Message-ID: <9172f8c5-3dd6-a573-8e28-1b3ae4b1726b@tom.com>
Date:   Wed, 27 Sep 2017 10:46:40 +0800
From:   "Wei Hu (Xavier)" <xavier.huwei@....com>
To:     Doug Ledford <dledford@...hat.com>,
        Leon Romanovsky <leon@...nel.org>
Cc:     "Wei Hu (Xavier)" <xavier.huwei@...wei.com>,
        linux-rdma@...r.kernel.org, lijun_nudt@....com, oulijun@...wei.com,
        charles.chenxin@...wei.com, liuyixian@...wei.com,
        xushaobo2@...wei.com, zhangxiping3@...wei.com,
        xavier.huwei@....com, linuxarm@...wei.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH for-next 05/20] RDMA/hns: Add command queue support for
 hip08 RoCE driver



On 2017/9/27 0:18, Doug Ledford wrote:
> On 9/26/2017 9:13 AM, Wei Hu (Xavier) wrote:
>>
>> On 2017/9/26 1:36, Doug Ledford wrote:
>>> On Mon, 2017-09-25 at 20:18 +0300, Leon Romanovsky wrote:
>>>> On Mon, Sep 25, 2017 at 01:06:53PM -0400, Doug Ledford wrote:
>>>>> On Wed, 2017-08-30 at 17:23 +0800, Wei Hu (Xavier) wrote:
>>>>>
>>>>>> +    /*
>>>>>> +     * If the command is sync, wait for the firmware to
>>>>>> write
>>>>>> back,
>>>>>> +     * if multi descriptors to be sent, use the first one to
>>>>>> check
>>>>>> +     */
>>>>>> +    if ((desc->flag) & HNS_ROCE_CMD_FLAG_NO_INTR) {
>>>>>> +        do {
>>>>>> +            if (hns_roce_cmq_csq_done(hr_dev))
>>>>>> +                break;
>>>>>> +            usleep_range(1000, 2000);
>>>>>> +            timeout++;
>>>>>> +        } while (timeout < priv->cmq.tx_timeout);
>>>>>> +    }
>>>>> then we spin here for a maximum amount of time between 200 and
>>>>> 400ms,
>>>>> so 1/4 to 1/2 a second.  All the time we are holding the bh lock on
>>>>> this CPU.  That seems excessive to me.  If we are going to spin
>>>>> that
>>>>> long, can you find a way to allocate/reserve your resources, send
>>>>> the
>>>>> command, then drop the bh lock while you spin, and retake it before
>>>>> you
>>>>> complete once the spinning is done?
>>>> They don't allocate anything in this loop, but checking the pointers
>>>> are
>>>> the same, see hns_roce_cmq_csq_done.
>>> I'm not sure I understand your intended implication of your comment.  I
>>> wasn't concerned about them allocating anything, only that if the
>>> hardware is hung, then this loop will hang out for 1/4 to 1/2 a second
>>> and hold up all bottom half processing on this CPU in the meantime.
>>> That's the sort of things that provides poor overall system behavior.
>>>
>>> Now, since they are really only checking to see if the hardware has
>>> gotten around to their particular command, and their command is part of
>>> a ring structure, it's possible to record the original head command,
>>> and our new head command, and then release the spin_lock_bh around the
>>> entire do{ }while construct, and in hns_roce_cmd_csq_done() you could
>>> check that head is not in the range old_head:new_head.  That would
>>> protect you in case something in the bottom half processing queued up
>>> some more commands and from one sleep to the next the head jumped from
>>> something other than the new_head to something past new_head, so that
>>> head == priv->cmq.csq.next_to_use ends up being perpetually false.
>>> But, that's just from a quick read of the code, I could easily be
>>> missing something here...
>> Hi, Doug
>>      Driver issues the cmds in cmq, and firmware gets and processes them.
>>      The firmware process only one cmd at the same time, and it will take
>>      about serveral to 200 us in one cmd currently, so the driver need
>>      not to use stream mode to issue cmd.
> I'm not sure I understand your response here.
>
> I get that the driver issues cmds in the cmq, and that the firmware gets
> them and processes them.
>
> I get that the firmware will only work on one command at a time and only
> move to the next one once the current one is complete.
>
> I get that commands take anywhere from a few usec to a couple hundred usec.
>
> I also get that because you are sleeping for somewhere in between 1000
> and 2000 usecs, that the driver could easily finish a whole slew of
> commands.  It could do 10 slow commands, or 100 or more fast commands.
> What this tells me is that the only reason your current implementation
> of hns_roce_cmq_csq_done() works at all is because you keep the device
> locked out from any other commands being put on the queue.  As far as I
> can tell, that's the only way you can guarantee that at some point you
> will wake up and the head pointer will be exactly at csq->next_to_use.
> Otherwise, if you didn't block them out, then you could sleep with the
> head pointer before csq->next_to_use and wake up the next time with it
> already well past csq->next_to_use.  Am I right about that?  While you
> are waiting on this command queue, any other commands are blocked from
> being placed on the command queue?
Hi, Doug,
you are right.
And one "hns_x" ib device only has one command queue in hip08,
other commands will be blocked when waiting on the command queue.
>
> I don't understand what you mean by "so the driver need not to use
> stream mode to issue cmd".
Sorry, my expression error.
stream -> pipeline

And if you argee, after this patchset has been accepted we will send a 
following up patch :
     In hns_roce_cmq_send function, replace
         usleep_range(1000, 2000);
     with the following statement:
          udelay(1);
     And if so, we can avoid using usleep_range function in spin_lock_bh 
spin region,
     because it probally cause calltrace.

     Best regards
Wei Hu
>>      Regards
>> Wei Hu
>>>>>> +#define HNS_ROCE_CMQ_TX_TIMEOUT            200
>>>>> or you could reduce the size of this define...
>>>>>
>>>>> -- 
>>>>> Doug Ledford <dledford@...hat.com>
>>>>>       GPG KeyID: B826A3330E572FDD
>>>>>       Key fingerprint = AE6B 1BDA 122B 23B4 265B  1274 B826 A333 0E57
>>>>> 2FDD
>>>>>
>>>>> -- 
>>>>> To unsubscribe from this list: send the line "unsubscribe linux-
>>>>> rdma" in
>>>>> the body of a message to majordomo@...r.kernel.org
>>>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>
>

Powered by blists - more mailing lists