linux-kernel - Re: [patch 2/7] timer: Remove FIFO guarantee

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Message-ID: <20150609053930.25020.qmail@ns.horizon.com>
Date:	9 Jun 2015 01:39:30 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	linux@...izon.com, tglx@...utronix.de
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org,
	viresh.kumar@...aro.org
Subject: Re: [patch 2/7] timer: Remove FIFO guarantee

Thomas Gleixner wrote:
> It does. Depending on when you enqueue the timer because the thing is
> calculated from the delta (expires - jiffies).

Ah, right.  If slack > 0, the slack amount is absolute and the rounding
will be consistent.

But if slack < 0, which is the default, it's a percentage of remaining
jiffies.  Since slack only delays timeouts, an earlier-scheduled
timeout could easily be delayed more.

(There are only six calls to set_timer_slack() to change the default
to something positive in the kernel.)

>>    I'm not disagreeing with the change, but it's not clear to me that
>>    it's as safe as you think.

> After thinking more about it, I'm even more sure that any code which
> relies on the FIFO "guarantee" is broken today.

Indeed, I am completely convinced.  All I might request is a reassignment
of blame in the commit message.



Thank you for your comments on my other blue-sky ideas, too.

I need to look into why we're using wheels, and what the point is.
How much of an advantage do they have over an efficient priority queue
like a pairing heap?
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