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

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Message-Id: <20160617004051.GR3923@linux.vnet.ibm.com>
Date:	Thu, 16 Jun 2016 17:40:51 -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 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.

							Thanx, Paul