Re: [PATCH] sched/fair: Fix misuse of available_idle_cpu()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Sep 08, 2022 at 11:36:32AM +0100, Mel Gorman wrote:
> On Thu, Sep 08, 2022 at 04:07:02PM +0800, Abel Wu wrote:
> > The function available_idle_cpu() was introduced to distinguish
> > between the code paths that cares if the vCPU is preempted and
> > the ones don't care. In general, available_idle_cpu() is used in
> > selecting cpus for immediate use, e.g. ttwu. While idle_cpu() is
> > used in the paths that only cares about the cpu is idle or not,
> > and __update_idle_core() is one of them.
> > 
> > Use idle_cpu() instead in the idle path to make has_idle_core
> > a better hint.
> > 
> > Fixes: 943d355d7fee (sched/core: Distinguish between idle_cpu() calls based on desired effect, introduce available_idle_cpu())
> > Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
> 
> Seems fair. As vCPU preemption is specific to virtualisation, it is very
> unlikely that SMT is exposed to the guest so the impact of the patch is

Right; only pinned guests typically expose such topology information
(anything else would be quite broken).

> minimal but I still think it's right so;

I'm not convinced; all of select_idle_sibling() seems to use
available_idle_cpu(), and that's the only consumer of
__update_idle_core(), so in that respect the current state makes sense.