linux-kernel - Re: [RFC][PATCH 12/13] stop

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Date:	Wed, 24 Jun 2015 06:39:31 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>, tj@...nel.org,
	mingo@...hat.com, linux-kernel@...r.kernel.org, der.herr@...r.at,
	dave@...olabs.net, riel@...hat.com, viro@...IV.linux.org.uk,
	torvalds@...ux-foundation.org
Subject: Re: [RFC][PATCH 12/13] stop_machine: Remove lglock

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

> 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

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