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

From: Tejun Heo
Date: Thu Apr 18 2024 - 12:14:42 EST


Hello,

On Thu, Apr 18, 2024 at 03:23:40PM +0800, linan666@xxxxxxxxxxxxxxx wrote:
> From: Li Nan <linan122@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxx>

Thanks.

--
tejun