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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 2017-04-19 17:00 +0200
Message-ID <txZfm-4Gn-79@gated-at.bofh.it> (permalink)
References (5 earlier) <tvR2y-6iE-11@gated-at.bofh.it> <tvRvz-6Kr-7@gated-at.bofh.it> <tvRFf-6Or-9@gated-at.bofh.it> <tvRYC-6WU-9@gated-at.bofh.it> <txXQd-3X1-7@gated-at.bofh.it>
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On Wed, Apr 19, 2017 at 03:22:26PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 13, 2017 at 11:42:32AM -0700, Paul E. McKenney wrote:
> 
> > I believe that you are missing the fact that RCU grace-period
> > initialization and cleanup walks through the rcu_node tree breadth
> > first, using rcu_for_each_node_breadth_first().
> 
> Indeed. That is the part I completely missed.
> 
> >                                                 This macro (shown below)
> > implements this breadth-first walk using a simple sequential traversal of
> > the ->node[] array that provides the structures making up the rcu_node
> > tree.  As you can see, this scan is completely independent of how CPU
> > numbers might be mapped to rcu_data slots in the leaf rcu_node structures.
> 
> So this code is clearly not a hotpath, but still its performance
> matters?
> 
> Seems like you cannot win here :/

Welcome to my world!!!  ;-)

But yes, running on 4096-CPU systems can put some serious stress on
some surprising areas.  Especially when those systems have cache-miss
latencies well in excess of a microsecond, and the users are nevertheless
expecting scheduling latencies well below 100 microseconds.

It was a fun challenge, I grant you that!

							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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