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

Wei Hu (Xavier)
2017-09-26T13:13:05Z

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.

Regards
Wei Hu
>>>> +#define HNS_ROCE_CMQ_TX_TIMEOUT 200
>>> or you could reduce the size of this define...
>>>
>>> --
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