Re: [PATCH 4/6] sched_ext: bypass mode shouldn't depend on ops.select_cpu()

[Tejun Heo <tj@kernel.org>]

Hello,

On Thu, Oct 10, 2024 at 01:15:17PM -0500, David Vernet wrote:
> On Wed, Oct 09, 2024 at 11:41:00AM -1000, Tejun Heo wrote:
> > Bypass mode was depending on ops.select_cpu() which can't be trusted as with
> > the rest of the BPF scheduler. Always enable and use scx_select_cpu_dfl() in
> > bypass mode.
> 
> Could you please clarify why we can't trust ops.select_cpu()? Even if it
> returns a bogus, offline, etc, CPU, shouldn't core.c take care of
> finding a valid CPU for us in select_fallback_rq()?

For example, if select_cpu() returns the same CPU for all threads on a
loaded system, that CPU can get very overloaded which can lead to RCU and
workqueue stalls which can then cascade to other failures.

> Assuming we really do require a valid CPU here in bypass mode, do we
> need to reset the state of the idle masks for the case of
> !scx_builtin_idle_enabled? The masks won't necessarily reflect the set
> of online CPUs if we haven't been updating it, right?

I think resched_cpu() after switching each CPU into bypass mode is enough.
That guarantees that the CPU leaves the idle state, clearing the idle state
if set, and if the CPU is idle, it goes back into idle, setting the bit, so
at the end, it ends up synchronized.

Thanks.

-- 
tejun