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Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

From "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Newsgroups linux.kernel
Subject Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick
Date 2017-04-13 19:40 +0200
Message-ID <tvQSS-6ep-31@gated-at.bofh.it> (permalink)
References (2 earlier) <tvJ50-Rx-5@gated-at.bofh.it> <tvPtM-5kd-33@gated-at.bofh.it> <tvPN8-5sD-15@gated-at.bofh.it> <tvQg9-5H7-1@gated-at.bofh.it> <tvQpQ-62k-11@gated-at.bofh.it>
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On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 13, 2017 at 09:55:16AM -0700, Paul E. McKenney wrote:
> > > > To avoid people tuning huge machines having to wait for me to give
> > > > them an answer as to why they are suffering lock contention after
> > > > cranking up the value of RCU_FANOUT_LEAF.
> 
> So is there a good reason to increase FANOUT_LEAF ?

Increasing it reduces the number of rcu_node structures, and thus the
number of cache misses during grace-period initialization and cleanup.
This has proven necessary in the past on large machines having long
memory latencies.  And there are starting to be some pretty big machines
running in production, and even for typical commerical workloads.

> > > > Or am I missing your point?
> > > 
> > > Your answer should be: don't do that then. Not provide them a shady work
> > > around.
> > > 
> > > tick skew isn't pretty and has other problems (there's a reason its not
> > > on by default). You're then doing two things you shouldn't.
> > 
> > The tick skew problem that I know of is energy efficiency for light
> > workloads.  This doesn't normally apply to the large heavily loaded
> > systems on which people skew ticks.
> > 
> > So what are the other problems?
> 
> If the jiffy updater bounces between CPUs (as is not uncommon) the
> duration of the jiffy becomes an average (I think we fixed it where it
> could go too fast by always jumping to a CPU which has a short jiffy,
> but I'm not sure).
> 
> This further complicates some of the jiffy based loops (which arguably
> should go away anyway).

Fair enough.  And I am OK with jiffies going away as long as there
is a low-overhead rough-and-ready timing mechanism replacing it.

> And I have vague memories of it actually causing lock contention, but
> I've forgotten how that worked.

That is a new one on me.  I can easily see how not skewing ticks could
cause serious lock contention, but am missing how skewed ticks would
do so.

