Messages in this thread | | | Date | Wed, 3 Sep 2014 09:38:52 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu] Eliminate deadlock between CPU hotplug and expedited grace periods |
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On Wed, Sep 03, 2014 at 05:28:50PM +0200, Peter Zijlstra wrote: > 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?
As you say below, to reduce RCU grace-period latency on small systems. And one level of abstraction's why is another level's how. ;-)
> 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.
See rcu_expedited_store() in kernel/ksysfs.c. Or the rcu_expedited module_param() in kernel/rcu/update.c.
> > > 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 ;-)
If a given use case can tolerate the latency of normal calls, then the normal calls certainly are preferable, no two ways about it.
> Making them less IPI happy would be good indeed.
Priority of this task duly increased. ;-)
Thanx, Paul
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