Re: [dpdk-dev] [PATCH] ixgbe:Add write memory barrier for recv pkts.

[Wang Dong <dong.wang.pro@hotmail.com>]

>
>
>> -----Original Message-----
>> From: outlook_739db8e1c4bc6fae@outlook.com [mailto:outlook_739db8e1c4bc6fae@outlook.com] On Behalf Of Dong.Wang
>> Sent: Wednesday, April 15, 2015 2:46 PM
>> To: Ananyev, Konstantin; dev@dpdk.org
>> Subject: Re: [dpdk-dev] [PATCH] ixgbe:Add write memory barrier for recv pkts.
>>
>>
>>
>>> Hi,
>>>
>>>> -----Original Message-----
>>>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of WangDong
>>>> Sent: Saturday, April 11, 2015 4:34 PM
>>>> To: dev@dpdk.org
>>>> Subject: [dpdk-dev] [PATCH] ixgbe:Add write memory barrier for recv pkts.
>>>>
>>>> Like transmit packets, before update receive descriptor's tail pointer, rte_wmb() should be added after writing recv descriptor.
>>>>
>>>> Signed-off-by: Dong Wang <dong.wang.pro@hotmail.com>
>>>> ---
>>>>    lib/librte_pmd_ixgbe/ixgbe_rxtx.c | 5 +++++
>>>>    1 file changed, 5 insertions(+)
>>>>
>>>> diff --git a/lib/librte_pmd_ixgbe/ixgbe_rxtx.c b/lib/librte_pmd_ixgbe/ixgbe_rxtx.c
>>>> index 9da2c7e..d504688 100644
>>>> --- a/lib/librte_pmd_ixgbe/ixgbe_rxtx.c
>>>> +++ b/lib/librte_pmd_ixgbe/ixgbe_rxtx.c
>>>> @@ -1338,6 +1338,9 @@ ixgbe_recv_pkts(void *rx_queue, struct rte_mbuf **rx_pkts,
>>>>    		 */
>>>>    		rx_pkts[nb_rx++] = rxm;
>>>>    	}
>>>> +
>>>> +	rte_wmb();
>>>> +
>>>
>>> Why do you think it is necessary?
>>> I can't see any good reason to put wmb() here.
>>> I would understand if, at least you'll try to insert it just before updating RDT:
>>>    rx_id = (uint16_t) ((rx_id == 0) ?
>>>                                        (rxq->nb_rx_desc - 1) : (rx_id - 1));
>>> + rte_wmb();
>>> IXGBE_PCI_REG_WRITE(rxq->rdt_reg_addr, rx_id);
>>>
>>> That is not needed IA with current implementation, but would make sense for machines with relaxed memory ordering.
>>> Though right now DPDK IXGBE PMD is supported only on IA,  anyway.
>>> Same for ixgbe_recv_scattered_pkts().
>>>
>>> Konstantin
>>
>> Yes, current implementation works well with IA, and the transmit packets
>> function's rte_wmb() is also unneccessary.
>>
>> But there are two reasons for adding rte_wmb() in recv pkts function:
>> 1) The memory barrier in recv pkts function and xmit pkts function are
>> inconsistent, rte_wmb() should be added to recv pkts function or be
>> removed from xmit pkts function.
>> 2) DPDK will support PowerPC processor (Other developers are working on
>> it), I check the memory ordering of PowerPC, there was no mention of
>> store-store instruction's principle in MPC8544 Reference Manual, only
>> said it is weak memory ordering.
>>
>> So, I think it is neccessary to add rte_wmb() to recv pkts function.
>>
>> Dong
>
> What I was trying to say:
>
> 1. I think you put barrier in a wrong place.
> Even for machines with weak memory ordering, we need a barrier only when we are goint to update RDT, i.e:
> if (nb_hold > rxq->rx_free_thresh) { ... ; barrier; IXGBE_PCI_REG_WRITE(rxq->rdt_reg_addr, ...); }
Yes, I put it in a wrong place, it will reduce performance. It's better 
to place it in that you suggested.
>
> 2. Even with putting wmb() here, you wouldn't fix  ixgbe_recv_pkts() to work on machines with weak memory ordering.
> I think that to make it work properly, you'll need an rmb() bewtween reading DD bit and rest of RXD:
>
> rxdp = &rx_ring[rx_id];
>   staterr = rxdp->wb.upper.status_error;
> + rte_rmb();
>   if (! (staterr & rte_cpu_to_le_32(IXGBE_RXDADV_STAT_DD)))
>                          break;
>   rxd = *rxdp;
Yes, it seems wmb is not enough for weak memory ordering processor. Both 
rmb and wmb are needed.
>
> 3. As Stephen pointed in his mail, we shouldn't penalise IA implementation with unnecessary barriers
> As was discussed at that thread:  http://dpdk.org/ml/archives/dev/2015-March/015202.html
> probably the best is to introduce a new macros: rte_smp_*mb (or something) that would be architecture dependent:
> compiler_barrier on IA, proper HW barrier on machines with weak memory ordering and update the code to use it.
>
> So, if you like to fix that issue, please do that in  a proper way.
>
> BTW, I think that for PPC support even before touching ixgbe or any other PMD,
> step 3 (or similar) need to be done on rte_ring enqueue/dequeue code.
>
> Konstantin
Yes,  a new set of macros should be introduced first, then we can update 
PMD code. Did anyone are working on it now ?

Dong
>
>>>
>>>
>>>>    	rxq->rx_tail = rx_id;
>>>>
>>>>    	/*
>>>> @@ -1595,6 +1598,8 @@ ixgbe_recv_scattered_pkts(void *rx_queue, struct rte_mbuf **rx_pkts,
>>>>    		first_seg = NULL;
>>>>    	}
>>>>
>>>> +	rte_wmb();
>>>> +
>>>>    	/*
>>>>    	 * Record index of the next RX descriptor to probe.
>>>>    	 */
>>>> --
>>>> 1.9.1
>>>