Re: [RFC][PATCH 12/13] stop_machine: Remove lglock

[Paul E. McKenney]

On Wed, Jun 24, 2015 at 10:42:48AM +0200, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.  ;-)

> 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?

							Thanx, Paul

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/