linux-kernel - [PATCH 3/9] lib/group_cpus: relax atomicity requirement in grp_spread_init

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Date: Fri, 19 Jan 2024 18:50:47 -0800
From: Yury Norov <yury.norov@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ming Lei <ming.lei@...hat.com>,
	linux-kernel@...r.kernel.org
Cc: Yury Norov <yury.norov@...il.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Breno Leitao <leitao@...ian.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Zi Yan <ziy@...dia.com>
Subject: [PATCH 3/9] lib/group_cpus: relax atomicity requirement in grp_spread_init_one()

Because nmsk and irqmsk are stable, extra atomicity is not required.

Signed-off-by: Yury Norov <yury.norov@...il.com>
NAKed-by: Ming Lei <ming.lei@...hat.com>
---

Regarding the NAK discussion:

  > > > > I think this kind of change should be avoided, here the code is
  > > > > absolutely in slow path, and we care code cleanness and readability
  > > > > much more than the saved cycle from non atomicity.
  > > >
  > > > Atomic ops have special meaning and special function. This 'atomic' way
  > > > of moving a bit from one bitmap to another looks completely non-trivial
  > > > and puzzling to me.
  > > >
  > > > A sequence of atomic ops is not atomic itself. Normally it's a sing of
  > > > a bug. But in this case, both masks are stable, and we don't need
  > > > atomicity at all.
  > >
  > > Here we don't care the atomicity.
  > >
  > > >
  > > > It's not about performance, it's about readability.
  > >
  > > __cpumask_clear_cpu() and __cpumask_set_cpu() are more like private
  > > helper, and more hard to follow.
  >
  > No that's not true. Non-atomic version of the function is not a
  > private helper of course.
  >
  > > [@linux]$ git grep -n -w -E "cpumask_clear_cpu|cpumask_set_cpu" ./ | wc
  > >     674    2055   53954
  > > [@linux]$ git grep -n -w -E "__cpumask_clear_cpu|__cpumask_set_cpu" ./ | wc
  > >      21      74    1580
  > >
  > > I don't object to comment the current usage, but NAK for this change.
  >
  > No problem, I'll add you NAK.
  
  You can add the following words meantime:
  
  __cpumask_clear_cpu() and __cpumask_set_cpu() are added in commit 6c8557bdb28d
  ("smp, cpumask: Use non-atomic cpumask_{set,clear}_cpu()") for fast code path(
  smp_call_function_many()).
  
  We have ~670 users of cpumask_clear_cpu & cpumask_set_cpu, lots of them
  fall into same category with group_cpus.c(doesn't care atomicity, not in fast
  code path), and needn't change to __cpumask_clear_cpu() and __cpumask_set_cpu().
  Otherwise, this way may encourage to update others into the __cpumask_* version.

 lib/group_cpus.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/lib/group_cpus.c b/lib/group_cpus.c
index 063ed9ae1b8d..0a8ac7cb1a5d 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 */
@@ -36,8 +36,8 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
 			if (cpus_per_grp-- == 0)
 				return;
 
-			cpumask_clear_cpu(sibl, nmsk);
-			cpumask_set_cpu(sibl, irqmsk);
+			__cpumask_clear_cpu(sibl, nmsk);
+			__cpumask_set_cpu(sibl, irqmsk);
 		}
 	}
 }
-- 
2.40.1