[PATCH] cfq-iosched: Fix a gcc 4.5 warning and put some comments

[PATCH] cfq-iosched: Fix a gcc 4.5 warning and put some comments

[Vivek Goyal]

- Andi encountedred following warning with gcc 4.5

  linux/block/cfq-iosched.c: In function ‘cfq_dispatch_requests’:
  linux/block/cfq-iosched.c:2156:3: warning: array subscript is above array
  bounds

- Warning happens due to following code.

  slice = group_slice * count /
		max_t(unsigned, cfqg->busy_queues_avg[cfqd->serving_prio],
		cfq_group_busy_queues_wl(cfqd->serving_prio, cfqd, cfqg));

  gcc is complaining about cfqg->busy_queues_avg[] being indexed by CFQ
  prio classes (RT, BE and IDLE) while the array size is only 2.

- At run time, we never access cfqg->busy_queues_avg[IDLE] and return from
  function before this code hits.

- To fix warning increase the array size though it will remain unused. This
  patch also puts some comments to clarify some of the confusions.

- I have taken Jens's patch and modified it a bit.

- Compile tested with gcc 4.4 and boot tested. I don't have gcc 4.5
  running, Andi can you please test it with gcc 4.5 to make sure it
  worked.

Reported-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 block/cfq-iosched.c |   16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

Index: linux-2.6-block/block/cfq-iosched.c
===================================================================
--- linux-2.6-block.orig/block/cfq-iosched.c	2010-10-21 13:27:33.000000000 -0400
+++ linux-2.6-block/block/cfq-iosched.c	2010-10-21 13:35:48.132946331 -0400
@@ -160,6 +160,7 @@ enum wl_prio_t {
 	BE_WORKLOAD = 0,
 	RT_WORKLOAD = 1,
 	IDLE_WORKLOAD = 2,
+	CFQ_PRIO_NR,
 };
 
 /*
@@ -184,10 +185,19 @@ struct cfq_group {
 	/* number of cfqq currently on this group */
 	int nr_cfqq;
 
-	/* Per group busy queus average. Useful for workload slice calc. */
-	unsigned int busy_queues_avg[2];
 	/*
-	 * rr lists of queues with requests, onle rr for each priority class.
+	 * Per group busy queus average. Useful for workload slice calc. We
+	 * create the array for each prio class but at run time it is used
+	 * only for RT and BE class and slot for IDLE class remains unused.
+	 * This is primarily done to avoid confusion and a gcc warning.
+	 */
+	unsigned int busy_queues_avg[CFQ_PRIO_NR];
+	/*
+	 * rr lists of queues with requests. We maintain service trees for
+	 * RT and BE classes. These trees are subdivided in subclasses
+	 * of SYNC, SYNC_NOIDLE and ASYNC based on workload type. For IDLE
+	 * class there is no subclassification and all the cfq queues go on
+	 * a single tree service_tree_idle.
 	 * Counts are embedded in the cfq_rb_root
 	 */
 	struct cfq_rb_root service_trees[2][3];

Re: [PATCH] cfq-iosched: Fix a gcc 4.5 warning and put some comments

[Jeff Moyer]

Vivek Goyal <vgoyal@redhat.com> writes:

> - Andi encountedred following warning with gcc 4.5
>
>   linux/block/cfq-iosched.c: In function ‘cfq_dispatch_requests’:
>   linux/block/cfq-iosched.c:2156:3: warning: array subscript is above array
>   bounds
>
> - Warning happens due to following code.
>
>   slice = group_slice * count /
> 		max_t(unsigned, cfqg->busy_queues_avg[cfqd->serving_prio],
> 		cfq_group_busy_queues_wl(cfqd->serving_prio, cfqd, cfqg));
>
>   gcc is complaining about cfqg->busy_queues_avg[] being indexed by CFQ
>   prio classes (RT, BE and IDLE) while the array size is only 2.
>
> - At run time, we never access cfqg->busy_queues_avg[IDLE] and return from
>   function before this code hits.
>
> - To fix warning increase the array size though it will remain unused. This
>   patch also puts some comments to clarify some of the confusions.
>
> - I have taken Jens's patch and modified it a bit.
>
> - Compile tested with gcc 4.4 and boot tested. I don't have gcc 4.5
>   running, Andi can you please test it with gcc 4.5 to make sure it
>   worked.
>
> Reported-by: Andi Kleen <ak@linux.intel.com>
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>

Acked-by: Jeff Moyer <jmoyer@redhat.com>

Re: [PATCH] cfq-iosched: Fix a gcc 4.5 warning and put some comments

[Jens Axboe]

