linux-kernel - Re: [patch 00/20] timer: Refactor the timer wheel

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Message-Id: <20160615202659.GS3923@linux.vnet.ibm.com>
Date:	Wed, 15 Jun 2016 13:26:59 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chris Mason <clm@...com>,
	Arjan van de Ven <arjan@...radead.org>, rt@...utronix.de
Subject: Re: [patch 00/20] timer: Refactor the timer wheel

On Wed, Jun 15, 2016 at 07:02:38PM +0200, Thomas Gleixner wrote:
> On Wed, 15 Jun 2016, Paul E. McKenney wrote:
> > On Mon, Jun 13, 2016 at 09:15:14AM -0700, Paul E. McKenney wrote:
> > > On Mon, Jun 13, 2016 at 08:40:50AM -0000, Thomas Gleixner wrote:
> > > > The current timer wheel has some drawbacks:
> > > > 
> > > > 1) Cascading
> > > > 
> > > >    Cascading can be an unbound operation and is completely pointless in most
> > > >    cases because the vast majority of the timer wheel timers are canceled or
> > > >    rearmed before expiration.
> > > > 
> > > > 2) No fast lookup of the next expiring timer
> > > > 
> > > >    In NOHZ scenarios the first timer soft interrupt after a long NOHZ period
> > > >    must fast forward the base time to current jiffies. As we have no way to
> > > >    find the next expiring timer fast, the code loops and increments the base
> > > >    time by one and checks for expired timers in each step. I've observed loops
> > > >    lasting 1 ms!
> > > > 
> > > > There are some other issues caused by the above, but they are minor compare to
> > > > those.
> > > 
> > > For SMP configurations, this passes light rcutorture testing.  For UP
> > > builds, it complains about undefined symbols.  Builds succeed with
> > > the following kneejerk patch.  Am retesting rcutorture.
> > 
> > And with the patch below, testing goes as well with your patch stack as
> > it does without it.  So, with that patch (or equivalent):
> > 
> > Tested-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > 
> > There were some complaints about increasing the size of the tiny
> > configuration, FYI.
> 
> I know. The extra storage space for the deferrable stuff makes it larger along
> with the extra code for avoiding all the crap which the current wheel suffers
> from :) Do the tiny people need NOHZ?

If it makes the code bigger, I would hope that they don't need it.

> > So, just out of curiosity, does anyone still run -rt on single-CPU systems?
> 
> Of course :)

Hey, had to ask!  ;-)

								Thanx, Paul