linux-kernel - Re: [patch 05/52] lglock: introduce special lglock and brlock spin locks

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Date:	Fri, 25 Jun 2010 11:50:02 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Nick Piggin <npiggin@...e.de>
cc:	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	John Stultz <johnstul@...ibm.com>,
	Frank Mayhar <fmayhar@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch 05/52] lglock: introduce special lglock and brlock spin
 locks

On Fri, 25 Jun 2010, Nick Piggin wrote:
> On Thu, Jun 24, 2010 at 08:15:54PM +0200, Thomas Gleixner wrote:
> > On Thu, 24 Jun 2010, npiggin@...e.de wrote:
> > 
> > > +#define DEFINE_LGLOCK(name)						\
> > > +									\
> > > + DEFINE_PER_CPU(arch_spinlock_t, name##_lock);				\
> > 
> > Uuurgh. You want to make that an arch_spinlock ? Just to avoid the
> > preempt_count overflow when you lock all cpu locks nested ?
> 
> Yep, and the lockdep wreckage too :)
> 
> Actually it's nice to avoid the function call too (lglock/brlocks
> are already out of line). Calls aren't totally free, especially
> on small chips without RSBs. And even with RSBs it's helpful not
> to overflow them, although Nehalem seems to have 12-16 entries.
>  
> 
> > I'm really not happy about that, it's going to be a complete nightmare
> > for RT. If you wanted to make this a present for RT giving the
> > scalability stuff massive testing, then you failed miserably :)
> 
> Heh, it's a present for -rt because the locking is quite isolated
> (I did the same thing with hashtable bitlocks, added a new structure
> for them, in case you prefer spinlocks than bit spinlocks there).

Sure, bitlocks are equally horrible.

> -rt already changes locking primitives, so in the worst case you
> might have to tweak this. My previous patches were open coding
> these locks in fs/ so I can understand why that was a headache.

I agree that having the code isolated makes my life easier, but I'm a
bit worried about the various new locking primitives which pop up in
all corners of the kernel.

> I think I'll keep it as is for now, it's hard enough to keep single
> threaded performance up. But it should be much easier to override
> this in -rt and I'll be happy to try restructuring things to help rt
> if and when it's possible.

Ok, lets see how bad it gets :)

Thanks,

	tglx
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