Re: [PATCH 3/3] sched: Update ->next_balance correctly during newidle balance

[Ingo Molnar <mingo@kernel.org>]

* Joel Fernandes (Google) <joel@joelfernandes.org> wrote:

> From: "Vineeth Pillai (Google)" <vineeth@bitbyteword.org>
> 
> When newidle balancing triggers, we see that it constantly clobbers 
> rq->next_balance even when there is no newidle balance happening due to 
> the cost estimates.  Due to this, we see that periodic load balance 
> (rebalance_domains) may trigger way more often when the CPU is going in 
> and out of idle at a high rate but is no really idle. Repeatedly 
> triggering load balance there is a bad idea as it is a heavy operation. 
> It also causes increases in softirq.
> 
> Another issue is ->last_balance is not updated after newidle balance 
> causing mistakes in the ->next_balance calculations.
> 
> Fix by updating last_balance when a newidle load balance actually happens 
> and then updating next_balance. This is also how it is done in other load 
> balance paths.
> 
> Testing shows a significant drop in softirqs when running:
> cyclictest -i 100 -d 100 --latency=1000 -D 5 -t -m  -q
> 
> Goes from ~6k to ~800.
> 
> Cc: Suleiman Souhlal <suleiman@google.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Vineeth Pillai (Google) <vineeth@bitbyteword.org>
> Co-developed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  kernel/sched/fair.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8e276d12c3cb..b147ad09126a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12076,11 +12076,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  
>  	if (!READ_ONCE(this_rq->rd->overload) ||
>  	    (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
> -
> -		if (sd)
> -			update_next_balance(sd, &next_balance);
>  		rcu_read_unlock();
> -
>  		goto out;
>  	}
>  	rcu_read_unlock();
> @@ -12095,8 +12091,6 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  		int continue_balancing = 1;
>  		u64 domain_cost;
>  
> -		update_next_balance(sd, &next_balance);
> -
>  		if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost)
>  			break;
>  
> @@ -12109,6 +12103,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  			t1 = sched_clock_cpu(this_cpu);
>  			domain_cost = t1 - t0;
>  			update_newidle_cost(sd, domain_cost);
> +			sd->last_balance = jiffies;
> +			update_next_balance(sd, &next_balance);
>  
>  			curr_cost += domain_cost;
>  			t0 = t1;

Okay, I'm applying patches #2 and #3, without #1: it should be safe
out of order, but let me know if I missed something ...

Thanks,

	Ingo