Message-Id: <4sur0l$12a3ma@fmsmga001.fm.intel.com>
From: "Chen, Kenneth W" <kenneth.w.chen@intel.com>
To: "'Con Kolivas'" <kernel@kolivas.org>, <tim.c.chen@linux.intel.com>
Cc: <linux-kernel@vger.kernel.org>, <mingo@elte.hu>
Subject: RE: Regression seen for patch "sched:dont decrease idle sleep avg"
Date: Thu, 11 May 2006 17:04:11 -0700
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Tim Chen writes:
> See patch:
> http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=e72ff0bb2c163eb13014ba113701bd42dab382fe 

Con Kolivas wrote on Monday, May 08, 2006 5:43 PM
> This patch corrects a bug in the original code which unintentionally dropped 
> the priority of tasks that were idle but were already high priority on other 
> merits. It doesn't further increase the priority.


This got me to take a non-casual look at that particular git commit.  The
first portion of the change log description says perfectly about the intent,
but after studying the code, I have to say that the actual code does not
implement what people say it will do.  In recalc_task_prio(), if a task's
sleep_time is more than INTERACTIVE_SLEEP, it will bump up p->sleep_avg all
the way to near maximum (at MAX_SLEEP_AVG - DEF_TIMESLICE), which according
to my calculation, it will have a priority bonus of 4 (out of max 5).

IOW, for a prolonged sleep, a task will immediately get near maximum priority
boost. Is that what the real intent is?  Seems to be on the contrary to what
the source code comments as well.

I think in the if (sleep_time > INTERACTIVE_SLEEP) block, p->sleep_avg should
be treated similarly like what the "else" block is doing: scale it proportionally
with past sleep time, perhaps not the immediate previously prolonged sleep
because that would for sure bump up priority too fast.  A better method might
be p->sleep_avg *= 2 or something like that.

- Ken
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