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Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context

From Peter Zijlstra <peterz@infradead.org>
Newsgroups linux.kernel
Subject Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context
Date 2017-06-06 18:10 +0200
Message-ID <tPpdo-2Df-35@gated-at.bofh.it> (permalink)
References <tP8me-aN-23@gated-at.bofh.it> <tP8vT-e0-7@gated-at.bofh.it> <tPknp-7Bq-47@gated-at.bofh.it> <tPmpd-OH-45@gated-at.bofh.it>
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On Tue, Jun 06, 2017 at 03:08:50PM +0200, Paolo Bonzini wrote:
> 
> 
> On 06/06/2017 12:53, Peter Zijlstra wrote:
> > On Mon, Jun 05, 2017 at 03:09:50PM -0700, Paul E. McKenney wrote:
> >> There would be a slowdown if 1) fast this_cpu_inc is not available and
> >> cannot be implemented (this usually means that atomic_inc has implicit
> >> memory barriers),
> > 
> > I don't get this.
> > 
> > How is per-cpu crud related to being strongly ordered?
> > 
> > this_cpu_ has 3 forms:
> > 
> > 	x86:		single instruction
> > 	arm64,s390:	preempt_disable()+atomic_op
> > 	generic:	local_irq_save()+normal_op
> > 
> > Only s390 is TSO, arm64 is very much a weak arch.
> 
> Right, and thus arm64 can implement a fast this_cpu_inc using LL/SC.
> s390 cannot because its atomic_inc has implicit memory barriers.

I'm not sure the ordering makes a useful differentiator between a fast
and non-fast this_cpu implementation.

For TSO archs making their atomic primitives fully ordered isn't _that_
much of a burden over their normal ordering requirements. And LL/SC
archs can have very slow (weak) atomics (PPC for example).

(Now theoretically a TSO arch could have weak(er) atomics, but I'm not
aware of any that do this).

I realize we're splitting hairs and slightly off topic :-)

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Thread

[PATCH RFC tip/core/rcu 0/2] srcu: All SRCU readers from both  process and irq "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 00:10 +0200
  [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 00:20 +0200
    Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Peter Zijlstra <peterz@infradead.org> - 2017-06-06 13:00 +0200
      Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 15:00 +0200
      Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Paolo Bonzini <pbonzini@redhat.com> - 2017-06-06 15:10 +0200
        Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Christian Borntraeger <borntraeger@de.ibm.com> - 2017-06-06 16:50 +0200
          Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Heiko Carstens <heiko.carstens@de.ibm.com> - 2017-06-06 17:30 +0200
            Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Christian Borntraeger <borntraeger@de.ibm.com> - 2017-06-06 17:40 +0200
              Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 18:00 +0200
            Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Peter Zijlstra <peterz@infradead.org> - 2017-06-06 18:20 +0200
              Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 19:10 +0200
              Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Heiko Carstens <heiko.carstens@de.ibm.com> - 2017-06-06 19:30 +0200
          Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Peter Zijlstra <peterz@infradead.org> - 2017-06-06 18:20 +0200
        Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Peter Zijlstra <peterz@infradead.org> - 2017-06-06 18:10 +0200
    Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Peter Zijlstra <peterz@infradead.org> - 2017-06-06 13:10 +0200
      Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Paolo Bonzini <pbonzini@redhat.com> - 2017-06-06 14:10 +0200
        Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 15:00 +0200
          Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Peter Zijlstra <peterz@infradead.org> - 2017-06-06 18:00 +0200
            Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 18:00 +0200
    Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Peter Zijlstra <peterz@infradead.org> - 2017-06-06 19:30 +0200
      Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 20:00 +0200
        Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context Peter Zijlstra <peterz@infradead.org> - 2017-06-06 20:10 +0200
          Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU  from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 20:30 +0200
  [PATCH RFC tip/core/rcu 2/2] srcu: Allow use of Classic SRCU from both process and interrupt context "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 00:20 +0200
  Re: [PATCH RFC tip/core/rcu 0/2] srcu: All SRCU readers from both  process and irq "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-06-06 19:10 +0200

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