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From: John Baldwin <jhb@freebsd.org>
To: freebsd-hackers@freebsd.org
Subject: Re: preemptive kernel
Date: Mon, 1 Apr 2013 14:33:23 -0400
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 (bigwig.baldwin.cx); Mon, 01 Apr 2013 15:02:04 -0400 (EDT)
Cc: Adrian Chadd <adrian@freebsd.org>,
 vasanth rao naik sabavat <vasanth.raonaik@gmail.com>
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On Friday, March 22, 2013 4:10:16 pm vasanth rao naik sabavat wrote:
> Hi Adrian,
> 
> Just to clarify, is the kernel pre-emption involuntary?
> 
> Let say I have a kernel thread processing a huge list of entries, would
> this thread get involuntarily context switched out because of kernel
> preemption?
> 
> What is the time slice after which a kernel thread can involuntarily
> context switched out?
> 
> Could you please point to the file in the source code which handles the
> kernel pre-emption.

In-kernel preemption is driven by interrupts, not time slices.  If an 
interrupt arrives that awakens a higher priority thread (e.g. an interrupt 
thread), or if your thread awakens a thread that has higher priority (e.g. due 
to wakeup() or cv_signal()), then your thread will be preempted.

In general time-based preemptions are only done for user threads.

-- 
John Baldwin