Re: [PATCH 4/9] blkcg: implement REQ_CGROUP_PUNT

[Jan Kara <jack@suse.cz>]

On Sat 15-06-19 11:24:48, Tejun Heo wrote:
> When a shared kthread needs to issue a bio for a cgroup, doing so
> synchronously can lead to priority inversions as the kthread can be
> trapped waiting for that cgroup.  This patch implements
> REQ_CGROUP_PUNT flag which makes submit_bio() punt the actual issuing
> to a dedicated per-blkcg work item to avoid such priority inversions.
> 
> This will be used to fix priority inversions in btrfs compression and
> should be generally useful as we grow filesystem support for
> comprehensive IO control.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Reviewed-by: Josef Bacik <josef@toxicpanda.com>
> Cc: Chris Mason <clm@fb.com>

...

> +bool __blkcg_punt_bio_submit(struct bio *bio)
> +{
> +	struct blkcg_gq *blkg = bio->bi_blkg;
> +
> +	/* consume the flag first */
> +	bio->bi_opf &= ~REQ_CGROUP_PUNT;
> +
> +	/* never bounce for the root cgroup */
> +	if (!blkg->parent)
> +		return false;
> +
> +	spin_lock_bh(&blkg->async_bio_lock);
> +	bio_list_add(&blkg->async_bios, bio);
> +	spin_unlock_bh(&blkg->async_bio_lock);
> +
> +	queue_work(blkcg_punt_bio_wq, &blkg->async_bio_work);
> +	return true;
> +}
> +

So does this mean that if there is some inode with lots of dirty data for a
blkcg that is heavily throttled, that blkcg can occupy a ton of workers all
being throttled in submit_bio()? Or what is constraining a number of
workers one blkcg can consume?

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR