Re: tree rcu: call_rcu scalability problem?

[Paul E. McKenney]

On Wed, Sep 02, 2009 at 09:17:44PM +0200, Peter Zijlstra wrote:
> On Wed, 2009-09-02 at 14:27 +0200, Nick Piggin wrote:
> 
> > It seems like nearly 2/3 of the cost is here:
> >         /* Add the callback to our list. */
> >         *rdp->nxttail[RCU_NEXT_TAIL] = head; <<<
> >         rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
> > 
> > In loading the pointer to the next tail pointer. If I'm reading the profile
> > correctly. Can't see why that should be a probem though...
> > 
> > ffffffff8107dee0 <__call_rcu>: /* __call_rcu total: 320971 100.000 */
> >    697  0.2172 :ffffffff8107dee0:       push   %r12
> 
> >    921  0.2869 :ffffffff8107df57:       push   %rdx
> >    151  0.0470 :ffffffff8107df58:       popfq
> > 183507 57.1725 :ffffffff8107df59:       mov    0x50(%rbx),%rax
> >    995  0.3100 :ffffffff8107df5d:       mov    %rdi,(%rax)
> 
> I'd guess at popfq to be the expensive op here.. skid usually causes the
> attribution to be a few ops down the line.

I believe that Nick's workload is routinely driving the number of
callbacks queued on a given CPU above 10,000, which would provoke numerous
(and possibly inlined) calls to force_quiescent_state().  Like about
400,000 such calls per second.  Hey, I was naively assuming that no one
would see more than 10,000 callbacks queued on a single CPU unless there
was some sort of major emergency underway, and coded accordingly.  ;-)

I offer the attached experimental (untested, might not even compile) patch.

							Thanx, Paul

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