On Wed, Sep 03, 2014 at 08:03:06AM -0700, Paul E. McKenney wrote: > > > Normal RCU grace periods avoid this by synchronizing on a lock acquired by > > > the RCU CPU-hotplug notifiers, but this does not work for the expedited > > > grace periods because the outgoing CPU can be running random tasks for > > > quite some time after RCU's notifier executes. So the fix is just to > > > drop back to a normal grace period when there is a CPU-hotplug operation > > > in progress. > > > > So why are we 'normally' doing an expedited call here anyhow? > > Presumably because they set either the boot parameter or > the sysfs variable that causes synchronize_sched() to so > synchronize_sched_expedited().
That's not a why but a how. Why does that option exist, why are we doing this? I cannot actually find a sysfs variable that controls this though; only the rcu_pm_notifier. It seems to favour doing an expedited call when suspending on 'small' machines. > > But those are not within hotplug bits. Also weren't we removing them? I > > thought we didn't appreciate spraying IPIs like they do? > > I hadn't heard anything about removing them, but making the > expedited primitives a bit less IPI-happy is on my list. I had some recollections of removing a fair number of expedited calls, but its was a long while ago so what do I know ;-) Making them less IPI happy would be good indeed. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/