* Paul E. McKenney <paul...@linux.vnet.ibm.com> wrote:
> On Wed, Jun 24, 2015 at 10:42:48AM +0200, Ingo Molnar wrote:
> >
> > * Peter Zijlstra <pet...@infradead.org> wrote:
> >
> > > On Tue, Jun 23, 2015 at 11:26:26AM -0700, Paul E. McKenney wrote:
> > > > >
> > > > > I really think you're making that expedited nonsense far too
> > > > > accessible.
> > > >
> > > > This has nothing to do with accessibility and everything to do with
> > > > robustness. And with me not becoming the triage center for too many
> > > > non-RCU
> > > > bugs.
> > >
> > > But by making it so you're rewarding abuse instead of flagging it :-(
> >
> > Btw., being a 'triage center' is the bane of APIs that are overly
> > successful,
> > so we should take that burden with pride! :-)
>
> I will gladly accept that compliment.
>
> And the burden. But, lazy as I am, I intend to automate it. ;-)
lol :)
> > Lockdep (and the scheduler APIs as well) frequently got into such
> > situations as
> > well, and we mostly solved it by being more informative with debug splats.
> >
> > I don't think a kernel API should (ever!) stay artificially silent, just
> > for fear
> > of flagging too many problems in other code.
>
> I agree, as attested by RCU CPU stall warnings, lockdep-RCU, sparse-based
> RCU checks, and the object-debug-based checks for double call_rcu().
> That said, in all of these cases, including your example of lockdep,
> the diagnostic is a debug splat rather than a mutex-contention meltdown.
> And it is the mutex-contention meltdown that I will continue making
> synchronize_sched_expedited() avoid.
>
> But given the change from bulk try_stop_cpus() to either stop_one_cpu() or
> IPIs, it would not be hard to splat if a given CPU didn't come back fast
> enough. The latency tracer would of course provide better information,
> but synchronize_sched_expedited() could do a coarse-grained job with
> less setup required.
>
> My first guess for the timeout would be something like 500 milliseconds.
> Thoughts?
So I'd start with 5,000 milliseconds and observe the results first ...
Thanks,
Ingo
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