linux-kernel - Re: [PATCH] sched_ext: idle: Handle migration-disabled tasks in BPF code

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Message-ID: <aNF-6bmzGgY5QaEp@slm.duckdns.org>
Date: Mon, 22 Sep 2025 06:52:57 -1000
From: Tejun Heo <tj@...nel.org>
To: Andrea Righi <arighi@...dia.com>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	sched-ext@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched_ext: idle: Handle migration-disabled tasks in BPF
 code

On Sat, Sep 20, 2025 at 03:26:21PM +0200, Andrea Righi wrote:
> When scx_bpf_select_cpu_dfl()/and() kfuncs are invoked outside of
> ops.select_cpu() we can't rely on @p->migration_disabled to determine if
> migration is disabled for the task @p.
> 
> In fact, migration is always disabled for the current task while running
> BPF code: __bpf_prog_enter() disables migration and __bpf_prog_exit()
> re-enables it.
> 
> To handle this, when @p->migration_disabled == 1, check whether @p is
> the current task. If so, migration was not disabled before entering the
> callback, otherwise migration was disabled.
> 
> This ensures correct idle CPU selection in all cases. The behavior of
> ops.select_cpu() remains unchanged, because this callback is never
> invoked for the current task and migration-disabled tasks are always
> excluded.
> 
> Example: without this change scx_bpf_select_cpu_and() called from
> ops.enqueue() always returns -EBUSY; with this change applied, it
> correctly returns idle CPUs.
> 
> Fixes: 06efc9fe0b8de ("sched_ext: idle: Handle migration-disabled tasks in idle selection")
> Cc: stable@...r.kernel.org # v6.16+
> Signed-off-by: Andrea Righi <arighi@...dia.com>

Applied to sched_ext/for-6.17-fixes.

Thanks.

-- 
tejun