On Fri, Jun 20, 2014 at 12:04:34PM -0700, j...@joshtriplett.org wrote:
> On Fri, Jun 20, 2014 at 11:32:49AM -0700, Paul E. McKenney wrote:
> > Hello!
> >
> > This series contains changes to address the performance regressions
> > introduced by commit ac1bea85781e (Make cond_resched() report RCU
> > quiescent states), which was in turn fixing a problem where tasks looping
> > in the kernel could delay RCU grace periods. The changes in this series
> > are as follows:
> >
> > 1. Reduce the overhead of checking added to cond_resched() and friends.
> >
> > 2. Add a new cond_resched_rcu_qs() to provide RCU quiescent states
> > even if cond_resched() should stop doing so.
> >
> > 3. Add a new RCU_COND_RESCHED_QS to prevent cond_resched() from
> > reporting RCU quiescent states.
> >
> > 4. Prevent rcutorture testing from reporting spurious RCU CPU stall
> > warnings, and also to test RCU_COND_RESCHED_QS.
> >
> > 5. Provides a boot/sysfs rcutree.jiffies_till_cond_resched_qs
> > parameter to replace the magic "7".
>
> For all five patches:
>
> Reviewed-by: Josh Triplett <j...@joshtriplett.org>
Thank you, added!
> Glad to see this doesn't add any overhead to rcutiny.
I suppose I should explain why that is...
First, single-CPU systems tend not to have thousands of mass-storage
devices, processes with many thousands of open files, or terabytes
of memory. Of course, in theory, a single-CPU system -could- have all
those things, but in practice thus far, they don't. Therefore, the
looping-in-the-kernel behavior that these things can cause simply don't
happen on single-CPU systems. Maybe some day they will, at which point
we can simply re-enable TREE_RCU for !SMP systems, so that those huge
single-CPU systems can use TREE_RCU, which has the needed protections.
Small embedded systems would of course still be able to benefit from
TINY_RCU.
In addition, single-CPU systems by definition have but on CPU. This
means that having a single runnable process on that CPU for tens of
seconds is much less likely, which eliminates another class of possible
indefinite-grace-period-extension bugs. In addition, the situations
where a bunch of CPUs "gang up" on a single CPU, generating endless
cleanup work for that CPU, also cannot happen on a single-CPU system.
This in turn eliminates the "grace-period extension via unending
cleanup" class of bugs.
Make sense?
Thanx, Paul
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