The function may call cpumask_equal with tl->mask(cpu) == tl->mask(i),
even when cpu != i. In such case, cpumask_equal() would always return
true, and we can proceed to the next iteration immediately.

Valentin Schneider shares on it:

  PKG can potentially hit that condition, and so can any
  sched_domain_mask_f that relies on the node masks...
  
  I'm thinking ideally we should have checks in place to
  ensure all node_to_cpumask_map[] masks are disjoint,
  then we could entirely skip the levels that use these
  masks in topology_span_sane(), but there's unfortunately
  no nice way to flag them... Also there would be cases
  where there's no real difference between PKG and NODE
  other than NODE is still based on a per-cpu cpumask and
  PKG isn't, so I don't see a nicer way to go about this.

v1: https://lore.kernel.org/lkml/ZrJk00cmVaUIAr4G@yury-ThinkPad/T/
v2:
 - defer initialization of 'mc' in patch #1 @Chen Yu;
 - more comments from Valentin Schneider.


Yury Norov (2):
  sched/topology: pre-compute topology_span_sane() loop params
  sched/topology: optimize topology_span_sane()

 kernel/sched/topology.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

-- 
2.43.0

Ping?

On Wed, Aug 07, 2024 at 12:05:19PM -0700, Yury Norov wrote:
> The function may call cpumask_equal with tl->mask(cpu) == tl->mask(i),
> even when cpu != i. In such case, cpumask_equal() would always return
> true, and we can proceed to the next iteration immediately.
> 
> Valentin Schneider shares on it:
> 
>   PKG can potentially hit that condition, and so can any
>   sched_domain_mask_f that relies on the node masks...
>   
>   I'm thinking ideally we should have checks in place to
>   ensure all node_to_cpumask_map[] masks are disjoint,
>   then we could entirely skip the levels that use these
>   masks in topology_span_sane(), but there's unfortunately
>   no nice way to flag them... Also there would be cases
>   where there's no real difference between PKG and NODE
>   other than NODE is still based on a per-cpu cpumask and
>   PKG isn't, so I don't see a nicer way to go about this.
> 
> v1: https://lore.kernel.org/lkml/ZrJk00cmVaUIAr4G@yury-ThinkPad/T/
> v2:
>  - defer initialization of 'mc' in patch #1 @Chen Yu;
>  - more comments from Valentin Schneider.
> 
> 
> Yury Norov (2):
>   sched/topology: pre-compute topology_span_sane() loop params
>   sched/topology: optimize topology_span_sane()
> 
>  kernel/sched/topology.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> -- 
> 2.43.0

Ping again?

On Wed, Aug 14, 2024 at 09:35:04AM -0700, Yury Norov wrote:
> Ping?
> 
> On Wed, Aug 07, 2024 at 12:05:19PM -0700, Yury Norov wrote:
> > The function may call cpumask_equal with tl->mask(cpu) == tl->mask(i),
> > even when cpu != i. In such case, cpumask_equal() would always return
> > true, and we can proceed to the next iteration immediately.
> > 
> > Valentin Schneider shares on it:
> > 
> >   PKG can potentially hit that condition, and so can any
> >   sched_domain_mask_f that relies on the node masks...
> >   
> >   I'm thinking ideally we should have checks in place to
> >   ensure all node_to_cpumask_map[] masks are disjoint,
> >   then we could entirely skip the levels that use these
> >   masks in topology_span_sane(), but there's unfortunately
> >   no nice way to flag them... Also there would be cases
> >   where there's no real difference between PKG and NODE
> >   other than NODE is still based on a per-cpu cpumask and
> >   PKG isn't, so I don't see a nicer way to go about this.
> > 
> > v1: https://lore.kernel.org/lkml/ZrJk00cmVaUIAr4G@yury-ThinkPad/T/
> > v2:
> >  - defer initialization of 'mc' in patch #1 @Chen Yu;
> >  - more comments from Valentin Schneider.
> > 
> > 
> > Yury Norov (2):
> >   sched/topology: pre-compute topology_span_sane() loop params
> >   sched/topology: optimize topology_span_sane()
> > 
> >  kernel/sched/topology.c | 20 ++++++++++++++++++--
> >  1 file changed, 18 insertions(+), 2 deletions(-)
> > 
> > -- 
> > 2.43.0

Le 28/08/2024 à 16:09, Yury Norov a écrit :
> Ping again?
> 

Hi,

The covert letter is v2, but the patch below it are without v2.

In patch 1/2, the Suggested-by: tags in v1 have been removed in this new 
version :(.

CJ

> On Wed, Aug 14, 2024 at 09:35:04AM -0700, Yury Norov wrote:
>> Ping?
>>
>> On Wed, Aug 07, 2024 at 12:05:19PM -0700, Yury Norov wrote:
>>> The function may call cpumask_equal with tl->mask(cpu) == tl->mask(i),
>>> even when cpu != i. In such case, cpumask_equal() would always return
>>> true, and we can proceed to the next iteration immediately.
>>>
>>> Valentin Schneider shares on it:
>>>
>>>    PKG can potentially hit that condition, and so can any
>>>    sched_domain_mask_f that relies on the node masks...
>>>    
>>>    I'm thinking ideally we should have checks in place to
>>>    ensure all node_to_cpumask_map[] masks are disjoint,
>>>    then we could entirely skip the levels that use these
>>>    masks in topology_span_sane(), but there's unfortunately
>>>    no nice way to flag them... Also there would be cases
>>>    where there's no real difference between PKG and NODE
>>>    other than NODE is still based on a per-cpu cpumask and
>>>    PKG isn't, so I don't see a nicer way to go about this.
>>>
>>> v1: https://lore.kernel.org/lkml/ZrJk00cmVaUIAr4G@yury-ThinkPad/T/
>>> v2:
>>>   - defer initialization of 'mc' in patch #1 @Chen Yu;
>>>   - more comments from Valentin Schneider.
>>>
>>>
>>> Yury Norov (2):
>>>    sched/topology: pre-compute topology_span_sane() loop params
>>>    sched/topology: optimize topology_span_sane()
>>>
>>>   kernel/sched/topology.c | 20 ++++++++++++++++++--
>>>   1 file changed, 18 insertions(+), 2 deletions(-)
>>>
>>> -- 
>>> 2.43.0
> 
> 

On Thu, Aug 29, 2024 at 08:11:41AM +0200, Christophe JAILLET wrote:
> Le 28/08/2024 à 16:09, Yury Norov a écrit :
> > Ping again?
> > 
> 
> Hi,
> 
> The covert letter is v2, but the patch below it are without v2.
> 
> In patch 1/2, the Suggested-by: tags in v1 have been removed in this new
> version :(.

My bad. I'll send v3.