linux-kernel - Re: Scheduling the highest priority task

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Message-ID: <46B1FC6D.7010202@felicis.org>
Date:	Thu, 02 Aug 2007 17:46:53 +0200
From:	Martin Roehricht <ml@...icis.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Scheduling the highest priority task

On 08/02/2007 05:19 PM, Ingo Molnar wrote:
> * Martin Roehricht <ml@...icis.org> wrote:
> 
>> That's fine with me, that within the same priority-queue any task can 
>> be chosen. But assume two tasks with highly different priorities, such 
>> as 105 and 135 are scheduled on the same processor and one of them is 
>> now to be migrated -- shouldn't be the queue with task P=105 
>> considered first for migration by this code? Both tasks would use 
>> different queues with their own linked lists, right?
> 
> yes. What makes you believe that the lower priority one (prio 135) is 
> chosen? [ as i said before, that will only be chosen if all tasks in the 
> higher-priority queue (prio 105) are either already running on a CPU or 
> have recently run so that the cache-hot logic skips them. ]

This believe is primarily based on my observations of multiple benchmark 
runs and also on your statement earlier: »in the SMP migration code, the 
'old scheduler' indeed picks the lowest priority one«.

Perhaps it is just an unfortunate coincidence that at ~90% of the time a 
migration decision is made, the higher priority process is currently 
cache hot whereas the lower priority process is not. That would be 
unlucky for me as I would like to decide upon specific runtime 
circumstances whether the highest or the lowest priority job of a 
runqueue should be migrated to another CPU. :-/

Martin
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