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Subject: Re: [f2fs-dev] [PATCH] f2fs: fix long latency due to discard during
 umount
From:   Chao Yu <yuchao0@huawei.com>
To:     Sahitya Tummala <stummala@codeaurora.org>
CC:     Jaegeuk Kim <jaegeuk@kernel.org>, <linux-kernel@vger.kernel.org>,
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Date:   Mon, 30 Mar 2020 18:16:40 +0800
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On 2020/3/30 16:38, Chao Yu wrote:
> Hi Sahitya,
> 
> Bad news, :( I guess we didn't catch the root cause, as after applying v3,
> I still can reproduce this issue:
> 
> generic/003 10s ...  30s

I use zram as backend device of fstest,

Call Trace:
 dump_stack+0x66/0x8b
 f2fs_submit_discard_endio+0x88/0xa0 [f2fs]
 generic_make_request_checks+0x70/0x5f0
 generic_make_request+0x3e/0x2e0
 submit_bio+0x72/0x140
 __submit_discard_cmd.isra.50+0x4a8/0x710 [f2fs]
 __issue_discard_cmd+0x171/0x3a0 [f2fs]

Does this mean zram uses single queue, so we may always fail to submit 'nowait'
IO due to below condition:

	/*
	 * Non-mq queues do not honor REQ_NOWAIT, so complete a bio
	 * with BLK_STS_AGAIN status in order to catch -EAGAIN and
	 * to give a chance to the caller to repeat request gracefully.
	 */
	if ((bio->bi_opf & REQ_NOWAIT) && !queue_is_mq(q)) {
		status = BLK_STS_AGAIN;
		goto end_io;
	}


