Virtualization - Oct 2022 - [RFC PATCH 01/21] block: add and use init tagset helper

On Wed, 5 Oct 2022 at 07:11, Damien Le Moal
<damien.lemoal at opensource.wdc.com> wrote:
>
> On 10/5/22 12:22, Chaitanya Kulkarni wrote:
> > Add and use the helper to initialize the common fields of the tag_set
> > such as blk_mq_ops, number of h/w queues, queue depth, command size,
> > numa_node, timeout, BLK_MQ_F_XXX flags, driver data. This
initialization
> > is spread all over the block drivers. This avoids the code repetation
of
> > the inialization code of the tag set in current block drivers and any
>
> s/inialization/initialization
> s/repetation/repetition
>
> > future ones.
> >
> > Signed-off-by: Chaitanya Kulkarni <kch at nvidia.com>
> > ---
> >  block/blk-mq.c                | 20 ++++++++++++++++++++
> >  drivers/block/null_blk/main.c | 10 +++-------
> >  include/linux/blk-mq.h        |  5 +++++
> >  3 files changed, 28 insertions(+), 7 deletions(-)
> >
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 8070b6c10e8d..e3a8dd81bbe2 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -4341,6 +4341,26 @@ static int blk_mq_alloc_tag_set_tags(struct
blk_mq_tag_set *set,
> >       return blk_mq_realloc_tag_set_tags(set, 0, new_nr_hw_queues);
> >  }
> >
> > +void blk_mq_init_tag_set(struct blk_mq_tag_set *set,
> > +             const struct blk_mq_ops *ops, unsigned int nr_hw_queues,
> > +             unsigned int queue_depth, unsigned int cmd_size, int
numa_node,
> > +             unsigned int timeout, unsigned int flags, void
*driver_data)
>
> That is an awful lot of arguments... I would be tempted to say pack all
> these into a struct but then that would kind of negate this patchset goal.
> Using a function with that many arguments will be error prone, and hard to
> review... Not a fan.
I completely agree.

But there is also another problem going down this route. If/when we
realize that there is another parameter needed in the blk_mq_tag_set.
Today that's quite easy to add (assuming the parameter can be
optional), without changing the blk_mq_init_tag_set() interface.
>
> > +{
> > +     if (!set)
> > +             return;
> > +
> > +     set->ops = ops;
> > +     set->nr_hw_queues = nr_hw_queues;
> > +     set->queue_depth = queue_depth;
> > +     set->cmd_size = cmd_size;
> > +     set->numa_node = numa_node;
> > +     set->timeout = timeout;
> > +     set->flags = flags;
> > +     set->driver_data = driver_data;
> > +}
> > +
>
[...]

Kind regards
Uffe

On 10/5/22 02:47, Ulf Hansson wrote:
> On Wed, 5 Oct 2022 at 07:11, Damien Le Moal <damien.lemoal at
opensource.wdc.com> wrote:
>> On 10/5/22 12:22, Chaitanya Kulkarni wrote:
>>> +void blk_mq_init_tag_set(struct blk_mq_tag_set *set,
>>> +             const struct blk_mq_ops *ops, unsigned int
nr_hw_queues,
>>> +             unsigned int queue_depth, unsigned int cmd_size, int
numa_node,
>>> +             unsigned int timeout, unsigned int flags, void
*driver_data)
>>
>> That is an awful lot of arguments... I would be tempted to say pack all
>> these into a struct but then that would kind of negate this patchset
goal.
>> Using a function with that many arguments will be error prone, and hard
to
>> review... Not a fan.
> 
> I completely agree.
> 
> But there is also another problem going down this route. If/when we
> realize that there is another parameter needed in the blk_mq_tag_set.
> Today that's quite easy to add (assuming the parameter can be
> optional), without changing the blk_mq_init_tag_set() interface.
Hi Chaitanya,

Please consider to drop the entire patch series. In addition to the 
disadvantages mentioned above I'd like to mention the following 
disadvantages:
* Replacing named member assignments with positional arguments in a
   function call makes code harder to read and harder to verify.
* This patch series makes tree-wide changes without improving the code
   in a substantial way.

Thanks,

Bart.