linux-kernel - Re: [patch V2 2/7] futex: Hash private futexes per process

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Message-ID: <20160527163617.GA28561@breakpoint.cc>
Date:	Fri, 27 May 2016 18:36:17 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Darren Hart <dvhart@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Darren Hart <darren@...art.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Davidlohr Bueso <dave@...olabs.net>, Chris Mason <clm@...com>,
	Carlos O'Donell <carlos@...hat.com>,
	Torvald Riegel <triegel@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [patch V2 2/7] futex: Hash private futexes per process

On 2016-05-07 10:44:39 [+0200], Thomas Gleixner wrote:
> On Fri, 6 May 2016, Darren Hart wrote:
> > On Thu, May 05, 2016 at 08:44:04PM -0000, Thomas Gleixner wrote:
> Sure, we need to serialize attempts to populate the hash. Especially in the
> non preallocated case. The thing with raw vs. non raw spinlocks is that the
> latter are expensive on RT and if there are just 5 instructions to protect it
> does not make any sense to chose the heavy version.
>  
> > > +config FUTEX_PRIVATE_HASH
> > > +	bool
> > > +	default FUTEX && SMP
> > > +
> > 
> > So no prompt, not user selectable. If you have SMP, you get this? I think
> > automatic is a good call... but is SMP the right criteria, or would NUMA be more
> > appropriate since I thought it was keeping the hash local to the NUMA node that
> > was the big win?
> 
> Yes, we can make it depend on NUMA. I even thought about making a run time
> decision for non preallocated ones when the machine is not numa. But for test
> coverage I wanted to have it as widely used as possible.

Do we want to change it to autodetect NUMA at runtime or do we want to
keep it as is for now?

Sebastian