> 
> Thanks,
> 
> On 2020/3/30 14:53, Sahitya Tummala wrote:
>> Hi Chao,
>>
>> On Fri, Mar 27, 2020 at 08:35:42AM +0530, Sahitya Tummala wrote:
>>> On Fri, Mar 27, 2020 at 09:51:43AM +0800, Chao Yu wrote:
>>>>
>>>> With this patch, most of xfstest cases cost 5 * n second longer than before.
>>>>
>>>> E.g. generic/003, during umount(), we looped into retrying one bio
>>>> submission.
>>>>
>>>> [61279.829724] F2FS-fs (zram1): Found nat_bits in checkpoint
>>>> [61279.885337] F2FS-fs (zram1): Mounted with checkpoint version = 5cf3cb8e
>>>> [61281.912832] submit discard bio start [23555,1]
>>>> [61281.912835] f2fs_submit_discard_endio [23555,1] err:-11
>>>> [61281.912836] submit discard bio end [23555,1]
>>>> [61281.912836] move dc to retry list [23555,1]
>>>>
>>>> ...
>>>>
>>>> [61286.881212] submit discard bio start [23555,1]
>>>> [61286.881217] f2fs_submit_discard_endio [23555,1] err:-11
>>>> [61286.881223] submit discard bio end [23555,1]
>>>> [61286.881224] move dc to retry list [23555,1]
>>>> [61286.905198] submit discard bio start [23555,1]
>>>> [61286.905203] f2fs_submit_discard_endio [23555,1] err:-11
>>>> [61286.905205] submit discard bio end [23555,1]
>>>> [61286.905206] move dc to retry list [23555,1]
>>>> [61286.929157] F2FS-fs (zram1): Issue discard(23555, 23555, 1) failed, ret: -11
>>>>
>>>> Could you take a look at this issue?
>>>
>>> Let me check and get back on this.
>>
>> I found the issue. The dc with multiple bios is getting requeued again and
>> again in case if one of its bio gets -EAGAIN error. Even the successfully
>> completed bios are getting requeued again resulting into long latency.
>> I have fixed it by splitting the dc in such case so that we can requeue only
>> the leftover bios into a new dc and retry that later within the 5 sec timeout.
>>
>> Please help to review v3 posted and if it looks good, I would like to request
>> you to test the earlier regression scenario with it to check the result again?
>>
>> thanks,
>>
>>>
>>> Thanks,
>>>
>>>>
>>>> Thanks,
>>>>
>>>>>
>>>>> Thanks,
>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>>> +				break;
>>>>>>> +			}
>>>>>>> +		}
>>>>>>>  
>>>>>>>  		atomic_inc(&dcc->issued_discard);
>>>>>>>  
>>>>>>> @@ -1463,6 +1477,40 @@ static unsigned int __issue_discard_cmd_orderly(struct f2fs_sb_info *sbi,
>>>>>>>  	return issued;
>>>>>>>  }
>>>>>>>  
>>>>>>> +static bool __should_discard_retry(struct f2fs_sb_info *sbi,
>>>>> s> > +		struct discard_policy *dpolicy)
>>>>>>> +{
>>>>>>> +	struct discard_cmd_control *dcc = SM_I(sbi)->dcc_info;
>>>>>>> +	struct discard_cmd *dc, *tmp;
>>>>>>> +	bool retry = false;
>>>>>>> +	unsigned long flags;
>>>>>>> +
>>>>>>> +	if (dpolicy->type != DPOLICY_UMOUNT)
>>>>>>> +		f2fs_bug_on(sbi, 1);
>>>>>>> +
>>>>>>> +	mutex_lock(&dcc->cmd_lock);
>>>>>>> +	list_for_each_entry_safe(dc, tmp, &(dcc->retry_list), list) {
>>>>>>> +		if (dpolicy->timeout != 0 &&
>>>>>>> +			f2fs_time_over(sbi, dpolicy->timeout)) {
>>>>>>> +			retry = false;
>>>>>>> +			break;
>>>>>>> +		}
>>>>>>> +
>>>>>>> +		spin_lock_irqsave(&dc->lock, flags);
>>>>>>> +		if (!dc->bio_ref) {
>>>>>>> +			dc->state = D_PREP;
>>>>>>> +			dc->error = 0;
>>>>>>> +			reinit_completion(&dc->wait);
>>>>>>> +			__relocate_discard_cmd(dcc, dc);
>>>>>>> +			retry = true;
>>>>>>> +		}
>>>>>>> +		spin_unlock_irqrestore(&dc->lock, flags);
>>>>>>> +	}
>>>>>>> +	mutex_unlock(&dcc->cmd_lock);
>>>>>>> +
>>>>>>> +	return retry;
>>>>>>> +}
>>>>>>> +
>>>>>>>  static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
>>>>>>>  					struct discard_policy *dpolicy)
>>>>>>>  {
>>>>>>> @@ -1470,12 +1518,13 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
>>>>>>>  	struct list_head *pend_list;
>>>>>>>  	struct discard_cmd *dc, *tmp;
>>>>>>>  	struct blk_plug plug;
>>>>>>> -	int i, issued = 0;
>>>>>>> +	int i, err, issued = 0;
>>>>>>>  	bool io_interrupted = false;
>>>>>>>  
>>>>>>>  	if (dpolicy->timeout != 0)
>>>>>>>  		f2fs_update_time(sbi, dpolicy->timeout);
>>>>>>>  
>>>>>>> +retry:
>>>>>>>  	for (i = MAX_PLIST_NUM - 1; i >= 0; i--) {
>>>>>>>  		if (dpolicy->timeout != 0 &&
>>>>>>>  				f2fs_time_over(sbi, dpolicy->timeout))
>>>>>>> @@ -1509,7 +1558,10 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
>>>>>>>  				break;
>>>>>>>  			}
>>>>>>>  
>>>>>>> -			__submit_discard_cmd(sbi, dpolicy, dc, &issued);
>>>>>>> +			err = __submit_discard_cmd(sbi, dpolicy, dc, &issued);
>>>>>>> +			if (err == -EAGAIN)
>>>>>>> +				congestion_wait(BLK_RW_ASYNC,
>>>>>>> +						DEFAULT_IO_TIMEOUT);
>>>>>>>  
>>>>>>>  			if (issued >= dpolicy->max_requests)
>>>>>>>  				break;
>>>>>>> @@ -1522,6 +1574,10 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
>>>>>>>  			break;
>>>>>>>  	}
>>>>>>>  
>>>>>>> +	if (!list_empty(&dcc->retry_list) &&
>>>>>>> +		__should_discard_retry(sbi, dpolicy))
>>>>>>> +		goto retry;
>>>>>>> +
>>>>>>>  	if (!issued && io_interrupted)
>>>>>>>  		issued = -1;
>>>>>>>  
>>>>>>> @@ -1613,6 +1669,12 @@ static unsigned int __wait_discard_cmd_range(struct f2fs_sb_info *sbi,
>>>>>>>  		goto next;
>>>>>>>  	}
>>>>>>>  
>>>>>>> +	if (dpolicy->type == DPOLICY_UMOUNT &&
>>>>>>> +		!list_empty(&dcc->retry_list)) {
>>>>>>> +		wait_list = &dcc->retry_list;
>>>>>>> +		goto next;
>>>>>>> +	}
>>>>>>> +
>>>>>>>  	return trimmed;
>>>>>>>  }
>>>>>>>  
>>>>>>> @@ -2051,6 +2113,7 @@ static int create_discard_cmd_control(struct f2fs_sb_info *sbi)
>>>>>>>  	for (i = 0; i < MAX_PLIST_NUM; i++)
>>>>>>>  		INIT_LIST_HEAD(&dcc->pend_list[i]);
>>>>>>>  	INIT_LIST_HEAD(&dcc->wait_list);
>>>>>>> +	INIT_LIST_HEAD(&dcc->retry_list);
>>>>>>>  	INIT_LIST_HEAD(&dcc->fstrim_list);
>>>>>>>  	mutex_init(&dcc->cmd_lock);
>>>>>>>  	atomic_set(&dcc->issued_discard, 0);
>>>>>>>
>>>>>
>>>
>>> -- 
>>> --
>>> Sent by a consultant of the Qualcomm Innovation Center, Inc.
>>> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
>>
> 
> 
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