linux-kernel - Re: [PATCH] blk-cgroup: Properly propagate the iostat update up the hierarchy

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Message-ID: <7ab67d43-84af-4328-8aca-9383afa2d6df@redhat.com>
Date: Wed, 15 May 2024 10:35:19 -0400
From: Waiman Long <longman@...hat.com>
To: Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
 Josef Bacik <josef@...icpanda.com>
Cc: cgroups@...r.kernel.org, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org, Dan Schatzberg <schatzberg.dan@...il.com>,
 Ming Lei <ming.lei@...hat.com>
Subject: Re: [PATCH] blk-cgroup: Properly propagate the iostat update up the
 hierarchy

On 5/15/24 10:30, Waiman Long wrote:
> During a cgroup_rstat_flush() call, the lowest level of nodes are flushed
> first before their parents. Since commit 3b8cc6298724 ("blk-cgroup:
> Optimize blkcg_rstat_flush()"), iostat propagation was still done to
> the parent. Grandparent, however, may not get the iostat update if the
> parent has no blkg_iostat_set queued in its lhead lockless list.
>
> Fix this iostat propagation problem by queuing the parent's global
> blkg->iostat into one of its percpu lockless lists to make sure that
> the delta will always be propagated up to the grandparent and so on
> toward the root blkcg.
>
> Note that successive calls to __blkcg_rstat_flush() are serialized by
> the cgroup_rstat_lock. So no special barrier is used in the reading
> and writing of blkg->iostat.lqueued.
>
> Fixes: 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()")
> Reported-by: Dan Schatzberg <schatzberg.dan@...il.com>
> Closes: https://lore.kernel.org/lkml/ZkO6l%2FODzadSgdhC@dschatzberg-fedora-PF3DHTBV/
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>   block/blk-cgroup.c | 19 ++++++++++++++++++-
>   1 file changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index 059467086b13..2a7624c32a1a 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -323,6 +323,7 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct gendisk *disk,
>   	blkg->q = disk->queue;
>   	INIT_LIST_HEAD(&blkg->q_node);
>   	blkg->blkcg = blkcg;
> +	blkg->iostat.blkg = blkg;
>   #ifdef CONFIG_BLK_CGROUP_PUNT_BIO
>   	spin_lock_init(&blkg->async_bio_lock);
>   	bio_list_init(&blkg->async_bios);
> @@ -1025,6 +1026,8 @@ static void __blkcg_rstat_flush(struct blkcg *blkcg, int cpu)
>   		unsigned int seq;
>   
>   		WRITE_ONCE(bisc->lqueued, false);
> +		if (bisc == &blkg->iostat)
> +			goto propagate_up; /* propagate up to parent only */
>   
>   		/* fetch the current per-cpu values */
>   		do {
> @@ -1034,10 +1037,24 @@ static void __blkcg_rstat_flush(struct blkcg *blkcg, int cpu)
>   
>   		blkcg_iostat_update(blkg, &cur, &bisc->last);
>   
> +propagate_up:
>   		/* propagate global delta to parent (unless that's root) */
> -		if (parent && parent->parent)
> +		if (parent && parent->parent) {
>   			blkcg_iostat_update(parent, &blkg->iostat.cur,
>   					    &blkg->iostat.last);
> +			/*
> +			 * Queue parent->iostat to its blkcg's lockless
> +			 * list to propagate up to the grandparent if the
> +			 * iostat hasn't been queued yet.
> +			 */
> +			if (!parent->iostat.lqueued) {
> +				struct llist_head *plhead;
> +
> +				plhead = per_cpu_ptr(parent->blkcg->lhead, cpu);
> +				llist_add(&parent->iostat.lnode, plhead);
> +				parent->iostat.lqueued = true;
> +			}
> +		}
>   	}
>   	raw_spin_unlock_irqrestore(&blkg_stat_lock, flags);
>   out:

Note that this patch will conflict with Ming's patch 
(https://lore.kernel.org/linux-block/20240515013157.443672-3-ming.lei@redhat.com/). 
Will send out an updated version later on to synchronize with his patch.

Cheers,
Longman