Messages in this thread | | | Subject | Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context | From | Paolo Bonzini <> | Date | Tue, 6 Jun 2017 15:08:50 +0200 |
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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.
s390's this_cpu_inc is *faster* than the generic one, but still pretty slow.
>> 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().
1 = arch-specific this_cpu_inc is available 2 = local_irq_save/restore as fast as preempt_disable/enable
If either 1 or 2 are true, this patch makes SRCU faster or equal
x86 (single instruction): 1 = true, 2 = false -> ok arm64 (weakly ordered): 1 = true, 2 = false -> ok powerpc: 1 = false, 2 = true -> ok s390: 1 = false, 2 = false -> slower
For other LL/SC architectures, notably arm, fast this_cpu_* ops not yet available, but could be written pretty easily.
>> 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.
Yes, of course, this is only for s390.
Alternatively, we could change the counters to atomic_t and use smp_mb__{before,after}_atomic, as in the (unnecessary) srcutiny patch. That should shave a few cycles on x86 too, since "lock inc" is faster than "inc; mfence". For srcuclassic (and stable) however I'd rather keep the simple __this_cpu_inc -> this_cpu_inc change.
Paolo
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