Re: [PATCH] CFQ: mark must_alloc flag when a cfqq doesn't allocate any request

[Jens Axboe <jens.axboe@oracle.com>]

On Fri, Aug 07 2009, Gui Jianfeng wrote:
> Hi Jens,
> 
> CFQ judges whether must_alloc flag is set, but never marks this flag.
> This patch set must_alloc flag when there is no request allocated by
> a cfqq. This change ensures cfqq will get at least one request even
> if the queue is full.
> Hope it helps.

The flag became redundant with the fix to not fail in set_request(). I
don't think your below patch is a very good idea, it would basically
mark any queue that has previously had a request allocated as a must
alloc queue. It would make more sense to mark an idling queue as must
alloc, since it's critical that this queue is allowed to allocate a
request once we idle a queue on its behalf. But we already check for the
wait_request flag in ->may_queue, so that part is already covered. So
I'd suggest we just delete any reference to must_alloc instead.


> 
> Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
> ---
>  block/cfq-iosched.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index fd7080e..531a404 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -2279,6 +2279,9 @@ static void cfq_put_request(struct request *rq)
>  		BUG_ON(!cfqq->allocated[rw]);
>  		cfqq->allocated[rw]--;
>  
> +		if (!(cfqq->allocated[0] + cfqq->allocated[1]))
> +			cfq_mark_cfqq_must_alloc(cfqq);
> +
>  		put_io_context(RQ_CIC(rq)->ioc);
>  
>  		rq->elevator_private = NULL;
> -- 
> 1.5.4.rc3
> 
> 

-- 
Jens Axboe