Re: FAILED: patch "[PATCH] sched_ext: idle: Handle migration-disabled tasks in BPF code" failed to apply to 6.16-stable tree

[Andrea Righi <arighi@nvidia.com>]

Hi Greg,

On Mon, Sep 29, 2025 at 01:40:52PM +0200, gregkh@linuxfoundation.org wrote:
> 
> The patch below does not apply to the 6.16-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.
> 
> To reproduce the conflict and resubmit, you may use the following commands:
> 
> git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.16.y
> git checkout FETCH_HEAD
> git cherry-pick -x 55ed11b181c43d81ce03b50209e4e7c4a14ba099
> # <resolve conflicts, build, test, etc.>
> git commit -s
> git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2025092952-wooing-result-72e9@gregkh' --subject-prefix 'PATCH 6.16.y' HEAD^..
> 
> Possible dependencies:

This patch depends on upstream commit 353656eb84fe ("sched_ext: Make
scx_idle_cpu() and related helpers static").

To resolve the conflict I think the best would be to apply commit
353656eb84fef ("sched_ext: idle: Make local functions static in
ext_idle.c") to 6.16-stable as well.

This commit only makes some functions static (no functional changes), so it
should be safe for stable and it'd keep the code more aligned with
upstream.

Thanks,
-Andrea

> 
> 
> 
> thanks,
> 
> greg k-h
> 
> ------------------ original commit in Linus's tree ------------------
> 
> From 55ed11b181c43d81ce03b50209e4e7c4a14ba099 Mon Sep 17 00:00:00 2001
> From: Andrea Righi <arighi@nvidia.com>
> Date: Sat, 20 Sep 2025 15:26:21 +0200
> Subject: [PATCH] sched_ext: idle: Handle migration-disabled tasks in BPF code
> 
> 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@vger.kernel.org # v6.16+
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> Acked-by: Changwoo Min <changwoo@igalia.com>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> 
> diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
> index 7174e1c1a392..537c6992bb63 100644
> --- a/kernel/sched/ext_idle.c
> +++ b/kernel/sched/ext_idle.c
> @@ -856,6 +856,32 @@ static bool check_builtin_idle_enabled(void)
>  	return false;
>  }
>  
> +/*
> + * Determine whether @p is a migration-disabled task in the context of BPF
> + * code.
> + *
> + * We can't simply check whether @p->migration_disabled is set in a
> + * sched_ext callback, because migration is always disabled for the current
> + * task while running BPF code.
> + *
> + * The prolog (__bpf_prog_enter) and epilog (__bpf_prog_exit) respectively
> + * disable and re-enable migration. For this reason, the current task
> + * inside a sched_ext callback is always a migration-disabled task.
> + *
> + * Therefore, when @p->migration_disabled == 1, check whether @p is the
> + * current task or not: if it is, then migration was not disabled before
> + * entering the callback, otherwise migration was disabled.
> + *
> + * Returns true if @p is migration-disabled, false otherwise.
> + */
> +static bool is_bpf_migration_disabled(const struct task_struct *p)
> +{
> +	if (p->migration_disabled == 1)
> +		return p != current;
> +	else
> +		return p->migration_disabled;
> +}
> +
>  static s32 select_cpu_from_kfunc(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
>  				 const struct cpumask *allowed, u64 flags)
>  {
> @@ -898,7 +924,7 @@ static s32 select_cpu_from_kfunc(struct task_struct *p, s32 prev_cpu, u64 wake_f
>  	 * selection optimizations and simply check whether the previously
>  	 * used CPU is idle and within the allowed cpumask.
>  	 */
> -	if (p->nr_cpus_allowed == 1 || is_migration_disabled(p)) {
> +	if (p->nr_cpus_allowed == 1 || is_bpf_migration_disabled(p)) {
>  		if (cpumask_test_cpu(prev_cpu, allowed ?: p->cpus_ptr) &&
>  		    scx_idle_test_and_clear_cpu(prev_cpu))
>  			cpu = prev_cpu;
>