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 15:45:09 +0800
From:	Wanpeng Li <wanpeng.li@...ux.intel.com>
To:	Jason Low <jason.low2@...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, Mar 27, 2015 at 12:58:22AM -0700, Jason Low wrote:
>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.

Will do.

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

Thanks, ;)

Regards,
Wanpeng Li
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