linux-kernel - Re: [PATCH] sched/fair: fix update the nohz.next_balance even if we haven't done any load balance

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Date:	Fri, 27 Mar 2015 00:58:22 -0700
From:	Jason Low <jason.low2@...com>
To:	Wanpeng Li <wanpeng.li@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, jason.low2@...com
Subject: Re: [PATCH] sched/fair: fix update the nohz.next_balance even if we
 haven't done any load balance

On Fri, 2015-03-27 at 15:25 +0800, Wanpeng Li wrote:
> As Srikar pointed out (https://lkml.org/lkml/2015/3/27/26):
> 
> | With the current code when the ilb cpus are not free: 
> | - We would be updating the nohz.next_balance even through we havent done
> |   any load balance.
> | - We might iterate thro the nohz.idle_cpus_mask()s to find balance_cpus.
> 
> This patch fix it by adding need_resched check with the idle check, and 
> keep the need_resched check in for loop to catch ilb get busy. 
> 
> Suggested-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...ux.intel.com>
> ---
>  kernel/sched/fair.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0576ce0..1d3e17f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7639,7 +7639,8 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>  	int balance_cpu;
>  
>  	if (idle != CPU_IDLE ||
> -	    !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
> +	    !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)) ||
> +		need_resched())
>  		goto end;

How about having:

	if (idle != CPU_IDLE || need_resched() ||
	    !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))

which wouldn't require adding a new line.

Besides that:

Reviewed-by: Jason Low <jason.low2@...com>

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