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SubjectRe: [PATCH] Introduce nodemask_t ADT [0/7]
Other ways to reduce cpumask copies:

1) Additional macros, for example a boolean cpus_intersect(x,y), which
is true iff x overlaps y, and saves the tmp cpumask in the code:

cpus_and(tmp, target_cpu_mask, allowed_mask);
if (!cpus_empty(tmp)) {

or a cpus_subset(x,y) "is x a subset of y" macro to replace this:

cpus_and(tmp, cpumask, cpu_callout_map);
BUG_ON(!cpus_equal(cpumask, tmp));

with this:

BUG_ON(!cpus_subset(cpumask, cpu_callout_map));

2) Using existing macros more carefully, for example using cpu_set() to
set the bit in the following, and saving the copy by assignment:

pending_irq_balance_cpumask[irq] = cpumask_of_cpu(new_cpu);

or using the existing cpu_isset() macro, replacing the code (seen
in part above ;):

cpus_and(allowed_mask, cpu_online_map, irq_affinity[selected_irq]);
target_cpu_mask = cpumask_of_cpu(min_loaded);
cpus_and(tmp, target_cpu_mask, allowed_mask);
if (!cpus_empty(tmp)) {

with the code:

if (cpu_isset(min_loaded, cpu_online_map) && cpu_isset(min_loaded, irq_affinity[selected_irq]) {

The current nested and ifdef'd complexity of the cpumask macro headers works
against such efficient coding - it's non-trivial to see just what macros are
available. The youngins reading this may not appreciate the value of reducing
brain load; but the oldins might.

2) Pass a cpumask pointer to a function that generates a cpumask instead of
taking one as a return value, letting the called function fill in the memory
so referenced. We should not be trying to hide such details, unless we can
do so seamlessly and consistent with traditional 'C' semantics.

3) Passing a const cpumask pointer to a function that examines a cpumask
instead of passing the cpumask by value (on small systems, its one word
either way - on big systems, it saves copying a multiword mask on the

4) Throwing away dead code:

static int physical_balance = 0;
cpumask_t tmp;
cpus_shift_right(tmp, cpu_online_map, 2);
if (smp_num_siblings > 1 && !cpus_empty(tmp)
physical_balance = 1;

The above code fragments are from arch/i386 in 2.6.3-rc1-mm1.

I won't rest till it's the best ...
Programmer, Linux Scalability
Paul Jackson <> 1.650.933.1373
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