Re: [PATCH 7/8] xprtrdma: Split the completion queue

Sagi Grimberg
2014-04-16T14:35:32Z

On 4/16/2014 5:25 PM, Steve Wise wrote:
>
>> -----Original Message-----
>> From: Sagi Grimberg [mailto:sagig-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org]
>> Sent: Wednesday, April 16, 2014 9:13 AM
>> To: Steve Wise; Chuck Lever; linux-nfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org; linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
>> Subject: Re: [PATCH 7/8] xprtrdma: Split the completion queue
>>
>> On 4/16/2014 4:30 PM, Steve Wise wrote:
>>> On 4/16/2014 7:48 AM, Sagi Grimberg wrote:
>>>> On 4/15/2014 1:23 AM, Chuck Lever wrote:
>>>>> The current CQ handler uses the ib_wc.opcode field to distinguish
>>>>> between event types. However, the contents of that field are not
>>>>> reliable if the completion status is not IB_WC_SUCCESS.
>>>>>
>>>>> When an error completion occurs on a send event, the CQ handler
>>>>> schedules a tasklet with something that is not a struct rpcrdma_rep.
>>>>> This is never correct behavior, and sometimes it results in a panic.
>>>>>
>>>>> To resolve this issue, split the completion queue into a send CQ and
>>>>> a receive CQ. The send CQ handler now handles only struct rpcrdma_mw
>>>>> wr_id's, and the receive CQ handler now handles only struct
>>>>> rpcrdma_rep wr_id's.
>>>> Hey Chuck,
>>>>
>>>> So 2 suggestions related (although not directly) to this one.
>>>>
>>>> 1. I recommend suppressing Fastreg completions - no one cares that
>>>> they succeeded.
>>>>
>>> Not true. The nfsrdma client uses frmrs across re-connects for the
>>> same mount and needs to know at any point in time if a frmr is
>>> registered or invalid. So completions of both fastreg and invalidate
>>> need to be signaled. See:
>>>
>>> commit 5c635e09cec0feeeb310968e51dad01040244851
>>> Author: Tom Tucker <tom-/Yg/VP3ZvrM@public.gmane.org>
>>> Date: Wed Feb 9 19:45:34 2011 +0000
>>>
>>> RPCRDMA: Fix FRMR registration/invalidate handling.
>>>
>> Hmm, But if either FASTREG or LINV failed the QP will go to error state
>> and you *will* get the error wc (with a rain of FLUSH errors).
>> AFAICT it is safe to assume that it succeeded as long as you don't get
>> error completions.
> But if an unsignaled FASTREG is posted and silently succeeds, then the next signaled work request fails, I believe the FASTREG will be completed with FLUSH status, yet the operation actually completed in the hw.

Actually if (any) WR successfully completed and SW got it as FLUSH error
it seems like a bug to me.
Once the HW processed the WQ entry it should update the consumer index
accordingly thus should not happen.

> So the driver would mark the frmr as INVALID, and a subsequent FASTREG for this frmr would fail because the frmr is in the VALID state.
>
>> Moreover, FASTREG on top of FASTREG are not allowed indeed, but AFAIK
>> LINV on top of LINV are allowed.
>> It is OK to just always do LINV+FASTREG post-list each registration and
>> this way no need to account for successful completions.
> Perhaps always posting a LINV+FASTREG would do the trick.
>
> Regardless, I recommend we don't muddle this particular patch which fixes a bug by using separate SQ and RQ CQs with tweaking how frmr registration is managed. IE this should be a separate patch for review/testing/etc.

Agree, as I said it wasn't directly related to this patch.

Sagi.
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