linux-kernel - Re: [PATCH 3/6] sched_ext: idle: Accept an arbitrary cpumask in scx_select_cpu

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Message-ID: <Z-uIpnfjWq6LXSX7@gpd3>
Date: Tue, 1 Apr 2025 08:33:10 +0200
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	Joel Fernandes <joelagnelf@...dia.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] sched_ext: idle: Accept an arbitrary cpumask in
 scx_select_cpu_dfl()

On Mon, Mar 31, 2025 at 11:56:36AM -1000, Tejun Heo wrote:
> On Fri, Mar 21, 2025 at 11:10:49PM +0100, Andrea Righi wrote:
> ...
> > +	/*
> > +	 * If @prev_cpu is not in the allowed domain, try to assign a new
> > +	 * arbitrary CPU usable by the task in the allowed domain.
> > +	 */
> > +	if (!cpumask_test_cpu(prev_cpu, allowed)) {
> > +		cpu = cpumask_any_and_distribute(p->cpus_ptr, allowed);
> > +		if (cpu < nr_cpu_ids) {
> > +			prev_cpu = cpu;
> > +		} else {
> > +			cpu = -EBUSY;
> > +			goto out_enable;
> > +		}
> > +	}
> 
> Would it be better to clear it to -1 and disable @prev_cpu optimizations if
> negative? Not a big deal, so please feel free to push back but things like
> wake_sync optimization become a bit weird with @prev_cpu set to some random
> CPU and down the line if we want to allow e.g. preferring previous idle CPU
> even when the sibling CPU isn't idle which seems to help with some
> workloads, this can become tricky.

Maybe a better strategy would be to try with prev_cpu = smp_processor_id(),
if it's in the subset p->cpus_ptr & allowed, which might be beneficial for
some waker->wakee scenarios, otherwise jump directly to the end, with
cpu = scx_pick_idle_cpu(allowed, node, flags).

Thanks,
-Andrea