linux-kernel - Re: [PATCH] af_packet: don't pass empty blocks for PACKET

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Date:	Fri, 06 Feb 2015 07:49:36 +0300
From:	Alexander Drozdov <al.drozdov@...il.com>
To:	Guy Harris <guy@...m.mit.edu>,
	Willem de Bruijn <willemb@...gle.com>
CC:	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Network Development <netdev@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Dan Collins <dan@...llins.co.nz>
Subject: Re: [PATCH] af_packet: don't pass empty blocks for PACKET_V3

On 06.02.2015 00:16:30 +0300 Guy Harris <guy@...m.mit.edu> wrote:
> On Feb 5, 2015, at 12:01 PM, Willem de Bruijn <willemb@...gle.com> wrote:
>
>> On Wed, Feb 4, 2015 at 9:58 PM, Alexander Drozdov <al.drozdov@...il.com> wrote:
>>> Don't close an empty block on timeout. Its meaningless to
>>> pass it to the user. Moreover, passing empty blocks wastes
>>> CPU & buffer space increasing probability of packets
>>> dropping on small timeouts.
>>>
>>> Side effect of this patch is indefinite user-space wait
>>> in poll on idle links. But, I believe its better to set
>>> timeout for poll(2) when needed than to get empty blocks
>>> every millisecond when not needed.
>> This change would break existing applications that have come
>> to depend on the periodic signal.
>>
>> I don't disagree with the argument that the data ready signal
>> should be sent only when a block is full or a timer expires and
>> at least some data is waiting, but that is moot at this point.
> For what it's worth, the BPF packet capture mechanism (which really needs a new name, to distinguish itself from the BPF packet filter language and its implementation(s), but I digress) has the same issue - when the timer expires, a wakeup is delivered even if there are no packets to read.
>
> *However*, if there are no packets available, the buffers aren't rotated, so the empty buffer is left around to be filled up with packets, rather than being made the hold buffer.
>
> Given that before the previous TPACKET_V3 change, wakeups were delivered when packets arrived rather than when a block was closed, presumably code using TPACKET_V3 was capable of dealing with wakeups being delivered when no new blocks had been made available to userland; could TPACKET_V3 work a bit more like BPF and deliver a wakeup when the timer expires *without* closing the empty block?
Thank you all for your comments! I'll try to create two patches:
1. Wakeup by timeout without closing the empty block
2. Allow to not wakeup by timeout (the feature should be explicitly requested by a user)
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