linux-kernel - Re: [patch 6/13] signal/timer/event fds v9

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Date:	Mon, 02 Apr 2007 19:47:44 +0200
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 6/13] signal/timer/event fds v9 - timerfd core ...

On Mon, 2007-04-02 at 10:30 -0700, Davide Libenzi wrote:

> > There is no inaccuracy when you rearm the timer on read: hrtimer_forward
> > takes care, that the period is accurate. It does not start the timer out
> > of the periodic order, i.e. on a different time frame.
> > 
> > Where is the win of keeping the timer running, when nobody cares about
> > the expiry at all ? It just generates interrupts and events for nothing.
> 
> Then you'd lose the ability to know if you lost one or more (yes, you 
> could figure it out by reading the time and with a few calculations). I 
> think that the capping (to a sane value) idea solves the DoS issue and at 
> the same time have the ability to report you missed ticks. What are your 
> strong points against that solution?

Err, the read function 

	ticks = hrtimer_forward(&ctx->tmr, ktime_get(),
                                ctx->tintv);

does give you the number of (lost) ticks.

tmr->expires holds the absolute expiry time of the last event.
hrtimer_forward() adds N intervals to tmr->expires, so that the new
tmr->expires value is greater than now (ktime_get()). It returns N.

So the number of lost ticks is N - 1. No time reading and no magic
math :)

	tglx


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