On 3 February 2017 at 19:38, Ananyev, Konstantin
<[email protected]> wrote:
>
>
>> -----Original Message-----
>> From: Jianbo Liu [mailto:[email protected]]
>> Sent: Friday, February 3, 2017 6:22 AM
>> To: Ananyev, Konstantin <[email protected]>
>> Cc: [email protected]; Zhang, Helin <[email protected]>;
>> [email protected]
>> Subject: Re: [PATCH 1/2] net/ixgbe: calculate the correct number of received
>> packets in bulk alloc function
>>
>> On 2 February 2017 at 00:19, Ananyev, Konstantin
>> <[email protected]> wrote:
>> > Hi,
>> >
>> >> -----Original Message-----
>> >> From: Jianbo Liu [mailto:[email protected]]
>> >> Sent: Monday, December 19, 2016 6:09 AM
>> >> To: [email protected]; Zhang, Helin <[email protected]>; Ananyev,
>> >> Konstantin <[email protected]>;
>> >> [email protected]
>> >> Cc: Jianbo Liu <[email protected]>
>> >> Subject: [PATCH 1/2] net/ixgbe: calculate the correct number of received
>> >> packets in bulk alloc function
>> >>
>> >> To get better performance, Rx bulk alloc recv function will scan 8
>> >> descriptors
>> >> in one time, but the statuses are not consistent on ARM platform because
>> >> the memory allocated for Rx descriptors is cacheable hugepages.
>> >> This patch is to calculate the number of received packets by scanning DD
>> >> bit
>> >> sequentially, and stops when meeting the first packet with DD bit unset.
>> >>
>> >> Signed-off-by: Jianbo Liu <[email protected]>
>> >> ---
>> >> drivers/net/ixgbe/ixgbe_rxtx.c | 12 ++++++++----
>> >> 1 file changed, 8 insertions(+), 4 deletions(-)
>> >>
>> >> diff --git a/drivers/net/ixgbe/ixgbe_rxtx.c
>> >> b/drivers/net/ixgbe/ixgbe_rxtx.c
>> >> index b2d9f45..2866bdb 100644
>> >> --- a/drivers/net/ixgbe/ixgbe_rxtx.c
>> >> +++ b/drivers/net/ixgbe/ixgbe_rxtx.c
>> >> @@ -1402,17 +1402,21 @@ ixgbe_rx_scan_hw_ring(struct ixgbe_rx_queue *rxq)
>> >> for (i = 0; i < RTE_PMD_IXGBE_RX_MAX_BURST;
>> >> i += LOOK_AHEAD, rxdp += LOOK_AHEAD, rxep += LOOK_AHEAD) {
>> >> /* Read desc statuses backwards to avoid race condition */
>> >> - for (j = LOOK_AHEAD-1; j >= 0; --j)
>> >> + for (j = LOOK_AHEAD - 1; j >= 0; --j) {
>> >> s[j] =
>> >> rte_le_to_cpu_32(rxdp[j].wb.upper.status_error);
>> >> -
>> >> - for (j = LOOK_AHEAD - 1; j >= 0; --j)
>> >> pkt_info[j] = rte_le_to_cpu_32(rxdp[j].wb.lower.
>> >> lo_dword.data);
>> >> + }
>> >> +
>> >> + rte_smp_rmb();
>> >
>> > If reads can be reordered, shouldn't we fill pkt_info[] after smp_rmb()
>> > here?
>>
>> The barrier is to forbid the reordering from the following readings,
>> which will count the number of actual received packets.
>
> What I meant is that if you'll keep reading from both rxdp[].wb.lower and
> rxdp[].wb.upper
> before rmb, then nothing would prevent cpu from reorder these reads in any
> way it likes
> (if we are talking about cpus with read reordering allowed), right?
> So it can end up with the following order:
>
> rxdp[N].wb.lower
> rxdp[N].wb.upper
>
> or even:
>
> rxdp[N-1].wb.lower
> rxdp[N].wb.lower
> rxdp[N-1].wb.upper
> rxdp[N].wb.upper
>
> In such cases pkt_info[] may contain invalid data.
Yes, it's possible. I'll send v2.
Thanks!
>
>> And as wb.uper and wb.lower of one descriptor are in the same
>> cacheline, could it be better to read them at the same time?.
>
> It could be, but I think for the sake of data integrity we have to make sure
> that
> cpu would never read any other RXD field before wb.upper. status_error, see
> above.
>
> BTW, the following code might re-read both wb.upper and wb.lower anyway.
> So I don't think you'll save many cycles here anyway.
>
>>
>> > As another nit - with rmb() in and because you are looking the first gap
>> > in s[] now,
>> > no need to read TXDs in backward order.
>>
>> Reading backward is just to keep as it is for x86 platform.
>
> With the change you introducing, I don't think it is necessary any more.
>
> Konstantin
>
>>
>> > How it looks to me (as a suggestion):
>> >
>> > for (j = 0; j != LOOK_AHEAD; j++)
>> > s[j] = rte_le_to_cpu_32(rxdp[j].wb.upper.status_error);
>> >
>> > rte_smp_rmb();
>> >
>> > for (j = 0; j < LOOK_AHEAD && (s[j] & IXGBE_RXDADV_STAT_DD) != 0; j++)
>> > ;
>> >
>> > for (j = 0; j < nb_dd; ++j) {
>> > pkt_info[j] = rte_le_to_cpu_32(rxdp[j].wb.lower.lo_dword.data);
>> > ....
>> >
>> > Konstantin
>> >
>> >
>> >>
>> >> /* Compute how many status bits were set */
>> >> nb_dd = 0;
>> >> for (j = 0; j < LOOK_AHEAD; ++j)
>> >> - nb_dd += s[j] & IXGBE_RXDADV_STAT_DD;
>> >> + if (s[j] & IXGBE_RXDADV_STAT_DD)
>> >> + ++nb_dd;
>> >> + else
>> >> + break;
>> >>
>> >> nb_rx += nb_dd;
>> >>
>> >> --
>> >> 2.4.11
>> >
