linux-kernel - Re: [PATCH 4/6] sched_ext: idle: Introduce scx_bpf_select_cpu

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Z-sQTkz8mOuUKar7@slm.duckdns.org>
Date: Mon, 31 Mar 2025 11:59:42 -1000
From: Tejun Heo <tj@...nel.org>
To: Andrea Righi <arighi@...dia.com>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	Joel Fernandes <joelagnelf@...dia.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] sched_ext: idle: Introduce scx_bpf_select_cpu_and()

Hello,

On Fri, Mar 21, 2025 at 11:10:50PM +0100, Andrea Righi wrote:
...
> +__bpf_kfunc s32 scx_bpf_select_cpu_and(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
> +				       const struct cpumask *cpus_allowed, u64 flags)
> +{
> +	s32 cpu;
> +
> +	if (!ops_cpu_valid(prev_cpu, NULL))
> +		return -EINVAL;
> +
> +	if (!check_builtin_idle_enabled())
> +		return -EBUSY;
> +
> +	if (!scx_kf_allowed(SCX_KF_SELECT_CPU | SCX_KF_ENQUEUE))
> +		return -EPERM;
> +
> +#ifdef CONFIG_SMP
> +	cpu = scx_select_cpu_dfl(p, prev_cpu, wake_flags, cpus_allowed, flags);
> +#else
> +	cpu = -EBUSY;
> +#endif
> +
> +	return cpu;
> +}

Later in the series, I find scx_bpf_select_cpu_and() being called with
p->cpus_ptr really confusing. scx_bpf_select_cpu_and() is always constrained
by p->cpus_ptr (except for the currently buggy case where p->nr_cpus_allowed
is used while p->cpus_ptr is overridden), so what does it mean to call
scx_bpf_select_cpu_and() with p->cpus_ptr as @cpus_allowed? I'd much prefer
if the convention in such cases is calling with NULL @cpus_allowed.
@cpus_allowed is the extra mask to and to p->cpus_ptr when searching for an
idle CPU. If we're going to use p->cpus_ptr, we just don't have the extra
cpumask to and.

Thanks.

-- 
tejun