							Thanx, Paul

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[PATCH tip/core/rcu 0/13] Miscellaneous fixes for 4.12 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-12 19:00 +0200
  [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-12 19:00 +0200
    Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick Peter Zijlstra <peterz@infradead.org> - 2017-04-13 11:20 +0200
      Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-13 18:10 +0200
        Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick Peter Zijlstra <peterz@infradead.org> - 2017-04-13 18:30 +0200
          Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-13 19:00 +0200
            Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick Peter Zijlstra <peterz@infradead.org> - 2017-04-13 19:10 +0200
              Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-13 19:40 +0200
                Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick Peter Zijlstra <peterz@infradead.org> - 2017-04-13 19:50 +0200
                Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-13 20:20 +0200
                Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick Peter Zijlstra <peterz@infradead.org> - 2017-04-13 20:30 +0200
                Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-13 20:50 +0200
                Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick Peter Zijlstra <peterz@infradead.org> - 2017-04-19 15:30 +0200
                Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick Peter Zijlstra <peterz@infradead.org> - 2017-04-19 15:50 +0200
                Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 17:10 +0200
                Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick Peter Zijlstra <peterz@infradead.org> - 2017-04-19 17:50 +0200
                Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 18:20 +0200
                Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 17:00 +0200
                Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick Peter Zijlstra <peterz@infradead.org> - 2017-04-13 20:40 +0200
                Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text  more explicit about skew_tick "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-13 21:50 +0200
  [PATCH tip/core/rcu 13/13] rcu: Fix typo in PER_RCU_NODE_PERIOD header comment "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-12 19:00 +0200
  [PATCH tip/core/rcu 09/13] mm: Use static initialization for "srcu" "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-12 19:00 +0200
  [PATCH tip/core/rcu 12/13] rcu: Use true/false in assignment to bool "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-12 19:00 +0200
  [PATCH tip/core/rcu 05/13] rcu: Remove obsolete comment from rcu_future_gp_cleanup() header "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-12 19:00 +0200
  [PATCH tip/core/rcu 03/13] types: Update obsolete callback_head comment "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-12 19:00 +0200
  [PATCH tip/core/rcu 11/13] rcu: Use bool value directly "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-12 19:00 +0200
  [PATCH tip/core/rcu 10/13] torture: Use correct path for Kconfig fragment for duplicates "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-12 19:00 +0200
  [PATCH tip/core/rcu 02/13] lockdep: Use "WARNING" tag on lockdep splats "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-12 19:00 +0200
    Re: [PATCH tip/core/rcu 02/13] lockdep: Use "WARNING" tag on lockdep  splats Peter Zijlstra <peterz@infradead.org> - 2017-04-13 11:20 +0200
      Re: [PATCH tip/core/rcu 02/13] lockdep: Use "WARNING" tag on lockdep  splats "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-13 18:10 +0200
  [PATCH tip/core/rcu 06/13] hlist_add_tail_rcu disable sparse warning "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-12 19:00 +0200
  [PATCH tip/core/rcu 01/13] mm: Rename SLAB_DESTROY_BY_RCU to SLAB_TYPESAFE_BY_RCU "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-12 19:10 +0200
    Re: [PATCH tip/core/rcu 01/13] mm: Rename SLAB_DESTROY_BY_RCU to  SLAB_TYPESAFE_BY_RCU Peter Zijlstra <peterz@infradead.org> - 2017-04-13 11:20 +0200
      Re: [PATCH tip/core/rcu 01/13] mm: Rename SLAB_DESTROY_BY_RCU to  SLAB_TYPESAFE_BY_RCU Vlastimil Babka <vbabka@suse.cz> - 2017-04-13 13:10 +0200
        Re: [PATCH tip/core/rcu 01/13] mm: Rename SLAB_DESTROY_BY_RCU to  SLAB_TYPESAFE_BY_RCU "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-13 18:10 +0200
        Re: [PATCH tip/core/rcu 01/13] mm: Rename SLAB_DESTROY_BY_RCU to  SLAB_TYPESAFE_BY_RCU Peter Zijlstra <peterz@infradead.org> - 2017-04-13 18:20 +0200
          Re: [PATCH tip/core/rcu 01/13] mm: Rename SLAB_DESTROY_BY_RCU to  SLAB_TYPESAFE_BY_RCU "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-13 19:30 +0200
          Re: [PATCH tip/core/rcu 01/13] mm: Rename SLAB_DESTROY_BY_RCU to SLAB_TYPESAFE_BY_RCU Eric Dumazet <edumazet@google.com> - 2017-04-13 23:40 +0200
            Re: [PATCH tip/core/rcu 01/13] mm: Rename SLAB_DESTROY_BY_RCU to  SLAB_TYPESAFE_BY_RCU Peter Zijlstra <peterz@infradead.org> - 2017-04-14 10:50 +0200
              Re: [PATCH tip/core/rcu 01/13] mm: Rename SLAB_DESTROY_BY_RCU to  SLAB_TYPESAFE_BY_RCU "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-14 15:50 +0200
  [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-18 01:30 +0200
    [PATCH v2 tip/core/rcu 11/11] rcu: Fix typo in PER_RCU_NODE_PERIOD header comment "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-18 01:30 +0200
