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

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Message-ID: <20150624140243.GC3892@linux.vnet.ibm.com>
Date:	Wed, 24 Jun 2015 07:03:11 -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 03:43:37PM +0200, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > 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.  ;-)
> 
> 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 ...

Sounds good, especially when I recall that the default RCU CPU stall
warning timeout is 21,000 milliseconds...  ;-)

							Thanx, Paul

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