Messages in this thread | | | Date | Wed, 13 Dec 2023 08:06:48 +0800 | From | Ming Lei <> | Subject | Re: [PATCH v3 2/7] lib/group_cpus: optimize inner loop in grp_spread_init_one() |
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On Tue, Dec 12, 2023 at 09:04:19AM -0800, Yury Norov wrote: > On Tue, Dec 12, 2023 at 05:46:53PM +0800, Ming Lei wrote: > > On Mon, Dec 11, 2023 at 08:21:02PM -0800, Yury Norov wrote: > > > The loop starts from the beginning every time we switch to the next > > > sibling mask. This is the Schlemiel the Painter's style of coding > > > because we know for sure that nmsk is clear up to current CPU, and we > > > can just continue from the next CPU. > > > > > > Also, we can do it nicer if leverage the dedicated for_each() iterator, > > > and simplify the logic of clearing a bit in nmsk. > > > > > > Signed-off-by: Yury Norov <yury.norov@gmail.com> > > > --- > > > lib/group_cpus.c | 13 ++++++------- > > > 1 file changed, 6 insertions(+), 7 deletions(-) > > > > > > diff --git a/lib/group_cpus.c b/lib/group_cpus.c > > > index ee272c4cefcc..10dead3ab0e0 100644 > > > --- a/lib/group_cpus.c > > > +++ b/lib/group_cpus.c > > > @@ -30,14 +30,13 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk, > > > > > > /* If the cpu has siblings, use them first */ > > > siblmsk = topology_sibling_cpumask(cpu); > > > - for (sibl = -1; cpus_per_grp > 0; ) { > > > - sibl = cpumask_next(sibl, siblmsk); > > > - if (sibl >= nr_cpu_ids) > > > - break; > > > - if (!cpumask_test_and_clear_cpu(sibl, nmsk)) > > > - continue; > > > + sibl = cpu + 1; > > > > It doesn't have to 'cpu + 1', see below comment. > > > > > + > > > + for_each_cpu_and_from(sibl, siblmsk, nmsk) { > > > + cpumask_clear_cpu(sibl, nmsk); > > > cpumask_set_cpu(sibl, irqmsk); > > > - cpus_per_grp--; > > > + if (cpus_per_grp-- == 0) > > > > if (--cpus_per_grp == 0) > > That's right, I'll send a new version this weekend. > > > > + return; > > > > I think for_each_cpu_and() should work just fine, cause cpu has been cleared > > from nmsk, so the change can be something like, then patch 1 isn't > > necessary. > > It works just fine except that it's O(N^2), where O(N) is easily > achievable. Again, it's not about performance, it's about coding > habits.
Both for_each_cpu_and() and for_each_cpu_and_from() are O(N), aren't they? Given both two are based on find_next_and_bit().
for_each_cpu_and() is simpler and more readable, and more importantly, we can save one single-user public helper.
> > > for_each_cpu_and(sibl, siblmsk, nmsk) { > > cpumask_clear_cpu(sibl, nmsk); > > cpumask_set_cpu(sibl, irqmsk); > > if (--cpus_per_grp == 0) > > return; > > }
Thanks, Ming
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