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Date:   Thu, 13 Apr 2023 11:23:21 +0800
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Subject: Re: [PATCH 1/2] blk-stat: fix QUEUE_FLAG_STATS clear
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To:     Tejun Heo <tj@kernel.org>
Cc:     axboe@kernel.dk, josef@toxicpanda.com, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        Chengming Zhou <zhouchengming@bytedance.com>
References: <20230412160754.1981705-1-chengming.zhou@linux.dev>
 <ZDbmlYIrRpkWRZla@slm.duckdns.org>
From:   Chengming Zhou <chengming.zhou@linux.dev>
In-Reply-To: <ZDbmlYIrRpkWRZla@slm.duckdns.org>
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On 2023/4/13 01:12, Tejun Heo wrote:
> On Thu, Apr 13, 2023 at 12:07:53AM +0800, chengming.zhou@linux.dev wrote:
>> From: Chengming Zhou <zhouchengming@bytedance.com>
>>
>> We need to set QUEUE_FLAG_STATS for two cases:
>> 1. blk_stat_enable_accounting()
>> 2. blk_stat_add_callback()
>>
>> So we should clear it only when ((q->stats->accounting == 0) &&
>> list_empty(&q->stats->callbacks)).
>>
>> blk_stat_disable_accounting() only check if q->stats->accounting
>> is 0 before clear the flag, this patch fix it.
>>
>> Also add list_empty(&q->stats->callbacks)) check when enable, or
>> the flag is already set.
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> 
> Acked-by: Tejun Heo <tj@kernel.org>
> 
> It'd be useful to explicitly illustrate the buggy behavior in the
> description (e.g. if you do X, Y and then Z, then X incorrectly loses

Yes, I will add below buggy behavior in the next version:

This bug can be reproduced as below on kernel without BLK_DEV_THROTTLING
(since it will unconditionally enable accounting, see the second patch).

# cat /sys/block/sr0/queue/scheduler
none mq-deadline [bfq]

# cat /sys/kernel/debug/block/sr0/state
SAME_COMP|IO_STAT|INIT_DONE|STATS|REGISTERED|NOWAIT|30

# echo none > /sys/block/sr0/queue/scheduler

# cat /sys/kernel/debug/block/sr0/state
SAME_COMP|IO_STAT|INIT_DONE|REGISTERED|NOWAIT

# cat /sys/block/sr0/queue/wbt_lat_usec
75000

We can see that after changing elevator from "bfq" to "none", "STATS" flag
is lost even though WBT callback still need it.


> accounting). Can you also please add the appropriate stable cc?

Ok, will do.

Thanks.

> 
> Thanks.
>