diff mbox series

[3/3] sched/nohz: Remove weird error handling from find_new_ilb()

Message ID 20231006102518.2452758-4-mingo@kernel.org
State New
Headers
Series sched/nohz: Misc minor cleanups to the NOHZ idle balancing code |

Commit Message

Ingo Molnar Oct. 6, 2023, 10:25 a.m. UTC 
  find_new_ilb() returns nr_cpu_ids on failure - which is a weird
choice in itself: not only is it a global variable, it is
a +1 out of bounds CPU index...

Its only user, kick_ilb(), then checks the return against nr_cpu_ids
to decide to return.

Instead of this, use a standard -1 return on failure to find an
idle CPU, as the argument is signed already.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org
---
 kernel/sched/fair.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

Comments

Valentin Schneider Oct. 9, 2023, 3:11 p.m. UTC | #1 
On 06/10/23 13:01, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@infradead.org> wrote:
>
>> On Fri, Oct 06, 2023 at 12:25:18PM +0200, Ingo Molnar wrote:
>> > find_new_ilb() returns nr_cpu_ids on failure - which is a weird
>> > choice in itself: not only is it a global variable, it is
>> > a +1 out of bounds CPU index...
>> 
>> FWIW this is what all the cpumask bitops return when they've exhausted
>> the mask. Eg. no bits left set etc..
>
> yeah, which then results in type-forcing uglies like:
>
>    kernel/events/core.c:	if ((unsigned)cpu >= nr_cpu_ids) {
>    kernel/events/core.c:   if ((unsigned)cpu >= nr_cpu_ids) {
>    kernel/smp.c:	   if ((unsigned)cpu >= nr_cpu_ids || !cpu_online(cpu)) {

I can't see why we'd want smp_call_function_single*() /
generic_exec_single() to take a signed int as input, shouldn't this just be
unsigned?

The perf thing does look like it wants signed though...

>
> :-/
>
> So I don't think this is a particularly well thought-out interface.
>
> Thanks,
>
> 	Ingo
diff mbox series

Patch

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d4e90d15bd77..dad60576cf56 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11572,7 +11572,7 @@  static inline int find_new_ilb(void)
 			return ilb_cpu;
 	}
 
-	return nr_cpu_ids;
+	return -1;
 }
 
 /*
@@ -11593,8 +11593,7 @@  static void kick_ilb(unsigned int flags)
 		nohz.next_balance = jiffies+1;
 
 	ilb_cpu = find_new_ilb();
-
-	if (ilb_cpu >= nr_cpu_ids)
+	if (ilb_cpu < 0)
 		return;
 
 	/*