Messages in this thread | | | Date | Thu, 7 Dec 2023 18:49:20 -0800 | From | Yury Norov <> | Subject | Re: [PATCH v2 2/6] lib/group_cpus: relax atomicity requirement in grp_spread_init_one() |
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On Fri, Dec 08, 2023 at 09:31:27AM +0800, Ming Lei wrote: > On Thu, Dec 07, 2023 at 12:38:56PM -0800, Yury Norov wrote: > > Because nmsk and irqmsk are stable, extra atomicity is not required. > > > > Signed-off-by: Yury Norov <yury.norov@gmail.com> > > --- > > lib/group_cpus.c | 9 ++++----- > > 1 file changed, 4 insertions(+), 5 deletions(-) > > > > diff --git a/lib/group_cpus.c b/lib/group_cpus.c > > index ee272c4cefcc..8eb18c6bbf3b 100644 > > --- a/lib/group_cpus.c > > +++ b/lib/group_cpus.c > > @@ -24,8 +24,8 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk, > > if (cpu >= nr_cpu_ids) > > return; > > > > - cpumask_clear_cpu(cpu, nmsk); > > - cpumask_set_cpu(cpu, irqmsk); > > + __cpumask_clear_cpu(cpu, nmsk); > > + __cpumask_set_cpu(cpu, irqmsk); > > cpus_per_grp--; > > > > /* If the cpu has siblings, use them first */ > > @@ -34,9 +34,8 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk, > > sibl = cpumask_next(sibl, siblmsk); > > if (sibl >= nr_cpu_ids) > > break; > > - if (!cpumask_test_and_clear_cpu(sibl, nmsk)) > > - continue; > > - cpumask_set_cpu(sibl, irqmsk); > > + __cpumask_clear_cpu(sibl, nmsk); > > + __cpumask_set_cpu(sibl, irqmsk); > > cpus_per_grp--; > > Here the change isn't simply to remove atomicity, and the test > part of cpumask_test_and_clear_cpu() is removed, so logic is changed, > I feel the correct change should be: > > if (cpumask_test_cpu(sibl, nmsk)) { > __cpumask_clear_cpu(sibl, nmsk); > __cpumask_set_cpu(sibl, irqmsk); > cpus_per_grp--; > }
Ohh. My mistake is that I put this patch prior to the #3, so people bisecting the kernel may hit this problem...
You're right here, but check the following patch: it switches the for() loop to for_each_cpu_and_from(sibl, siblmsk, nmsk), and it means that inside the loop sibl indexes set bits in both siblmsk and nmsk.
Now, because both masks are stable when the grp_spread_init_one() is called, there's no chance to get nmks.sibl cleared suddenly, and it means we can just drop the check.
Does this makes sense to you?
I can send v3 with a proper order of patches, if needed.
Thanks, Yury
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