On Tue, 2012-01-31 at 14:24 -0800, Paul E. McKenney wrote: > > > Can we get it back to speed by scheduling a work function on all cpus? > > > wouldn't that force a quiescent state and allow call_srcu() to fire? > > > > > > In kvm's use case synchronize_srcu_expedited() is usually called when no > > > thread is in a critical section, so we don't have to wait for anything > > > except the srcu machinery. > > > > OK, I'll try and come up with means of making it go fast again ;-) > > I cannot resist suggesting a kthread to do the call_srcu(), which > would allow synchronize_srcu_expedited() to proceed with its current > brute-force speed. Right, so I really don't like to add a kthread per srcu instance. Sharing a kthread between all SRCUs will be problematic since these sync things can take forever and so the thread will become a bottlneck. Also, I'd really like to come up with a better means of sync for SRCU and not hammer the entire machine (3 times). One of the things I was thinking of is adding a sequence counter in the per-cpu data. Using that we could do something like: unsigned int seq1 = 0, seq2 = 0, count = 0; int cpu, idx; idx = ACCESS_ONCE(sp->completions) & 1; for_each_possible_cpu(cpu) seq1 += per_cpu(sp->per_cpu_ref, cpu)->seq; for_each_possible_cpu(cpu) count += per_cpu(sp->per_cpu_ref, cpu)->c[idx]; for_each_possible_cpu(cpu) seq2 += per_cpu(sp->per_cpu_ref, cpu)->seq; /* * there's no active references and no activity, we pass */ if (seq1 == seq2 && count == 0) return; synchronize_srcu_slow(); This would add a fast-path which should catch the case Avi outlined where we call sync_srcu() when there's no other SRCU activity. The other thing I was hoping to be able to pull off is add a copy of idx into the same cacheline as c[] and abuse cache-coherency to avoid some of the sync_sched() calls, but that's currently hurting my brain. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
