linux-kernel - Re: [PATCH v2 03/11] sched_ext: Add an event, 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: Windows password security audit tool. GUI, reports in PDF.

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

Message-ID: <fe75af77-f832-4edd-bd33-40e047f89210@igalia.com>
Date: Thu, 30 Jan 2025 15:22:08 +0900
From: Changwoo Min <changwoo@...lia.com>
To: Tejun Heo <tj@...nel.org>
Cc: void@...ifault.com, arighi@...dia.com, kernel-dev@...lia.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 03/11] sched_ext: Add an event, SELECT_CPU_FALLBACK

Hello,

On 25. 1. 28. 05:01, Tejun Heo wrote:
> Hello,
> 
> On Sun, Jan 26, 2025 at 07:16:06PM +0900, Changwoo Min wrote:
> ...
>>   struct scx_event_stats {
>> +	/*
>> +	 * If ops.select_cpu() returns a CPU which can't be used by the task,
>> +	 * the core scheduler code silently picks a fallback CPU.
>> +	 */
>> +	u64		SELECT_CPU_FALLBACK;
> 
> As C has one global namespace, we're gonna have to prefix these. Hopefully,
> something not too long. SCX_EV_?

Sure, I will add SCX_EV_ as a prefix for all events.

> 
>> @@ -3663,6 +3668,10 @@ static int select_task_rq_scx(struct task_struct *p, int prev_cpu, int wake_flag
>>   		cpu = SCX_CALL_OP_TASK_RET(SCX_KF_ENQUEUE | SCX_KF_SELECT_CPU,
>>   					   select_cpu, p, prev_cpu, wake_flags);
>>   		*ddsp_taskp = NULL;
>> +
>> +		if (unlikely(!is_cpu_allowed(p, cpu)))
>> +			__scx_add_event(SELECT_CPU_FALLBACK, 1);
> 
> This is trying to guess what select_task_rq() is going to do and then count
> that as an event, which doesn't seem great. Can't we just record the picked
> CPU here into a field in p->scx and then compare that against cpu_of(rq) in
> enqueue_task_scx()? That way, we can reliably count events where CPU
> selection was modified by the fallback logic regardless of why or how that
> happens.

That sounds good. I will add a field (say, selected_cpu?) to struct 
sched_ext_entity.

Regards,
Changwoo Min