linux-kernel - Re: [patch 2/3] scheduler: add full memory barriers upon task switch at runqueue lock/unlock

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Date:	Mon, 1 Feb 2010 21:49:01 +1100
From:	Nick Piggin <npiggin@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Nicholas Miell <nmiell@...cast.net>, laijs@...fujitsu.com,
	dipankar@...ibm.com, josh@...htriplett.org, dvhltc@...ibm.com,
	niv@...ibm.com, tglx@...utronix.de, Valdis.Kletnieks@...edu,
	dhowells@...hat.com
Subject: Re: [patch 2/3] scheduler: add full memory barriers upon task
 switch at runqueue lock/unlock

On Mon, Feb 01, 2010 at 11:36:01AM +0100, Peter Zijlstra wrote:
> On Mon, 2010-02-01 at 21:11 +1100, Nick Piggin wrote:
> > All, but one at a time, no? How much of a DoS really is taking these
> > locks for a handful of cycles each, per syscall?
> 
> I was more worrying about the cacheline trashing than lock hold times
> there.

Well, same issue really. Look at all the unprived files in /proc
for example that can look through all per-cpu cachelines. It just
takes a single read syscall to do a lot of them too.

 
> > I mean, we have LOTS of syscalls that take locks, and for a lot longer,
> > (look at dcache_lock).
> 
> Yeah, and dcache is a massive pain, isn't it ;-)

My point is, I don't think it is something we can realistically
care much about and it is nowhere near a new or unique problem
being added by this one patch.

It is really a RoS, reduction of service, rather than a DoS. And
any time we allow an unpriv user on our system, we have RoS potential :)

 
> > I think we basically just have to say that locking primitives should be
> > somewhat fair, and not be held for too long, it should more or less
> > work.
> 
> Sure, it'll more of less work, but he's basically making rq->lock a
> global lock instead of a per-cpu lock.
> 
> > If the locks are getting contended, then the threads calling
> > sys_membarrier are going to be spinning longer too, using more CPU time,
> > and will get scheduled away...
> 
> Sure, and increased spinning reduces the total throughput.
> 
> > If there is some particular problem on -rt because of the rq locks,
> > then I guess you could consider whether to add more overhead to your
> > ctxsw path to reduce the problem, or simply not support sys_membarrier
> > for unprived users in the first place.
> 
> Right, for -rt we might need to do that, but its just that rq->lock is a
> very hot lock, and adding basically unlimited trashing to it didn't seem
> like a good idea.
> 
> Also, I'm thinking making it a priv syscall basically renders it useless
> for Mathieu.

Well I just mean that it's something for -rt to work out. Apps can
still work if the call is unsupported completely.
 

> Anyway, it might be I'm just paranoid... but archs with large core count
> and lazy tlb flush seem particularly vulnerable.
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