On 2010-10-21 20:24, Vivek Goyal wrote:
> - Andi encountedred following warning with gcc 4.5
> 
>   linux/block/cfq-iosched.c: In function ‘cfq_dispatch_requests’:
>   linux/block/cfq-iosched.c:2156:3: warning: array subscript is above array
>   bounds
> 
> - Warning happens due to following code.
> 
>   slice = group_slice * count /
> 		max_t(unsigned, cfqg->busy_queues_avg[cfqd->serving_prio],
> 		cfq_group_busy_queues_wl(cfqd->serving_prio, cfqd, cfqg));
> 
>   gcc is complaining about cfqg->busy_queues_avg[] being indexed by CFQ
>   prio classes (RT, BE and IDLE) while the array size is only 2.
> 
> - At run time, we never access cfqg->busy_queues_avg[IDLE] and return from
>   function before this code hits.
> 
> - To fix warning increase the array size though it will remain unused. This
>   patch also puts some comments to clarify some of the confusions.
> 
> - I have taken Jens's patch and modified it a bit.
> 
> - Compile tested with gcc 4.4 and boot tested. I don't have gcc 4.5
>   running, Andi can you please test it with gcc 4.5 to make sure it
>   worked.
> 
> Reported-by: Andi Kleen <ak@linux.intel.com>
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>

Thanks, I'll put this one in. BTW, you can't just add a signed-off-by
from me (or anyone else, for that matter), they have to be provided
explicitly by each individual.

-- 
Jens Axboe

Re: [PATCH] cfq-iosched: Fix a gcc 4.5 warning and put some comments

[Vivek Goyal]

On Fri, Oct 22, 2010 at 09:46:19AM +0200, Jens Axboe wrote:
> On 2010-10-21 20:24, Vivek Goyal wrote:
> > - Andi encountedred following warning with gcc 4.5
> > 
> >   linux/block/cfq-iosched.c: In function ‘cfq_dispatch_requests’:
> >   linux/block/cfq-iosched.c:2156:3: warning: array subscript is above array
> >   bounds
> > 
> > - Warning happens due to following code.
> > 
> >   slice = group_slice * count /
> > 		max_t(unsigned, cfqg->busy_queues_avg[cfqd->serving_prio],
> > 		cfq_group_busy_queues_wl(cfqd->serving_prio, cfqd, cfqg));
> > 
> >   gcc is complaining about cfqg->busy_queues_avg[] being indexed by CFQ
> >   prio classes (RT, BE and IDLE) while the array size is only 2.
> > 
> > - At run time, we never access cfqg->busy_queues_avg[IDLE] and return from
> >   function before this code hits.
> > 
> > - To fix warning increase the array size though it will remain unused. This
> >   patch also puts some comments to clarify some of the confusions.
> > 
> > - I have taken Jens's patch and modified it a bit.
> > 
> > - Compile tested with gcc 4.4 and boot tested. I don't have gcc 4.5
> >   running, Andi can you please test it with gcc 4.5 to make sure it
> >   worked.
> > 
> > Reported-by: Andi Kleen <ak@linux.intel.com>
> > Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> > Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> Thanks, I'll put this one in. BTW, you can't just add a signed-off-by
> from me (or anyone else, for that matter), they have to be provided
> explicitly by each individual.

Ok, sorry about that.

So in general, if I happen to pick somebody's patch, modify it and repost
it, how do I reflect the Signed-off-by of original author.

Thanks
Vivek

Re: [PATCH] cfq-iosched: Fix a gcc 4.5 warning and put some comments

[Jens Axboe]

On 2010-10-22 15:31, Vivek Goyal wrote:
> On Fri, Oct 22, 2010 at 09:46:19AM +0200, Jens Axboe wrote:
>> On 2010-10-21 20:24, Vivek Goyal wrote:
>>> - Andi encountedred following warning with gcc 4.5
>>>
>>>   linux/block/cfq-iosched.c: In function ‘cfq_dispatch_requests’:
>>>   linux/block/cfq-iosched.c:2156:3: warning: array subscript is above array
>>>   bounds
>>>
>>> - Warning happens due to following code.
>>>
>>>   slice = group_slice * count /
>>> 		max_t(unsigned, cfqg->busy_queues_avg[cfqd->serving_prio],
>>> 		cfq_group_busy_queues_wl(cfqd->serving_prio, cfqd, cfqg));
>>>
>>>   gcc is complaining about cfqg->busy_queues_avg[] being indexed by CFQ
>>>   prio classes (RT, BE and IDLE) while the array size is only 2.
>>>
>>> - At run time, we never access cfqg->busy_queues_avg[IDLE] and return from
>>>   function before this code hits.
>>>
>>> - To fix warning increase the array size though it will remain unused. This
>>>   patch also puts some comments to clarify some of the confusions.
>>>
>>> - I have taken Jens's patch and modified it a bit.
>>>
>>> - Compile tested with gcc 4.4 and boot tested. I don't have gcc 4.5
>>>   running, Andi can you please test it with gcc 4.5 to make sure it
>>>   worked.
>>>
>>> Reported-by: Andi Kleen <ak@linux.intel.com>
>>> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>
>> Thanks, I'll put this one in. BTW, you can't just add a signed-off-by
>> from me (or anyone else, for that matter), they have to be provided
>> explicitly by each individual.
> 
> Ok, sorry about that.
> 
> So in general, if I happen to pick somebody's patch, modify it and repost
> it, how do I reflect the Signed-off-by of original author.

What I usually do is leave the original signed-off-by, then describe my
changes, then add my signed-off-by. I think that is acceptable
behaviour. It's very different from adding a signed-off-by to something
that hasn't been signed-off by the original author yet that's legally an
issue. But hey, IANAL :-)

-- 
Jens Axboe