Re: [PATCH] sched/fair: Skip sched-domain traversal when wake affine is disabled

[Srikar Dronamraju <srikar@linux.ibm.com>]

* Kayra Cizmeci <kayracizmeci@gmail.com> [2026-11-09 21:10:46]:

> On Mon, Nov 09, 2026 at 10:01:20PM +0530, Srikar Dronamraju wrote:
> > * Kayra Cizmeci <kayracizmeci@gmail.com> [2026-11-09 18:09:56]:
> > 
> > Hi Kayra,
> > 
> > > 
> > > On Mon, Nov 09, 2026 at 05:35:22PM +0530, Srikar Dronamraju wrote:
> > > > In select_task_rq_fair(), TTWU wakeups for which wake_wide() returns
> > > > true, or where the waking CPU is not in the task's allowed CPU mask,
> > > > want_affine is set to false.
> > > > 
> > > 
> > > 
> > > This just changes the behavior.
> > > 
> > > Old Code WF_FORK 1, WF_TTWU 0:
> > > 
> > > want_affine = 0, so we skip the first if block in the loop then leave
> > > once we find a domain without our flag. And go with slow path.
> > > 
> > > 
> > > New Code:
> > > 
> > > want_affine is moved away and early exit is removed. So we enter the if
> > > block and try to find a domain with prev_cpu. Because WF_TTWU 
> > > is 0, we just return new_cpu. Or if we can't fimd the domain with
> > > prev_cpu, slow path.
> > > 
> > 
> > When WF_TTWU is set, we are always suppose take the fast path.
> > slowpath is for WF_FORK and WF_EXEC.
> 
> I know. And that is the exact problem with the above diff :-).
> 
> Like let's say prev_cpu is in the first domain we are looking, the
> domain has SD_WAKE_AFFINE and SD_BALANCE_FORK set and next domain has
> only SD_WAKE_AFFINE set:
> 
> Old Code:
> 
> sd = first domain and we enter the slow path with the first domain.
> 
> With your patch:
> 
> sd = NULL and we return new_cpu
> 

Oh thats a fair point, thanks for the reviewing it.

> 
> Or am I missing something?
> 
> Cheers,
> 
> Kayra

-- 
Thanks and Regards
Srikar Dronamraju