Re: [PATCH 1/2] sched/swait: allow swake_up() to return

[Peter Zijlstra]

On Fri, Nov 10, 2017 at 03:10:17PM +0800, Peter Xu wrote:
> I came to this when reading kvm_vcpu_wake_up(), so that only affects
> some statistic which may not be that critical.  However I don't know
> whether there would be any other real use case that we would like to
> know exactly whether a call to [s]wake_up() has really done something
> or just returned with a NOP.
> 
> Anyway, please let me know if you think the same change to wake_up()
> would be meaningful, otherwise I can drop this patch and post another
> KVM-only one to clean up the redundant callers of swait_active(),
> since even if we dropped that list check in 35a2897c2a30, we'll do
> that again in swake_up_locked().

See commits:

  8cd641e3c7cb ("sched/wait: Add swq_has_sleeper()")
  5e0018b3e39e ("kvm: Serialize wq active checks in kvm_vcpu_wake_up()")


In any case, I don't think we want the change you propose. The numbers
don't mean much and there's no point in making all the callers in the
kernel slower for it.