Re: [RFC][PATCH 12/13] stop_machine: Remove lglock

[Peter Zijlstra]

On Wed, Jun 24, 2015 at 07:50:42AM -0700, Paul E. McKenney wrote:
> On Wed, Jun 24, 2015 at 09:35:03AM +0200, Peter Zijlstra wrote:

> > I still don't see a problem here though; the stop_one_cpu() invocation
> > for the CPU that's suffering its preemption latency will take longer,
> > but so what?
> > 
> > How does polling and dropping back to sync_rcu() generate better
> > behaviour than simply waiting for the completion?
> 
> Because if there is too much delay, synchronize_rcu() is no slower
> than is synchronize_rcu_expedited(), plus synchronize_rcu() is much
> more efficient.

Still confused.. How is polling and then blocking more efficient than
just blocking in the first place? I'm seeing the polling as a waste of
cpu time.

The thing is, if we're stalled on a stop_one_cpu() call, the sync_rcu()
is equally stalled. The sync_rcu() cannot wait more efficient than we're
already waiting either.


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/