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SubjectRe: [PATCH 05/10] block: remove per-queue plugging
On 2011-04-11 23:14, NeilBrown wrote:
> On Mon, 11 Apr 2011 12:59:23 -0400 "hch@infradead.org" <hch@infradead.org>
> wrote:
>
>> On Mon, Apr 11, 2011 at 02:50:22PM +1000, NeilBrown wrote:
>>> diff --git a/block/blk-core.c b/block/blk-core.c
>>> index 273d60b..903ce8d 100644
>>> --- a/block/blk-core.c
>>> +++ b/block/blk-core.c
>>> @@ -2674,19 +2674,23 @@ static void flush_plug_list(struct blk_plug *plug)
>>> struct request_queue *q;
>>> unsigned long flags;
>>> struct request *rq;
>>> + struct list_head head;
>>>
>>> BUG_ON(plug->magic != PLUG_MAGIC);
>>>
>>> if (list_empty(&plug->list))
>>> return;
>>> + list_add(&head, &plug->list);
>>> + list_del_init(&plug->list);
>>>
>>> if (plug->should_sort)
>>> - list_sort(NULL, &plug->list, plug_rq_cmp);
>>> + list_sort(NULL, &head, plug_rq_cmp);
>>> + plug->should_sort = 0;
>>
>> As Jens mentioned this should be list_splice_init. But looking over
>> flush_plug_list the code there seems strange to me.
>>
>> What does the local_irq_save in flush_plug_list protect? Why don't
>> we need it over the list_sort? And do we still need it when first
>> splicing the list to a local one?
>>
>> It's one of these cases where I'd really like to see more comments
>> explaining why the code is doing what it's doing.
>
> My understanding of that was that the calling requirement of
> __elv_add_request is that the queue spinlock is held and that interrupts are
> disabled.
> So rather than possible enabling and disabling interrupts several times as
> different queue are handled, the code just disabled interrupts once, and
> then just take the spinlock once for each different queue.
>
> The whole point of the change to plugging was to take locks less often.
> Disabling interrupts less often is presumably an analogous goal.
>
> Though I agree that a comment would help.
>
> q = NULL;
> + /* Disable interrupts just once rather than using spin_lock_irq/sin_unlock_irq
> * variants
> */
> local_irq_save(flags);
>
>
> assuming my analysis is correct.

Yep that is correct, it's to avoid juggling irq on and off for multiple
queues. I will put a comment there.


--
Jens Axboe



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