linux-kernel - Re: [PATCH 1/2] sched/wait: add round robin wakeup mode

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Date:	Mon, 09 Feb 2015 23:06:17 -0500
From:	Jason Baron <jbaron@...mai.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	mingo@...hat.com, viro@...iv.linux.org.uk,
	akpm@...ux-foundation.org, normalperson@...t.net,
	davidel@...ilserver.org, mtk.manpages@...il.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/wait: add round robin wakeup mode

On 02/09/2015 04:50 PM, Peter Zijlstra wrote:
> On Mon, Feb 09, 2015 at 08:05:57PM +0000, Jason Baron wrote:
>> diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
>> index 852143a..17d1039 100644
>> --- a/kernel/sched/wait.c
>> +++ b/kernel/sched/wait.c
>> @@ -71,8 +71,11 @@ static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
>>  		unsigned flags = curr->flags;
>>  
>>  		if (curr->func(curr, mode, wake_flags, key) &&
>> -				(flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
>> +			       (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive) {
>> +			if (flags & WQ_FLAG_ROUND_ROBIN)
>> +				list_move_tail(&curr->task_list, &q->task_list);
>>  			break;
>> +		}
>>  	}
>>  }
> I think you meant to write something like:
>
> 	if (curr->func(curr, mode, wake_flags, key) &&
> 	    (flags & WQ_FLAG_EXCLUSIVE)) {
> 		if (flag & WQ_FLAG_ROUND_ROBIN)
> 			list_move_tail(&curr->task_list, &q->task_list);
> 		if (!--nr_exclusive)
> 			break;
> 	}
>
> Otherwise can only work for nr_exclusive==1.

Indeed. I'm also wondering if its worth avoiding the list_move_tail() for the case where nr_exclusive is initially 0. IE the wake all case, where we are just going to end up doing a bunch of list_move_tail() calls, but end up in the same state.

Thanks,

-Jason



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