Re: [RFC][PATCH] make global bitlock waitqueues per-node

[Hugh Dickins]

On Wed, 21 Dec 2016, Linus Torvalds wrote:
> On Wed, Dec 21, 2016 at 12:32 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > FWIW, here's mine.. compiles and boots on a NUMA x86_64 machine.
> 
> So I like how your patch is smaller, but your patch is also broken.
> 
> First off, the whole contention bit is *not* NUMA-specific. It should
> help non-NUMA too, by avoiding the stupid extra cache miss.
> 
> Secondly, CONFIG_NUMA is a broken thing to test anyway, since adding a
> bit for the NUMA case can overflow the page flags as far as I can tell
> (MIPS seems to support NUMA on 32-bit, for example, but I didn't
> really check the Kconfig details). Making it dependent on 64-bit might
> be ok (and would fix the issue above - I don't think we really need to
> care too much about 32-bit any more)
> 
> But making it conditional at all means that now you have those two
> different cases for this, which is a maintenance nightmare. So don't
> do it even if we could say "screw 32-bit".
> 
> Anyway, the conditional thing could be fixed by just taking Nick's
> patch 1/2, and your patch (with the conditional bits stripped out).

Yup.

> 
> I do think your approach of just re-using the existing bit waiting
> with just a page-specific waiting function is nicer than Nick's "let's
> just roll new waiting functions" approach. It also avoids the extra
> initcall.
> 
> Nick, comments?
> 
> Hugh - mind testing PeterZ's patch too? My comments about the
> conditional PG_waiters bit and page bit overflow are not relevant for
> your particular scenario, so you can ignore that part, and just take
> PaterZ's patch directly.

Right, I put them both through some loads yesterday and overnight:
Peter's patch and Nick's patch each work fine here, no issues seen
with either (but I didn't attempt to compare them, aesthetically
nor in performance).

Hugh