linux-kernel - Re: Preempt of BKL and with tickless systems

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Message-ID: <46438C54.20101@tmr.com>
Date:	Thu, 10 May 2007 17:19:16 -0400
From:	Bill Davidsen <davidsen@....com>
To:	Lee Revell <rlrevell@...-job.com>
CC:	Linux Kernel mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Preempt of BKL and with tickless systems

Lee Revell wrote:
> On 5/8/07, Bill Davidsen <davidsen@....com> wrote:
>> I think I have a reasonable grip on the voluntary and full preempt
>> models, can anyone give me any wisdom on the preempt of the BKL? I know
>> what it does, the question is where it might make a difference under
>> normal loads. Define normal as servers and desktops.
> 
> This was introduced by Ingo to solve a real problem that I found,
> where some codepath would hold the BKL for long enough to introduce
> excessive scheduling latencies - search list archive for details.  But
> I don't remember the code path (scrolling the FB console?  VT
> switching? reiser3?  misc. ioctl()s?).  Basically, taking the BKL
> disabled preemption which caused long latencies.
> 
> It's certainly possible that whatever issue led to this was solved in
> another way since.
> 
Anything is possible. I feel that using voluntary + bkl is probably good 
for most servers, forced preempt for desktop, although it really doesn't 
seem to do much beyond voluntary.

Thanks for the clarification.

-- 
Bill Davidsen <davidsen@....com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot
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