Date: Mon, 11 Jan 2010 16:15:39 +0100
From: Jens Axboe <jens.axboe@oracle.com>
To: Divyesh Shah <dpshah@google.com>
Cc: czoccolo@gmail.com, linux-kernel@vger.kernel.org, nauman@google.com,
       mikew@google.com, ctalbott@google.com
Subject: Re: [PATCH] cfq-iosched: Respect ioprio_class when preempting
Message-ID: <20100111151539.GP4489@kernel.dk>
References: <20100107025413.27610.48801.stgit@austin.mtv.corp.google.com>
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On Wed, Jan 06 2010, Divyesh Shah wrote:
> In cfq_should_preempt(), we currently allow some cases where a non-RT request
> can preempt an ongoing RT cfqq timeslice. This should not happen.
> Examples include:
> 
> o A sync_noidle wl type non-RT request pre-empting a sync_noidle wl type cfqq
>   on which we are idling.
> o Once we have per-cgroup async queues, a non-RT sync request pre-empting a RT
>   async cfqq.
> 
> Signed-off-by: Divyesh Shah<dpshah@google.com>
> ---
> 
>  block/cfq-iosched.c |    6 ++++++
>  1 files changed, 6 insertions(+), 0 deletions(-)
> 
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 7d6cef9..e9e6610 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -3104,6 +3104,12 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
>  		return true;
>  
>  	/*
> +	 * Don't allow a non-RT request to preempt an ongoing RT cfqq timeslice.
> +	 */
> +	if (cfq_class_rt(cfqq) && !cfq_class_rt(new_cfqq))
> +		return false;

It would be nice to just be able to do

        if (new_cfqq->ioprio_class > cfqq->ioprio_class)
                return false;

but that doesn't quite work given IOPRIO_CLASS_NONE < _RT. So I think
your patch is fine, I've added it. Thanks.

-- 
Jens Axboe

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