Re: [PATCH] Don't wait if queue already has requests.

[Gui Jianfeng <guijianfeng@cn.fujitsu.com>]

Justin TerAvest wrote:
> Commit 7667aa0630407bc07dc38dcc79d29cc0a65553c1 added logic to wait for
> the last queue of the group to become busy (have at least one request),
> so that the group does not lose out for not being continuously
> backlogged. The commit did not check for the condition that the last
> queue already has some requests. As a result, if the queue already has
> requests, wait_busy is set. Later on, cfq_select_queue() checks the
> flag, and decides that since the queue has a request now and wait_busy
> is set, the queue is expired.  This results in early expiration of the
> queue.
> 
> This patch fixes the problem by adding a check to see if queue already
> has requests. If it does, wait_busy is not set. As a result, time slices
> do not expire early.
> 
> The queues with more than one request are usually buffered writers.
> Testing shows improvement in isolation between buffered writers.
> 
> Signed-off-by: Justin TerAvest <teravest@google.com>

This patch looks good to me.

Reviewed-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>

Thanks,
Gui