linux-kernel - Re: [PATCH 4/9] blkcg: implement REQ_CGROUP

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Date:   Thu, 20 Jun 2019 09:42:29 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Jan Kara <jack@...e.cz>
Cc:     dsterba@...e.com, clm@...com, josef@...icpanda.com,
        axboe@...nel.dk, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 4/9] blkcg: implement REQ_CGROUP_PUNT

Hello, Jan.

On Thu, Jun 20, 2019 at 05:37:33PM +0200, Jan Kara wrote:
> > +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?

There's only one work item per blkcg-device pair, so the maximum
number of kthreads a blkcg can occupy on a filesystem would be one.
It's the same scheme as writeback work items.

Thanks.

-- 
tejun