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

Nick Piggin
2010-06-25T10:11:24Z

On Fri, Jun 25, 2010 at 11:50:02AM +0200, Thomas Gleixner wrote:
> 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@suse.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.

Be sure to shout at people for it :). Locking primitives really need
to be reviewed, and having them in common places lets other people
use them too.

> > 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 :)

Ok good.