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Date: Wed, 15 May 2024 12:58:00 -0400
From: Dan Schatzberg <schatzberg.dan@gmail.com>
To: Waiman Long <longman@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>, Tejun Heo <tj@kernel.org>,
	Josef Bacik <josef@toxicpanda.com>, cgroups@vger.kernel.org,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	Ming Lei <ming.lei@redhat.com>
Subject: Re: [PATCH] blk-cgroup: Properly propagate the iostat update up the
 hierarchy
Message-ID: <ZkTpmPoMb4PyPwCl@dschatzberg-fedora-PF3DHTBV>
References: <20240515143059.276677-1-longman@redhat.com>
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In-Reply-To: <20240515143059.276677-1-longman@redhat.com>

On Wed, May 15, 2024 at 10:30:59AM -0400, 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@gmail.com>
> Closes: https://lore.kernel.org/lkml/ZkO6l%2FODzadSgdhC@dschatzberg-fedora-PF3DHTBV/
> Signed-off-by: Waiman Long <longman@redhat.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:
> -- 
> 2.39.3
> 

I've tested and confirmed this patch fixes the original issue. Thanks!