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

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Message-ID: <da36c0c5-da61-4d27-9bb4-ef9f212102e2@igalia.com>
Date: Sun, 22 Dec 2024 13:32:45 +0900
From: Changwoo Min <changwoo@...lia.com>
To: Andrea Righi <arighi@...dia.com>
Cc: tj@...nel.org, void@...ifault.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()

Hi Andrea,

On 24. 12. 21. 06:30, Andrea Righi wrote:
> Hi Changwoo,
> 
> On Fri, Dec 20, 2024 at 03:20:21PM +0900, Changwoo Min wrote:
> ...
>> +	/*
>> +	 * If the rq clock is valid, use the cached rq clock.
>> +	 * Otherwise, return a fresh rq glock.
> 
> s/glock/clock/
Opps. My bad.

>> +	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);
>> +	}
> 
> I was wondering if we could use a special value for clock (like ~0ULL or
> similar) to mark the clock as invalid.
> 
> This way, we could get rid of the extra READ_ONCE(rq->scx.flags) logic for
> checking the clock validity. And if the actual clock happens to match the
> special value, we'd simply re-read the TSC, which shouldn't be a big issue
> in theory.


Thank you for the suggestion. In theory, the clock can overflow,
so it would be hard to reserve a specific value. I think it would
be better to keep the code as it is.

Regards,
Changwoo Min