Re: [PATCH] Avoid mask based num_possible_cpus and num_online_cpus-v5

[David Daney]

On 02/27/2012 02:07 PM, Andrew Morton wrote:
> On Mon, 27 Feb 2012 13:55:55 -0800
> David Daney<ddaney.cavm@xxxxxxxxx>  wrote:
>
> > On 01/31/2012 04:17 PM, Venkatesh Pallipadi wrote:
> > > Kernel's notion of possible cpus (from include/linux/cpumask.h)
> > >    *     cpu_possible_mask- has bit 'cpu' set iff cpu is populatable
> > >
> > >    *  The cpu_possible_mask is fixed at boot time, as the set of CPU id's
> > >    *  that it is possible might ever be plugged in at anytime during the
> > >    *  life of that system boot.
> > >
> > >    #define num_possible_cpus()     cpumask_weight(cpu_possible_mask)
> > >
> > > and on x86 cpumask_weight() calls hweight64 and hweight64 (on older kernels
> > > and systems with !X86_FEATURE_POPCNT) or a popcnt based alternative.
> > >
> > > i.e, We needlessly go through this mask based calculation everytime
> > > num_possible_cpus() is called.
> > >
> > > The problem is there with cpu_online_mask() as well, which is fixed value at
> > > boot time in !CONFIG_HOTPLUG_CPU case and should not change that often even
> > > in HOTPLUG case.
> > >
> > > Though most of the callers of these two routines are init time (with few
> > > exceptions of runtime calls), it is cleaner to use variables
> > > and not go through this repeated mask based calculation.
> > >
> > > Signed-off-by: Venkatesh Pallipadi<venki@xxxxxxxxxx>
> > > Acked-by: Srivatsa S. Bhat<srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> > > Acked-by: KOSAKI Motohiro<kosaki.motohiro@xxxxxxxxxxxxxx>
> >
> > How is it that this patch got merged to linux-next before all the
> > cleanup patches for nr_online_cpus?
>
> <spends five minutes searching mailing list archives>
>
> I for one do not have a clue what patches the term "cleanup patches for
> nr_online_cpus" refers to.  Patches have names - please use them!

Sorry about that.  I was a little hasty:

  From:	Venkatesh Pallipadi <venki@xxxxxxxxxx>
  Subject: [PATCH 0/3] Cleanup raw handling of online/possible map
  Date:	Tue, 14 Feb 2012 14:49:41 -0800
  Message-Id: <1329259784-20592-1-git-send-email-venki@xxxxxxxxxx>

I don't really know a better way to refer to them.

David Daney



> >    From the looks of your follow-on patches it would seem that all MIPS,
> > hexagon, and um are now broken.
> >
> > I know for a fact that MIPS doesn't boot because of this.
>
> I shall drop
> cpumask-avoid-mask-based-num_possible_cpus-and-num_online_cpus.patch.
>
> That patch was sent as a single standalone patch and the changelog had
> no mention of any needed preparatory patches.  If resending, please
> send *all* patches in a single sequence-numbered series.  We know how
> to do this.

It is possible that Venkatesh did not know about the breakage when the 
original patch was sent
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