Re: [PATCH 04/10] block: initial patch for on-stack per-task plugging

[Jeff Moyer <jmoyer@redhat.com>]

Jens Axboe <jaxboe@fusionio.com> writes:

This looks mostly good.  I just have a couple of questions, listed below.

> +/*
> + * Attempts to merge with the plugged list in the current process. Returns
> + * true if merge was succesful, otherwise false.
> + */
> +static bool check_plug_merge(struct task_struct *tsk, struct request_queue *q,
> +			     struct bio *bio)
> +{

Would a better name for this function be attempt_plug_merge?

> +	plug = current->plug;
> +	if (plug && !sync) {
> +		if (!plug->should_sort && !list_empty(&plug->list)) {
> +			struct request *__rq;
> +
> +			__rq = list_entry_rq(plug->list.prev);
> +			if (__rq->q != q)
> +				plug->should_sort = 1;

[snip]

> +static int plug_rq_cmp(void *priv, struct list_head *a, struct list_head *b)
> +{
> +	struct request *rqa = container_of(a, struct request, queuelist);
> +	struct request *rqb = container_of(b, struct request, queuelist);
> +
> +	return !(rqa->q == rqb->q);
> +}


> +static void __blk_finish_plug(struct task_struct *tsk, struct blk_plug *plug)
> +{

[snip]

> +	if (plug->should_sort)
> +		list_sort(NULL, &plug->list, plug_rq_cmp);

The other way to do this is to just keep track of which queues you need
to run after exhausting the plug list.  Is it safe to assume you've done
things this way to keep each request queue's data structures cache hot
while working on it?

> +static inline void blk_flush_plug(struct task_struct *tsk)
> +{
> +	struct blk_plug *plug = tsk->plug;
> +
> +	if (unlikely(plug))
> +		__blk_flush_plug(tsk, plug);
> +}

Why is that unlikely?

Cheers,
Jeff