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From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: netdev@vger.kernel.org, bpf@vger.kernel.org,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"David Miller" <davem@davemloft.net>,
	"Björn Töpel" <bjorn.topel@gmail.com>,
	"John Fastabend" <john.fastabend@gmail.com>,
	brouer@redhat.com
Subject: Re: [PATCH bpf-next 1/2] xdp: Move devmap bulk queue into struct net_device
Date: Fri, 10 Jan 2020 23:34:47 +0100	[thread overview]
Message-ID: <871rs7x7nc.fsf@toke.dk> (raw)
In-Reply-To: <20200110170824.7379adbf@carbon>

Jesper Dangaard Brouer <brouer@redhat.com> writes:

> On Fri, 10 Jan 2020 15:22:02 +0100
> Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
>> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
>> index 2741aa35bec6..1b2bc2a7522e 100644
>> --- a/include/linux/netdevice.h
>> +++ b/include/linux/netdevice.h
> [...]
>> @@ -1993,6 +1994,8 @@ struct net_device {
>>  	spinlock_t		tx_global_lock;
>>  	int			watchdog_timeo;
>>  
>> +	struct xdp_dev_bulk_queue __percpu *xdp_bulkq;
>> +
>>  #ifdef CONFIG_XPS
>>  	struct xps_dev_maps __rcu *xps_cpus_map;
>>  	struct xps_dev_maps __rcu *xps_rxqs_map;
>
> We need to check that the cache-line for this location in struct
> net_device is not getting updated (write operation) from different CPUs.
>
> The test you ran was a single queue single CPU test, which will not
> show any regression for that case.

Well, pahole says:

	/* --- cacheline 14 boundary (896 bytes) --- */
	struct netdev_queue *      _tx __attribute__((__aligned__(64))); /*   896     8 */
	unsigned int               num_tx_queues;        /*   904     4 */
	unsigned int               real_num_tx_queues;   /*   908     4 */
	struct Qdisc *             qdisc;                /*   912     8 */
	struct hlist_head  qdisc_hash[16];               /*   920   128 */
	/* --- cacheline 16 boundary (1024 bytes) was 24 bytes ago --- */
	unsigned int               tx_queue_len;         /*  1048     4 */
	spinlock_t                 tx_global_lock;       /*  1052     4 */
	int                        watchdog_timeo;       /*  1056     4 */

	/* XXX 4 bytes hole, try to pack */

	struct xdp_dev_bulk_queue * xdp_bulkq;           /*  1064     8 */
	struct xps_dev_maps *      xps_cpus_map;         /*  1072     8 */
	struct xps_dev_maps *      xps_rxqs_map;         /*  1080     8 */
	/* --- cacheline 17 boundary (1088 bytes) --- */


of those, tx_queue_len is the max queue len (so only set on init),
tx_global_lock is not used by multi-queue devices, watchdog_timeo also
seems to be a static value thats set on init, and the xps* pointers also
only seems to be set once on init. So I think we're fine?

I can run a multi-CPU test just to be sure, but I really don't see which
of those fields might be updated on TX...

-Toke


  reply	other threads:[~2020-01-10 22:34 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-10 14:22 [PATCH bpf-next 0/2] xdp: Introduce bulking for non-map XDP_REDIRECT Toke Høiland-Jørgensen
2020-01-10 14:22 ` [PATCH bpf-next 1/2] xdp: Move devmap bulk queue into struct net_device Toke Høiland-Jørgensen
2020-01-10 15:03   ` Björn Töpel
2020-01-10 15:26     ` Toke Høiland-Jørgensen
2020-01-10 16:08   ` Jesper Dangaard Brouer
2020-01-10 22:34     ` Toke Høiland-Jørgensen [this message]
2020-01-10 22:46       ` Eric Dumazet
2020-01-10 23:16         ` Toke Høiland-Jørgensen
2020-01-10 14:22 ` [PATCH bpf-next 2/2] xdp: Use bulking for non-map XDP_REDIRECT Toke Høiland-Jørgensen
2020-01-10 15:15   ` Björn Töpel
2020-01-10 15:30     ` Toke Høiland-Jørgensen
2020-01-10 15:54       ` Björn Töpel
2020-01-10 15:57         ` Toke Høiland-Jørgensen

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