Re: [patch] tlb flush_data: replace per_cpu with an array

Ingo Molnar
2009-01-13T00:01:37Z

* Mike Travis <travis@sgi.com> wrote:

> Frederik Deweerdt wrote:
> > On Mon, Jan 12, 2009 at 02:54:25PM -0800, Mike Travis wrote:
> >> Frederik Deweerdt wrote:
> >>> Hi,
> >>>
> >>> On x86_64 flush tlb data is stored in per_cpu variables. This is
> >>> unnecessary because only the first NUM_INVALIDATE_TLB_VECTORS entries
> >>> are accessed.
> >>> This patch aims at making the code less confusing (there's nothing
> >>> really "per_cpu") by using a plain array. It also would save some memory
> >>> on most distros out there (Ubuntu x86_64 has NR_CPUS=64 by default).
> >>>
> >>> Regards,
> >>> Frederik
> >>>
> >>> Signed-off-by: Frederik Deweerdt <frederik.deweerdt@xprog.eu>
> >> Here is the net change in memory usage with this patch on a allyesconfig
> >> with NR_CPUS=4096.
>
> > Yes, this point wrt. memory was based on my flawed understanding of how
> > per_cpu actually allocates the data. There is however 1) a confusing use
> > of per_cpu removed, 2) faster access to the flush data.
>
> Is this true? On a widely separated NUMA system, requiring all CPU's to
> access memory on NODE 0 for every tlb flush would seem expensive.
> That's another benefit of per_cpu data, it's local to the node's cpus.

That's irrelevant here: we only have 8 IPI slots for the TLB flush, so
we'll use the first 8 per_cpu areas (note how all access is cpu-nr modulo
8 in essence). That is going to be node 0 anyway in most NUMA setups.

> (And was it determined yet, that a cacheline has to be tossed around as
> well?)

No, that assertion of Andi is wrong.

Ingo