Re: [PATCH for-next 05/20] RDMA/hns: Add command queue support for hip08 RoCE driver

Doug Ledford
2017-09-26T16:18:56Z

[-- Attachment #1.1: Type: text/plain, Size: 5149 bytes --]

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?

I don't understand what you mean by "so the driver need not to use
stream mode to issue cmd".

>     Regards
> Wei Hu
>>>>> +#define HNS_ROCE_CMQ_TX_TIMEOUT            200
>>>> or you could reduce the size of this define...
>>>>
>>>> --
>>>> Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
>>>>      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-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
>>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>
>

--
Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
GPG Key ID: B826A3330E572FDD
Key fingerprint = AE6B 1BDA 122B 23B4 265B 1274 B826 A333 0E57 2FDD

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 884 bytes --]