linux-kernel - CFQ: Preemption/timeout logic reversed?

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Message-ID: <ed038eb20904191756y5a5bcf8fn53473405b3929de6@mail.gmail.com>
Date:	Mon, 20 Apr 2009 02:56:15 +0200
From:	Carl Henrik Lunde <chlunde@...g.uio.no>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:	Jens Axboe <jens.axboe@...cle.com>
Subject: CFQ: Preemption/timeout logic reversed?

Hi!

It seems the preemption "bonus" logic in CFQ is reversed, a preempted
process is given an additional delay in start time instead of a bonus.
 This seems unfair.  I'm not sure if it's a good idea to let
slice_resid grow without limit as shown below, but isn't this more
like the way it was intended to work?  Or did I misunderstand
something?

PS! The comment above cfq_preempt_queue seems outdated too.

Code not tested, just showing what I mean:

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 664ebfd..ea18d45 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -292,7 +292,8 @@ cfq_prio_to_slice(struct cfq_data *cfqd, struct
cfq_queue *cfqq)
 static inline void
 cfq_set_prio_slice(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 {
-       cfqq->slice_end = cfq_prio_to_slice(cfqd, cfqq) + jiffies;
+       cfqq->slice_end = cfq_prio_to_slice(cfqd, cfqq) +
cfqq->slice_resid + jiffies;
+       cfqq->slice_resid = 0;
        cfq_log_cfqq(cfqd, cfqq, "set_slice=%lu", cfqq->slice_end - jiffies);
 }

@@ -489,8 +490,6 @@ static void cfq_service_tree_add(struct cfq_data *cfqd,
                        rb_key += jiffies;
        } else if (!add_front) {
                rb_key = cfq_slice_offset(cfqd, cfqq) + jiffies;
-               rb_key += cfqq->slice_resid;
-               cfqq->slice_resid = 0;
        } else
                rb_key = 0;

@@ -1851,8 +1850,7 @@ cfq_should_preempt(struct cfq_data *cfqd, struct
cfq_queue *new_cfqq,
 }

 /*
- * cfqq preempts the active queue. if we allowed preempt with no slice left,
- * let it have half of its nominal slice.
+ * cfqq preempts the active queue.  start a new time slice.
  */
 static void cfq_preempt_queue(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 {

-- 
mvh
Carl Henrik
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