Re: [PATCH v3 3/5] sched_ext: Implement scx_bpf_clock_get_ns()

[Changwoo Min]

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