Re: cpu_exclusive sched domains fix broke ppc64

[Paul Jackson]

Paul Mackerras wrote:
> I'm not sure what the best way to fix this is

Thank-you for reporting this.  Likely the best way to fix this for now,
since we are late in a release (Linus will probably want to wack me
upside the head for breaking his build ;) is to leave the
node_to_cpumask and for_each_cpu_mask exactly as they are, and have the
code that my cpu_exclusive sched domain patch added make a local copy
of the cpumask.

I just sent off a patch to do this - quite untested so far.

I am trying now to get fire up crosstools to verify the build.
But if you can get it to build anytime soon, let me know.  My
crosstools are rusty -- it might take me a bit to resuscitate them.

I also am not sure what is the best way to fix this detail with
node_to_cpumask and for_each_cpu_mask in the long term.  The choices I
see are:

 1) Leave it be - which makes it easy trip the build bug I hit,
    due to the different styles of node_to_cpumask, inline or
    macro, on different archs.

 2) Make node_to_cpumask a macro on all archs, though that
    makes it even easier than it is now to write code that
    appears to modify a local variable, but actually modifies
    some global array of the per-node cpumasks, which could
    lead to some juicy runtime bugs.

 3) Make node_to_cpumask an inline on all archs, though that might
    force a local stack copy of a cpumask in places that might
    be performance critical on arch's with big cpumasks.

 4) Perhaps some more subtle combination of macros/inlines
    can be all things to all arch's.

I'm not going to unravel the above tonight.

> it seems unfortunate that for_each_cpu_mask
> requires the mask to be an lvalue, but that isn't documented anywhere
> that I can see.

Are you saying that it's unfortunate that for_each_cpu_mask requires
an lvalue, or that it's unfortunate that this isn't documented?

Or both ;).

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@xxxxxxx> 1.925.600.0401
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