linux-kernel - Re: [PATCH] blk-iocost: do not WARNING if iocg has already offlined

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Message-ID: <ZiFG5KtGGpYdOmnY@slm.duckdns.org>
Date: Thu, 18 Apr 2024 06:14:28 -1000
From: Tejun Heo <tj@...nel.org>
To: linan666@...weicloud.com
Cc: josef@...icpanda.com, axboe@...nel.dk, hch@....de,
	cgroups@...r.kernel.org, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, yukuai3@...wei.com,
	yi.zhang@...wei.com, houtao1@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH] blk-iocost: do not WARNING if iocg has already offlined

Hello,

On Thu, Apr 18, 2024 at 03:23:40PM +0800, linan666@...weicloud.com wrote:
> From: Li Nan <linan122@...wei.com>
> 
> In iocg_pay_debt(), warn is triggered if 'active_list' is empty, which
> is intended to confirm iocg is avitve when it has debt. However, warn
> can be triggered during removing cgroup controller, as

Maybe saying "a blkcg is being removed" is clearer?

> iocg_waitq_timer_fn() is awakened at that time.
> 
>   WARNING: CPU: 0 PID: 2344971 at block/blk-iocost.c:1402 iocg_pay_debt+0x14c/0x190
>   Call trace:
>   iocg_pay_debt+0x14c/0x190
>   iocg_kick_waitq+0x438/0x4c0
>   iocg_waitq_timer_fn+0xd8/0x130
>   __run_hrtimer+0x144/0x45c
>   __hrtimer_run_queues+0x16c/0x244
>   hrtimer_interrupt+0x2cc/0x7b0
> 
> The warn in this situation is meaningless. Since this iocg is being
> removed, the state of the 'active_list' is irrelevant, and 'waitq_timer'
> is canceled after removing 'active_list' in ioc_pd_free(), which ensure
> iocg is freed after iocg_waitq_timer_fn() returns.
> 
> Therefore, add the check if iocg has already offlined to avoid warn
> when removing cgroup controller.
> 
> Signed-off-by: Li Nan <linan122@...wei.com>
> ---
>  block/blk-iocost.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/blk-iocost.c b/block/blk-iocost.c
> index baa20c85799d..2e109c016a39 100644
> --- a/block/blk-iocost.c
> +++ b/block/blk-iocost.c
> @@ -1440,7 +1440,7 @@ static void iocg_pay_debt(struct ioc_gq *iocg, u64 abs_vpay,
>  	lockdep_assert_held(&iocg->waitq.lock);
>  
>  	/* make sure that nobody messed with @iocg */
> -	WARN_ON_ONCE(list_empty(&iocg->active_list));
> +	WARN_ON_ONCE(list_empty(&iocg->active_list) && iocg->pd.online);

Can you add a comment explaining why we need the pd.online test?

Other than the above nits, looks great to me. Please feel free to add

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

Thanks.

-- 
tejun