Re: [PATCH sched_ext/for-6.15] sched_ext: Implement SCX_OPS_ALLOW_QUEUED_WAKEUP

[Tejun Heo <tj@kernel.org>]

Hello,

On Thu, Feb 13, 2025 at 09:18:41AM +0100, Andrea Righi wrote:
> > Implement SCX_OPS_ALLOW_QUEUED_WAKEUP which allows BPF schedulers to choose
> > to enable ttwu_queue optimization.
> 
> I'm wondering whether it makes sense to introduce a new SCX_OPS flag for
> this, considering that we already have the TTWU_QUEUE sched feature, that
> determines this behavior.
> 
> Is this in perspective of a future scenario, when we may potentially have
> multiple scx schedulers running at the same time and they may want to set a
> different queued wakeup behavior?

It's more that it can be a breaking change for schedulers that expect
ops.select_cpu() to be called on wakeups. e.g. If we make this behavior the
default, scx_layered as of the last relesae would break in an unobvious but
serious way - it'd fail to occupy CPUs fully as it'd be skipping picking
idle CPU and pushing tasks there for a noticeable portion of wakeups on
multi-LLC machines. The same holds for most of the C schedulers.

Down the line, once everyone is onboard, we can maybe make it the default
and drop the flag but, for now, this needs to be opt-in.

Thanks.

-- 
tejun