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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ZTIyD8EuhSmF4JFr@gmail.com>
Date:   Fri, 20 Oct 2023 09:53:51 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        "Vineeth Pillai (Google)" <vineeth@...byteword.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH 3/3] sched: Update ->next_balance correctly during
 newidle balance


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

> From: "Vineeth Pillai (Google)" <vineeth@...byteword.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@...gle.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Frederic Weisbecker <frederic@...nel.org>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Signed-off-by: Vineeth Pillai (Google) <vineeth@...byteword.org>
> Co-developed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.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