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From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
Cc: netdev@vger.kernel.org, bpf@vger.kernel.org, davem@davemloft.net,
	anthony.l.nguyen@intel.com, kuba@kernel.org, bjorn@kernel.org,
	Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Subject: Re: [PATCH intel-next 2/2] ice: introduce XDP Tx fallback path
Date: Tue, 01 Jun 2021 14:38:03 +0200	[thread overview]
Message-ID: <87czt5dal0.fsf@toke.dk> (raw)
In-Reply-To: <20210601113236.42651-3-maciej.fijalkowski@intel.com>

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?


  reply	other threads:[~2021-06-01 12:38 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 [this message]
2021-06-08 12:12     ` Maciej Fijalkowski
2021-06-08 21:43       ` Toke Høiland-Jørgensen
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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