linux-kernel - Re: [PATCH 2/2] sched_ext/idle: Make scx_bpf_select_cpu

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Message-ID: <aCLXb-wGipqKbvXE@gpd3>
Date: Tue, 13 May 2025 07:23:59 +0200
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] sched_ext/idle: Make scx_bpf_select_cpu_and() usable
 from any context

On Mon, May 12, 2025 at 10:00:19AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Mon, May 12, 2025 at 09:07:49PM +0200, Andrea Righi wrote:
> ...
> > 	if (scx_kf_allowed_if_unlocked())
> > 		rq = task_rq_lock(p, &rf);
> > 	...
> > 	if (scx_kf_allowed_if_unlocked())
> > 		task_rq_unlock(rq, p, &rf);
> > 
> > Or at least it should cover all current use cases. The tricky one is
> > scx_bpf_select_cpu_and() being called via BPF test_run from a user-space
> > task (scx_rustland_core).
> > 
> > If we had a way to clearly identify a test_run context, we could restrict
> > this to BPF_PROG_TYPE_STRUCT_OPS and BPF_PROG_TYPE_TEST_RUN (but as far as
> > I can tell, the latter doesn't exist).
> 
> Shouldn't that work as-is? TEST_RUN isn't gonna set any kf_mask, so the same
> condition would work?

Oh yes, it works, my point is that, without any restrictions, the kfunc
might be used in problematic contexts, for example if it's called with
pi_lock held it could trigger a deadlock with task_rq_lock(). Limiting its
use to ops.select_cpu(), ops.enqueue(), ops.dispatch(), and test_run would
allow to cover all the use cases while being safe.

Thanks,
-Andrea