Re: [Query] increased latency observed in cpu hotplug path

[Khan, Imran]

On 8/1/2016 2:58 PM, Khan, Imran wrote:
> On 7/30/2016 7:54 AM, Akinobu Mita wrote:
>> 2016-07-28 22:18 GMT+09:00 Khan, Imran <kimran@xxxxxxxxxxxxxx>:
>>>
>>> Hi,
>>>
>>> Recently we have observed some increased latency in CPU hotplug
>>> event in CPU online path. For online latency we see that block
>>> layer is executing notification handler for CPU_UP_PREPARE event
>>> and this in turn waits for RCU grace period resulting (sometimes)
>>> in an execution time of 15-20 ms for this notification handler.
>>> This change was not there in 3.18 kernel but is present in 4.4
>>> kernel and was introduced by following commit:
>>>
>>>
>>> commit 5778322e67ed34dc9f391a4a5cbcbb856071ceba
>>> Author: Akinobu Mita <akinobu.mita@xxxxxxxxx>
>>> Date:   Sun Sep 27 02:09:23 2015 +0900
>>>
>>>     blk-mq: avoid inserting requests before establishing new mapping
>>
>> ...
>>
>>> Upon reverting this commit I could see an improvement of 15-20 ms in
>>> online latency. So I am looking for some help in analyzing the effects
>>> of reverting this or should some other approach to reduce the online
>>> latency must be taken.
>>
>> Can you observe the difference in online latency by removing
>> get_online_cpus() and put_online_cpus() pair in blk_mq_init_allocated_queue()
>> instead of full reverting the commit?
>>
> Hi Akinobu,
> I tried your suggestion but could not achieve any improvement. Actually the snippet that is causing the change in latency is the following one :
> 
> list_for_each_entry(q, &all_q_list, all_q_node) {
>                 blk_mq_freeze_queue_wait(q);
> 
>                 /*
>                  * timeout handler can't touch hw queue during the
>                  * reinitialization
>                  */
>                 del_timer_sync(&q->timeout);
>  }
> 
> I understand that this is getting executed now for CPU_UP_PREPARE as well resulting in 
> increased latency in the cpu online path. I am trying to reduce this latency while keeping the 
> purpose of this commit intact. I would welcome further suggestions/feedback in this regard.
> 
Hi Akinobu,

I am not able to reduce the cpu online latency with this patch, could you please let me know what
functionality will be broken, if we avoid this patch in our kernel. Also if you have some other 
suggestions towards improving this patch please let me know.

-- 
Imran Khan
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