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

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Message-ID: <Pine.LNX.4.64.0704021052070.26489@alien.or.mcafeemobile.com>
Date:	Mon, 2 Apr 2007 10:52:56 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Thomas Gleixner <tglx@...utronix.de>
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, 2 Apr 2007, Thomas Gleixner wrote:

> 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 :)

Yack, I missed that part :) Sounds fine then.



- Davide


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