Re: [PATCH v3 1/2] sched/fair: Add NOHZ balancer flag for nohz.next_balance updates

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Aug 23, 2021 at 12:16:59PM +0100, Valentin Schneider wrote:

> Gate NOHZ blocked load
> update by the presence of NOHZ_STATS_KICK - currently all NOHZ balance
> kicks will have the NOHZ_STATS_KICK flag set, so no change in behaviour is
> expected.

> @@ -10572,7 +10572,8 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>  	 * setting the flag, we are sure to not clear the state and not
>  	 * check the load of an idle cpu.
>  	 */
> -	WRITE_ONCE(nohz.has_blocked, 0);
> +	if (flags & NOHZ_STATS_KICK)
> +		WRITE_ONCE(nohz.has_blocked, 0);
>  
>  	/*
>  	 * Ensures that if we miss the CPU, we must see the has_blocked
> @@ -10594,13 +10595,15 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>  		 * balancing owner will pick it up.
>  		 */
>  		if (need_resched()) {
> -			has_blocked_load = true;
> +			if (flags & NOHZ_STATS_KICK)
> +				has_blocked_load = true;
>  			goto abort;
>  		}
>  
>  		rq = cpu_rq(balance_cpu);
>  
> -		has_blocked_load |= update_nohz_stats(rq);
> +		if (flags & NOHZ_STATS_KICK)
> +			has_blocked_load |= update_nohz_stats(rq);
>  
>  		/*
>  		 * If time for next balance is due,
> @@ -10631,8 +10634,9 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>  	if (likely(update_next_balance))
>  		nohz.next_balance = next_balance;
>  
> -	WRITE_ONCE(nohz.next_blocked,
> -		now + msecs_to_jiffies(LOAD_AVG_PERIOD));
> +	if (flags & NOHZ_STATS_KICK)
> +		WRITE_ONCE(nohz.next_blocked,
> +			   now + msecs_to_jiffies(LOAD_AVG_PERIOD));
>  
>  abort:
>  	/* There is still blocked load, enable periodic update */

I'm a bit puzzled by this; that function has:

  SCHED_WARN_ON((flags & NOHZ_KICK_MASK) == NOHZ_BALANCE_KICK);

Which:

 - isn't updated
 - implies STATS must be set when BALANCE

the latter gives rise to my confusion; why add that gate on STATS? It
just doesn't make sense to do a BALANCE and not update STATS.