linux-kernel - Re: report a bug about sched

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Message-ID: <4A6DAD16.7050809@billgatliff.com>
Date:	Mon, 27 Jul 2009 08:35:18 -0500
From:	Bill Gatliff <bgat@...lgatliff.com>
To:	Jamie Lokier <jamie@...reable.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	sen wang <wangsen.linux@...il.com>, mingo@...e.hu,
	akpm@...ux-foundation.org, kernel@...ivas.org, npiggin@...e.de,
	arjan@...radead.org, linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: report a bug about sched_rt

Jamie Lokier wrote:
> Bill Gatliff wrote:
>   
>> Jamie Lokier wrote:
>>     
>>> I agree with communicting the desire explicitly to the scheduler.
>>>
>>> In the above example, the exact desire is "give me as much CPU as I
>>> ask for, because my hardware servicing will be adversely but
>>> non-fatally affected if you don't, and the amount of CPU needed to
>>> service the hardware cannot be determined in advance, but prevent me
>>>       
>> >from blocking progress in the rest of the system by limiting my
>>     
>>> exclusive ownership of the CPU".
>>>
>>> How do you propose to communicate that to the scheduler, if not by
>>> something rather like RT-bandwidth with downgrading to SCHED_OTHER
>>> when a policy limit is exceeded?
>>>       
>> This is a great real-world problem.  And there's no one-size-fits-all 
>> answer, unfortunately.
>>
>> RT-bandwidth will give you the system behavior you are after, but it's a 
>> pretty blunt instrument.
>>     
>
> I'm under the impression that RT-bandwidth will *not* give the above
> system behaviour, and that is the whole reason for this thread.
>   

I think I misspoke.  What I meant to say is that RT-bandwidth will 
(probably) prevent the hardware handler from eating 100% of the CPU.  
But the system will suffer quite a, um, "discontinuity" when the 
throttling happens.

>   
>> I'd consider putting some throttling in your interrupt handler that 
>> prevents it from running more than a certain amount of calculation per 
>> interrupt event.
>>     
>
> There is no interrupt handler in my specification above...
>   

True.  But in practice, I think such devices are typically 
interrupt-driven at some level.


b.g.

-- 
Bill Gatliff
bgat@...lgatliff.com

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