined by invoking cond_resched().

But I still have questions, why cond_resched() does not yield no-op
while CONFIG_PREEMPT is set? And why does it deal with the
PREEMPT_ACTIVE flag anyway?

2007/2/22, Dong Feng <middle.fengdong@xxxxxxxxx>:
I have a question about cond_resched().

What is the condition under which I should invoke cond_resched() irreplaceably?

For example, I see the following code in ksoftirqd(),


But I do not understand why I should not write the following code,


Are the above two pieces of code equal in functionality?

On the other hand, I see cond_resched() check and set PREEMPT_ACTIVE.
I currently do not understand why it should do this, since I think
PREEMPT_ACTIVE is only used to be set in the return-from-interrupt
code in order to prevent schedule() from removing task from run queue
unpredictably. But for cond_resched(), which is a planned voluntary
switch, why does it also deal with this flag?

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