netdev - Re: XDP redirect measurements, gotchas and tracepoints

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Date:   Wed, 23 Aug 2017 07:51:44 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Michael Chan <michael.chan@...adcom.com>
Cc:     "Duyck, Alexander H" <alexander.h.duyck@...el.com>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "brouer@...hat.com" <brouer@...hat.com>,
        "pstaszewski@...are.pl" <pstaszewski@...are.pl>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "xdp-newbies@...r.kernel.org" <xdp-newbies@...r.kernel.org>,
        "andy@...yhouse.net" <andy@...yhouse.net>,
        "borkmann@...earbox.net" <borkmann@...earbox.net>
Subject: Re: XDP redirect measurements, gotchas and tracepoints

On Tue, Aug 22, 2017 at 11:59 PM, Michael Chan
<michael.chan@...adcom.com> wrote:
> On Tue, Aug 22, 2017 at 6:06 PM, Alexander Duyck
> <alexander.duyck@...il.com> wrote:
>> On Tue, Aug 22, 2017 at 1:04 PM, Michael Chan <michael.chan@...adcom.com> wrote:
>>>
>>> Right, but it's conceivable to add an API to "return" the buffer to
>>> the input device, right?
>>
>> You could, it is just added complexity. "just free the buffer" in
>> ixgbe usually just amounts to one atomic operation to decrement the
>> total page count since page recycling is already implemented in the
>> driver. You still would have to unmap the buffer regardless of if you
>> were recycling it or not so all you would save is 1.000015259 atomic
>> operations per packet. The fraction is because once every 64K uses we
>> have to bulk update the count on the page.
>>
>
> If the buffer is returned to the input device, the input device can
> keep the DMA mapping.  All it needs to do is to dma_sync it back to
> the input device when the buffer is returned.

That is what ixgbe is already doing. The Rx path doesn't free the
page, it just treats it as a bounce buffer and uses the page count to
make certain we don't use a section of the buffer that is already in
use.

- Alex