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From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Eric Dumazet <edumazet@google.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Chris Mason <clm@fb.com>, Arjan van de Ven <arjan@infradead.org>,
	rt@linutronix.de
Subject: Re: [patch 00/20] timer: Refactor the timer wheel
Date: Wed, 15 Jun 2016 13:26:59 -0700	[thread overview]
Message-ID: <20160615202659.GS3923@linux.vnet.ibm.com> (raw)
In-Reply-To: <alpine.DEB.2.11.1606151900460.5839@nanos>

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@linux.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

      reply	other threads:[~2016-06-15 20:27 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-06-13  8:40 [patch 00/20] timer: Refactor the timer wheel Thomas Gleixner
2016-06-13  8:40 ` [patch 01/20] timer: Make pinned a timer property Thomas Gleixner
2016-06-13  8:40 ` [patch 02/20] x86/apic/uv: Initialize timer as pinned Thomas Gleixner
2016-06-13  8:40 ` [patch 03/20] x86/mce: " Thomas Gleixner
2016-06-13  8:40 ` [patch 04/20] cpufreq/powernv: " Thomas Gleixner
2016-06-13 13:18   ` Arjan van de Ven
2016-06-13  8:40 ` [patch 05/20] driver/net/ethernet/tile: " Thomas Gleixner
2016-06-13  8:40 ` [patch 06/20] drivers/tty/metag_da: " Thomas Gleixner
2016-06-13 13:13   ` Arjan van de Ven
2016-06-13  8:40 ` [patch 07/20] drivers/tty/mips_ejtag: " Thomas Gleixner
2016-06-13  8:40 ` [patch 08/20] net/ipv4/inet: Initialize timers " Thomas Gleixner
2016-06-13  8:40 ` [patch 09/20] timer: Remove mod_timer_pinned Thomas Gleixner
2016-06-13  8:40 ` [patch 10/20] timer: Add a cascading tracepoint Thomas Gleixner
2016-06-13  8:40 ` [patch 11/20] hlist: Add hlist_is_last_node() helper Thomas Gleixner
2016-06-13 10:27   ` Paolo Bonzini
2016-06-13  8:40 ` [patch 12/20] timer: Give a few structs and members proper names Thomas Gleixner
2016-06-13  8:41 ` [patch 13/20] timer: Switch to a non cascading wheel Thomas Gleixner
2016-06-13 11:40   ` Peter Zijlstra
2016-06-13 12:30     ` Thomas Gleixner
2016-06-13 12:46       ` Eric Dumazet
2016-06-13 14:30         ` Thomas Gleixner
2016-06-14 10:11       ` Ingo Molnar
2016-06-14 16:28         ` Thomas Gleixner
2016-06-14 17:14           ` Arjan van de Ven
2016-06-14 18:05             ` Thomas Gleixner
2016-06-14 20:34               ` Peter Zijlstra
2016-06-14 20:42               ` Peter Zijlstra
2016-06-14 21:17                 ` Eric Dumazet
2016-06-15 14:53                   ` Thomas Gleixner
2016-06-15 14:55                     ` Arjan van de Ven
2016-06-15 16:43                       ` Thomas Gleixner
2016-06-16 15:43                         ` Thomas Gleixner
2016-06-16 16:02                           ` Paul E. McKenney
2016-06-16 18:14                             ` Peter Zijlstra
2016-06-17  0:40                               ` Paul E. McKenney
2016-06-17  4:04                                 ` Paul E. McKenney
2016-06-16 16:04                           ` Arjan van de Ven
2016-06-16 16:09                             ` Thomas Gleixner
2016-06-15 15:05                     ` Eric Dumazet
2016-06-13 14:36   ` Richard Cochran
2016-06-13 14:39     ` Thomas Gleixner
2016-06-13  8:41 ` [patch 15/20] timer: Move __run_timers() function Thomas Gleixner
2016-06-13  8:41 ` [patch 14/20] timer: Remove slack leftovers Thomas Gleixner
2016-06-13  8:41 ` [patch 16/20] timer: Optimize collect timers for NOHZ Thomas Gleixner
2016-06-13  8:41 ` [patch 17/20] tick/sched: Remove pointless empty function Thomas Gleixner
2016-06-13  8:41 ` [patch 18/20] timer: Forward wheel clock whenever possible Thomas Gleixner
2016-06-13 15:14   ` Richard Cochran
2016-06-13 15:18     ` Thomas Gleixner
2016-06-13  8:41 ` [patch 19/20] timer: Split out index calculation Thomas Gleixner
2016-06-13  8:41 ` [patch 20/20] timer: Optimization for same expiry time in mod_timer() Thomas Gleixner
2016-06-13 14:10 ` [patch 00/20] timer: Refactor the timer wheel Eric Dumazet
2016-06-13 16:15 ` Paul E. McKenney
2016-06-15 15:15   ` Paul E. McKenney
2016-06-15 17:02     ` Thomas Gleixner
2016-06-15 20:26       ` Paul E. McKenney [this message]

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