On Tue, Jul 29, 2014 at 09:55:36AM +0200, Peter Zijlstra wrote:
> On Mon, Jul 28, 2014 at 03:56:13PM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
> >
> > RCU-tasks requires the occasional voluntary context switch
> > from CPU-bound in-kernel tasks. In some cases, this requires
> > instrumenting cond_resched(). However, there is some reluctance
> > to countenance unconditionally instrumenting cond_resched() (see
> > http://lwn.net/Articles/603252/),
>
> No, if its a good reason mention it, if not ignore it.
Fair enough. ;-)
> > so this commit creates a separate
> > cond_resched_rcu_qs() that may be used in place of cond_resched() in
> > locations prone to long-duration in-kernel looping.
>
> Sounds like a pain and a recipe for mistakes. How is joe kernel hacker
> supposed to 1) know about this new api, and 2) decide which to use?
>
> Heck, even I wouldn't know, and I just read the damn patch.
When Joe Hacker gets stall warning messages due to loops in the kernel
that contain cond_resched(), that is a hint that cond_resched_rcu_qs()
is required. These stall warnings can occur when using RCU-tasks and when
using normal RCU in NO_HZ_FULL kernels in cases where the scheduling-clock
interrupt is left off while executing a long code path in the kernel.
(Of course, in both cases, another eminently reasonable fix is to shorten
the offending code path in the kernel.)
I should add words to that effect to Documentation/RCU/stallwarn.txt,
shouldn't I? Done.
Thanx, Paul
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