linux-kernel - Scheduling the highest priority task

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Message-ID: <46B19CA1.7050204@felicis.org>
Date:	Thu, 02 Aug 2007 10:58:09 +0200
From:	Martin Roehricht <ml@...icis.org>
To:	linux-kernel@...r.kernel.org
CC:	mingo@...e.hu
Subject: Scheduling the highest priority task

Hi,

perhaps someone can give me a hint what I should consider to look for in 
order to change the ("old" 2.6.21) scheduler such that it schedules the 
highest priority task of a given runqueue.
Given a multiprocessor system I currently observe that whenever there 
are two tasks on one CPU, the lower priority one is migrated to another 
CPU. But I don't realize why this happens. From looking at the source 
code I thought it should be the highest priority one (lowest bit set in 
the runqueue's bitmap) according to
	idx = sched_find_first_bit(array->bitmap);
within move_tasks(). The idx value is then used as an index (surprise) 
to the linked list of tasks of this particular priority and one task is 
picked:
	head = array->queue + idx;
         curr = head->prev;
         tmp = list_entry(curr, struct task_struct, run_list);

Is my assumption wrong? Using printk()s within this code section makes 
the system just hang completely quite soon. The schedstats do not notify 
me immediately. So I am a bit lost on how to track down or trace the 
problem.

Can anybody confirm that my observations are correct that the scheduler 
picks the lowest priority job of a runqueue for migration?
What needs to be changed in order to pick the highest priority one?

Martin
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