linux-kernel - Re: [PATCH v6 2/6] sched_ext: Implement scx_bpf_now

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Message-ID: <5eeef2e1-7dc4-4aa8-8be4-c6e0fc2b5267@igalia.com>
Date: Fri, 27 Dec 2024 09:38:37 +0900
From: Changwoo Min <changwoo@...lia.com>
To: Tejun Heo <tj@...nel.org>, Changwoo Min <multics69@...il.com>
Cc: void@...ifault.com, arighi@...dia.com, mingo@...hat.com,
 peterz@...radead.org, kernel-dev@...lia.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 2/6] sched_ext: Implement scx_bpf_now_ns()

Hello,

On 24. 12. 25. 06:47, Tejun Heo wrote:
> Hello,
> 
> On Fri, Dec 20, 2024 at 03:20:21PM +0900, Changwoo Min wrote:
> ...
>> +__bpf_kfunc u64 scx_bpf_now_ns(void)
> 
> Given that the default time unit is ns for the scheduler, the _ns suffix
> can probably go.

Ok. I will change is as suggested.

> 

>> +	if (!(READ_ONCE(rq->scx.flags) & SCX_RQ_CLK_VALID)) {
>> +		clock = sched_clock_cpu(cpu_of(rq));
>> +
>> +		/*
>> +		 * The rq clock is updated outside of the rq lock.
>> +		 * In this case, keep the updated rq clock invalid so the next
>> +		 * kfunc call outside the rq lock gets a fresh rq clock.
>> +		 */
>> +		scx_rq_clock_update(rq, clock, false);
> 
> Hmm... what does this update do?
It can be dropped as we do not track prev_clock.


> ...
>> +static inline void scx_rq_clock_update(struct rq *rq, u64 clock, bool valid)
>> +{
>> +	if (!scx_enabled())
>> +		return;
>> +	WRITE_ONCE(rq->scx.clock, clock);
>> +	if (valid)
>> +		WRITE_ONCE(rq->scx.flags, rq->scx.flags | SCX_RQ_CLK_VALID);
>> +}
> 
> Isn't rq->scx.clock used iff VALID is set? If so, why does !VALID read need
> to update rq->scx.clock?

If we drop the previous scx_rq_clock_update(.., false),  we can
drop the if condition checking the valid flag.

Regards,
Changwoo Min