    [PATCH v2 tip/core/rcu 03/11] types: Update obsolete callback_head comment "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-18 01:30 +0200
    [PATCH v2 tip/core/rcu 06/11] hlist_add_tail_rcu disable sparse warning "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-18 01:30 +0200
    [PATCH v2 tip/core/rcu 04/11] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-18 01:40 +0200
      Re: [PATCH v2 tip/core/rcu 04/11] rcu: Make RCU_FANOUT_LEAF help  text more explicit about skew_tick Josh Triplett <josh@joshtriplett.org> - 2017-04-18 02:20 +0200
        Re: [PATCH v2 tip/core/rcu 04/11] rcu: Make RCU_FANOUT_LEAF help  text more explicit about skew_tick "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-18 20:50 +0200
    [PATCH v2 tip/core/rcu 05/11] rcu: Remove obsolete comment from rcu_future_gp_cleanup() header "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-18 01:40 +0200
    [PATCH v2 tip/core/rcu 08/11] torture: Use correct path for Kconfig fragment for duplicates "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-18 01:40 +0200
    [PATCH v2 tip/core/rcu 09/11] rcu: Use bool value directly "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-18 01:40 +0200
    [PATCH v2 tip/core/rcu 10/11] rcu: Use true/false in assignment to bool "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-18 01:40 +0200
    [PATCH v2 tip/core/rcu 01/11] mm: Rename SLAB_DESTROY_BY_RCU to SLAB_TYPESAFE_BY_RCU "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-18 01:40 +0200
      Re: [PATCH v2 tip/core/rcu 01/11] mm: Rename SLAB_DESTROY_BY_RCU to  SLAB_TYPESAFE_BY_RCU David Rientjes <rientjes@google.com> - 2017-04-18 02:20 +0200
    [PATCH v2 tip/core/rcu 02/11] lockdep: Use "WARNING" tag on lockdep splats "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-18 01:40 +0200
      Re: [PATCH v2 tip/core/rcu 02/11] lockdep: Use "WARNING" tag on  lockdep splats Josh Triplett <josh@joshtriplett.org> - 2017-04-19 17:10 +0200
        Re: [PATCH v2 tip/core/rcu 02/11] lockdep: Use "WARNING" tag on  lockdep splats "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 18:30 +0200
    [PATCH v2 tip/core/rcu 07/11] rcu: Improve comments for hotplug/suspend/hibernate functions "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-18 01:40 +0200
    Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 Peter Zijlstra <peterz@infradead.org> - 2017-04-19 13:30 +0200
      Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 Peter Zijlstra <peterz@infradead.org> - 2017-04-19 13:40 +0200
      Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 Christian Borntraeger <borntraeger@de.ibm.com> - 2017-04-19 13:50 +0200
        Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 Peter Zijlstra <peterz@infradead.org> - 2017-04-19 14:10 +0200
          Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 Marc Zyngier <marc.zyngier@arm.com> - 2017-04-19 15:00 +0200
          Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 16:50 +0200
            Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 Josh Triplett <josh@joshtriplett.org> - 2017-04-19 17:00 +0200
              Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 Peter Zijlstra <peterz@infradead.org> - 2017-04-19 17:10 +0200
                Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 17:20 +0200
            Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 Peter Zijlstra <peterz@infradead.org> - 2017-04-19 17:00 +0200
              Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 17:20 +0200
        Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 15:30 +0200
          Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 Christian Borntraeger <borntraeger@de.ibm.com> - 2017-04-19 15:30 +0200
      Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 15:10 +0200
        Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 Peter Zijlstra <peterz@infradead.org> - 2017-04-19 15:20 +0200
          Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 17:40 +0200
            Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 Peter Zijlstra <peterz@infradead.org> - 2017-04-19 17:50 +0200
              Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 18:20 +0200
    Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 18:50 +0200
      [PATCH v3 tip/core/rcu 06/11] hlist_add_tail_rcu disable sparse warning "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 18:50 +0200
      [PATCH v3 tip/core/rcu 11/11] rcu: Fix typo in PER_RCU_NODE_PERIOD header comment "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 18:50 +0200
      [PATCH v3 tip/core/rcu 05/11] rcu: Remove obsolete comment from rcu_future_gp_cleanup() header "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 18:50 +0200
      [PATCH v3 tip/core/rcu 08/11] torture: Use correct path for Kconfig fragment for duplicates "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 18:50 +0200
      [PATCH v3 tip/core/rcu 03/11] types: Update obsolete callback_head comment "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 18:50 +0200
      [PATCH v3 tip/core/rcu 10/11] rcu: Use true/false in assignment to bool "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 18:50 +0200
      [PATCH v3 tip/core/rcu 07/11] rcu: Improve comments for hotplug/suspend/hibernate functions "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 18:50 +0200
      [PATCH v3 tip/core/rcu 04/11] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 19:00 +0200
      [PATCH v3 tip/core/rcu 01/11] mm: Rename SLAB_DESTROY_BY_RCU to SLAB_TYPESAFE_BY_RCU "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-04-19 19:00 +0200

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