linux-kernel - Re: [patch 13/20] timer: Switch to a non cascading wheel

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Message-Id: <20160617040406.GA4306@linux.vnet.ibm.com>
Date:	Thu, 16 Jun 2016 21:04:06 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjanvandeven@...il.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chris Mason <clm@...com>,
	Arjan van de Ven <arjan@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	George Spelvin <linux@...encehorizons.net>
Subject: Re: [patch 13/20] timer: Switch to a non cascading wheel

On Thu, Jun 16, 2016 at 05:40:51PM -0700, Paul E. McKenney wrote:
> On Thu, Jun 16, 2016 at 08:14:02PM +0200, Peter Zijlstra wrote:
> > On Thu, Jun 16, 2016 at 09:02:15AM -0700, Paul E. McKenney wrote:
> > > > 2) When we do that right, we can make the tick frequency a command line option
> > > >    and just have a compiled in default.
> > > 
> > > As long as there is something that tells RCU what the tick frequency
> > > actually is at runtime, this should not be a problem.  For example,
> > > in rcu_implicit_dynticks_qs(), the following:
> > > 
> > > 	rdp->rsp->jiffies_resched += 5;
> > > 
> > > Would instead need to be something like:
> > > 
> > > 	rdp->rsp->jiffies_resched += 5 * jiffies_per_tick;
> > > 
> > > Changing tick frequency at runtime would be a bit more tricky, as it would
> > > be tough to avoid some oddball false positives during the transition.
> > 
> > So the 'fun' part will be frequencies with non integer factors of 1000.
> > Like say HZ=300. For that we'll have to keep jiffies_remainder, and
> > add an extra jiffy every time that rolls over.
> > 
> > That would make your case slightly more interesting than you really
> > want I suspect.
> 
> My particular case is not that sensitive, so 1000/300 would be plenty
> accurate.

That said...  If someone were to set HZ=501, now -that- could be a
bit problematic.  :-/

							Thanx, Paul