linux-kernel - Re: [PATCH v3 3/5] sched_ext: Implement scx_bpf_clock_get

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Message-ID: <51eb6162-ebbf-4fbd-8422-f21ebf9726f7@igalia.com>
Date: Wed, 4 Dec 2024 14:17:20 +0900
From: Changwoo Min <changwoo@...lia.com>
To: Tejun Heo <tj@...nel.org>
Cc: void@...ifault.com, mingo@...hat.com, peterz@...radead.org,
 kernel-dev@...lia.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/5] sched_ext: Implement scx_bpf_clock_get_ns()

Hello,


On 24. 12. 4. 08:37, Tejun Heo wrote:
> Hello,
> 
> On Tue, Dec 03, 2024 at 11:28:00PM +0900, Changwoo Min wrote:
>> +__bpf_kfunc u64 scx_bpf_clock_get_ns(void)
>> +{
>> +	static DEFINE_PER_CPU(u64, prev_clk);
>> +	struct rq *rq = this_rq();
> 
> this_rq() is this_cpu_ptr(). Shouldn't this be below preempt_disable() if
> this function is allowed to be called from sleepable ops?

Thanks for pointing that out. You are right. I will change it in
the next version.

> 
>> +	u64 pr_clk, cr_clk;
>> +
>> +	preempt_disable();
>> +	pr_clk = __this_cpu_read(prev_clk);
> 
> Would it make sense to make the above rq->scx.prev_clk?

That will also work. I will change it as suggested in the next
version.

> 
>> +	/*
>> +	 * If the rq clock is invalid, start a new rq clock period
>> +	 * with a fresh sched_clock().
>> +	 */
>> +	if (!(rq->scx.flags & SCX_RQ_CLK_VALID)) {
>> +		cr_clk = sched_clock();
>> +		scx_rq_clock_update(rq, cr_clk);
>> +	}
>> +	/*
>> +	 * If the rq clock is valid, use the cached rq clock
>> +	 * whenever the clock does not go backward.
>> +	 */
> 
> Can you move the comments inside the if/else bodies so that "} else {" can
> stay on the same line?

Sure.

> 
>> +	else {
>> +		cr_clk = rq->scx.clock;
>> +		/*
>> +		 * If the clock goes backward, start a new rq clock period
>> +		 * with a fresh sched_clock().
>> +		 */
> 
> Can you please add comment explaining how this can happen?

Sure, I will add the explanation in the next version.


Thanks!
Changwoo Min