From owner-cvs-all@FreeBSD.ORG  Wed Jan 24 07:49:51 2007
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Date: Wed, 24 Jan 2007 15:49:49 +0800
From: David Xu <davidxu@freebsd.org>
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To: Jeff Roberson <jroberson@chesapeake.net>
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Cc: src-committers@FreeBSD.org, jhb@FreeBSD.org, cvs-src@FreeBSD.org,
	Jeff Roberson <jeff@FreeBSD.org>, cvs-all@FreeBSD.org,
	Julian Elischer <julian@FreeBSD.org>
Subject: Re: cvs commit: src/sys/kern sched_ule.c
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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.