Messages in this thread | | | Date | Wed, 15 Feb 2012 22:35:04 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu] rcu: direct algorithmic SRCU implementation |
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On Wed, Feb 15, 2012 at 01:59:23PM +0100, Peter Zijlstra wrote: > On Sun, 2012-02-12 at 18:09 -0800, Paul E. McKenney wrote: > > The current implementation of synchronize_srcu_expedited() can cause > > severe OS jitter due to its use of synchronize_sched(), which in turn > > invokes try_stop_cpus(), which causes each CPU to be sent an IPI. > > This can result in severe performance degradation for real-time workloads > > and especially for short-interation-length HPC workloads. Furthermore, > > because only one instance of try_stop_cpus() can be making forward progress > > at a given time, only one instance of synchronize_srcu_expedited() can > > make forward progress at a time, even if they are all operating on > > distinct srcu_struct structures. > > > > This commit, inspired by an earlier implementation by Peter Zijlstra > > (https://lkml.org/lkml/2012/1/31/211) and by further offline discussions, > > takes a strictly algorithmic bits-in-memory approach. This has the > > disadvantage of requiring one explicit memory-barrier instruction in > > each of srcu_read_lock() and srcu_read_unlock(), but on the other hand > > completely dispenses with OS jitter and furthermore allows SRCU to be > > used freely by CPUs that RCU believes to be idle or offline. > > > > The update-side implementation handles the single read-side memory > > barrier by rechecking the per-CPU counters after summing them and > > by running through the update-side state machine twice. > > Yeah, getting rid of that second memory barrier in srcu_read_lock() is > pure magic :-) > > > This implementation has passed moderate rcutorture testing on both 32-bit > > x86 and 64-bit Power. A call_srcu() function will be present in a later > > version of this patch. > > Goodness ;-)
Glad you like the magic and the prospect of call_srcu(). ;-)
> > @@ -131,10 +214,11 @@ int __srcu_read_lock(struct srcu_struct *sp) > > int idx; > > > > preempt_disable(); > > - idx = sp->completed & 0x1; > > - barrier(); /* ensure compiler looks -once- at sp->completed. */ > > - per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++; > > - srcu_barrier(); /* ensure compiler won't misorder critical section. */ > > + idx = rcu_dereference_index_check(sp->completed, > > + rcu_read_lock_sched_held()) & 0x1; > > + ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]) += > > + SRCU_USAGE_COUNT + 1; > > + smp_mb(); /* B */ /* Avoid leaking the critical section. */ > > preempt_enable(); > > return idx; > > } > > You could use __this_cpu_* muck to shorten some of that.
Ah, so something like this?
ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += SRCU_USAGE_COUNT + 1;
Now that you mention it, this does look nicer, applied here and to srcu_read_unlock().
> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Thanx, Paul
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