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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 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
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 15:00 +0200
Message-ID <tPmfv-vX-7@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>
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On Tue, Jun 06, 2017 at 12:53:43PM +0200, 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.
> 
> >                   and 2) local_irq_save/restore is slower than disabling
> > preemption.  The main architecture with these constraints is s390, which
> > however is already paying the price in __srcu_read_unlock and has not
> > complained.
> 
> IIRC only PPC (and hopefully soon x86) has a local_irq_save() that is as
> fast as preempt_disable().
> 
> > A valid optimization on s390 would be to skip the smp_mb;
> > AIUI, this_cpu_inc implies a memory barrier (!) due to its implementation.
> 
> You mean the s390 this_cpu_inc() in specific, right? Because
> this_cpu_inc() in general does not imply any such thing.

More generally, yes, the commit log needs some more help, good catch,
thank you!

Does the code itself also need more help?

							Thanx, Paul

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[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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