cvs commit: src/sys/kern sched_ule.c

[Jeff Roberson]

On Wed, 24 Jan 2007, David Xu wrote:

> Jeff Roberson wrote:
>> On Wed, 24 Jan 2007, David Xu wrote:
>> 
>>> Jeff Roberson wrote:
>>> 
>>>>> if idlethread is preempted, who will clear its idle bit in 
>>>>> idle_cpus_mask ?
>>>> 
>>>> 
>>>> 
>>>> idle_cpus_mask was broken before for all schedulers.  This commit didn't 
>>>> change that.  ULE doesn't use idle_cpus_mask and it's idlethread doesn't 
>>>> set or clear it.  The idle thread for the other schedulers remains 
>>>> unchanged.
>>>> 
>>> I havn't read ULE code, but 4BSD has following code:
>>> 
>> 
>> This is the very same code that was there before.  I didn't change it.  I 
>> just moved it into the schedulers.  It was always broken with preemption. I 
>> tried to use idle_cpus_mask in ULE but found that it always was set for 
>> most CPUs.  You will find that IPI_PREEMPT also had no effect but I fixed 
>> that recently.
>> 
>
> orignal code does not preempt per-cpu idle thread, so the idle threads
> have chance to clear their idle bits in idle_cpu_masks, but now because
> the threads can be preempted directly, so they don't have chance
> to clear the masks, unless sched_switch clears them( but the code is not
> there), without clearing them, very cpu will receive a broadcast IPI
> whenever a thread is added to runqueue for 4BSD scheduler, this is
> incorrect if a cpu is already running a non-idle thread.
>
>

The IPI_PREEMPT code was broken until I fixed it last week.  It didn't 
preempt any threads and it's not enabled by default in 4BSD.  That and 
maybe_preempt() are really the only pieces of code that would cause 
preemption.  I haven't changed maybe_preempt() but it allows for 
preemption of the idlethread and has for as long as I'm aware.

I don't disagree that idle_cpu_mask is broken, however, I don't think my 
commit had any effect on that or idlethread except within ULE.  In my 
opinion, the proper way to fix it is in sched_switch() anyhow.  Julian 
said he was going to look into that.

Cheers,
Jeff