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

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Date: Wed, 15 May 2024 07:00:14 -1000
From: Tejun Heo <tj@...nel.org>
To: Waiman Long <longman@...hat.com>
Cc: Jens Axboe <axboe@...nel.dk>, Josef Bacik <josef@...icpanda.com>,
	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 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@...il.com>
> Closes: https://lore.kernel.org/lkml/ZkO6l%2FODzadSgdhC@dschatzberg-fedora-PF3DHTBV/
> Signed-off-by: Waiman Long <longman@...hat.com>

Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun