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From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Cc: intel-wired-lan@lists.osuosl.org, netdev@vger.kernel.org,
	bpf@vger.kernel.org, davem@davemloft.net,
	anthony.l.nguyen@intel.com, kuba@kernel.org, bjorn@kernel.org,
Subject: Re: [PATCH intel-next 2/2] ice: introduce XDP Tx fallback path
Date: Tue, 08 Jun 2021 23:43:40 +0200	[thread overview]
Message-ID: <87o8cgnib7.fsf@toke.dk> (raw)
In-Reply-To: <20210608121259.GA1971@ranger.igk.intel.com>

Maciej Fijalkowski <maciej.fijalkowski@intel.com> writes:

> On Tue, Jun 01, 2021 at 02:38:03PM +0200, Toke Høiland-Jørgensen wrote:
>> Maciej Fijalkowski <maciej.fijalkowski@intel.com> writes:
>> > Under rare circumstances there might be a situation where a requirement
>> > of having a XDP Tx queue per core could not be fulfilled and some of the
>> > Tx resources would have to be shared between cores. This yields a need
>> > for placing accesses to xdp_rings array onto critical section protected
>> > by spinlock.
>> >
>> > Design of handling such scenario is to at first find out how many queues
>> > are there that XDP could use. Any number that is not less than the half
>> > of a count of cores of platform is allowed. XDP queue count < cpu count
>> > is signalled via new VSI state ICE_VSI_XDP_FALLBACK which carries the
>> > information further down to Rx rings where new ICE_TX_XDP_LOCKED is set
>> > based on the mentioned VSI state. This ring flag indicates that locking
>> > variants for getting/putting xdp_ring need to be used in fast path.
>> >
>> > For XDP_REDIRECT the impact on standard case (one XDP ring per CPU) can
>> > be reduced a bit by providing a separate ndo_xdp_xmit and swap it at
>> > configuration time. However, due to the fact that net_device_ops struct
>> > is a const, it is not possible to replace a single ndo, so for the
>> > locking variant of ndo_xdp_xmit, whole net_device_ops needs to be
>> > replayed.
>> >
>> > It has an impact on performance (1-2 %) of a non-fallback path as
>> > branches are introduced.
>> I generally feel this is the right approach, although the performance
>> impact is a bit unfortunately, obviously. Maybe it could be avoided by
>> the use of static_branch? I.e., keep a global refcount of how many
>> netdevs are using the locked path and only activate the check in the
>> fast path while that refcount is >0?
> This would be an ideal solution if we would be able to have it PF-scoped,
> which AFAICT is not possible as static key is per module, right?

Yeah, static_branch basically patches the kernel text when activated
(hence the low overhead), so it's a global switch...

> I checked that before the bank holiday here in Poland and indeed I was not
> observing perf drops. Only thing that is questionable is the fact that a
> single PF would affect all the others that ice driver is serving.
> OTOH I see that Jesper acked that work.
> Let me play with this a bit more as I'm in the middle of switching my HW
> lab, but I wanted to break the silence over here. I didn't manage to check
> that one fallback path will affect other PFs.
> Thanks Toke for that great idea :) any other opinions are more than
> welcome.

You're welcome! :)


  reply	other threads:[~2021-06-08 21:43 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-01 11:32 [PATCH intel-next 0/2] ice: bring up XDP_TX back to life Maciej Fijalkowski
2021-06-01 11:32 ` [PATCH intel-next 1/2] ice: unify xdp_rings accesses Maciej Fijalkowski
2021-06-01 11:32 ` [PATCH intel-next 2/2] ice: introduce XDP Tx fallback path Maciej Fijalkowski
2021-06-01 12:38   ` Toke Høiland-Jørgensen
2021-06-08 12:12     ` Maciej Fijalkowski
2021-06-08 21:43       ` Toke Høiland-Jørgensen [this message]
2021-06-03  0:52   ` Nguyen, Anthony L
2021-06-08 12:13     ` Maciej Fijalkowski
2021-06-03  8:42   ` Jesper Dangaard Brouer

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