Eric Dumazet <[email protected]> wrote:
>
> Andrew Morton a écrit :
> > Andi Kleen <[email protected]> wrote:
> >> On Thursday 09 February 2006 19:04, Andrew Morton wrote:
> >>> Ashok Raj <[email protected]> wrote:
> >>>> The problem was with ACPI just simply looking at the namespace doesnt
> >>>> exactly give us an idea of how many processors are possible in this platform.
> >>> We need to fix this asap - the performance penalty for HOTPLUG_CPU=y,
> >>> NR_CPUS=lots will be appreciable.
> >> What is this performance penalty exactly?
> >
> > All those for_each_cpu() loops will hit NR_CPUS cachelines instead of
> > hweight(cpu_possible_map) cachelines.
>
> You mean NR_CPUS bits, mostly all included in a single cacheline, and even in
> a single long word :) for most cases (NR_CPUS <= 32 or 64)
>
No, I mean cachelines:
static void recalc_bh_state(void)
{
int i;
int tot = 0;
if (__get_cpu_var(bh_accounting).ratelimit++ < 4096)
return;
__get_cpu_var(bh_accounting).ratelimit = 0;
for_each_cpu(i)
tot += per_cpu(bh_accounting, i).nr;
That's going to hit NR_CPUS cachelines even on a 2-way.
Or am I missing something really obvious here?
(Probably the most expensive ones will be get_page_state() and friends.
And argh, they're still hardwired to CPU_MASK_ALL).
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