netdev - Re: [PATCH bpf-next v2 06/10] xsk: propagate napi

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Message-ID: <585b011f-0817-a684-d1db-125bb55741fe@intel.com>
Date:   Mon, 16 Nov 2020 14:24:56 +0100
From:   Björn Töpel <bjorn.topel@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Björn Töpel <bjorn.topel@...il.com>,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        magnus.karlsson@...el.com, ast@...nel.org, daniel@...earbox.net,
        maciej.fijalkowski@...el.com, sridhar.samudrala@...el.com,
        jesse.brandeburg@...el.com, qi.z.zhang@...el.com, kuba@...nel.org,
        edumazet@...gle.com, jonathan.lemon@...il.com, maximmi@...dia.com,
        intel-wired-lan@...ts.osuosl.org, netanel@...zon.com,
        akiyano@...zon.com, michael.chan@...adcom.com,
        sgoutham@...vell.com, ioana.ciornei@....com,
        ruxandra.radulescu@....com, thomas.petazzoni@...tlin.com,
        mcroce@...rosoft.com, saeedm@...dia.com, tariqt@...dia.com,
        aelior@...vell.com, ecree@...arflare.com,
        ilias.apalodimas@...aro.org, grygorii.strashko@...com,
        sthemmin@...rosoft.com, kda@...ux-powerpc.org
Subject: Re: [PATCH bpf-next v2 06/10] xsk: propagate napi_id to XDP socket Rx
 path

On 2020-11-16 13:42, Michael S. Tsirkin wrote:
> On Mon, Nov 16, 2020 at 01:01:40PM +0100, BjÃ¶rn TÃ¶pel wrote:
>>
>> On 2020-11-16 12:55, Michael S. Tsirkin wrote:
>>> On Mon, Nov 16, 2020 at 12:04:12PM +0100, BjÃfÂ¶rn TÃfÂ¶pel wrote:
>>>> From: BjÃfÂ¶rn TÃfÂ¶pel <bjorn.topel@...el.com>
>>>>
>>>> Add napi_id to the xdp_rxq_info structure, and make sure the XDP
>>>> socket pick up the napi_id in the Rx path. The napi_id is used to find
>>>> the corresponding NAPI structure for socket busy polling.
>>>>
>>>> Acked-by: Ilias Apalodimas <ilias.apalodimas@...aro.org>
>>>> Signed-off-by: BjÃfÂ¶rn TÃfÂ¶pel <bjorn.topel@...el.com>
>>>
>>> A bunch of drivers just pass in 0. could you explain when
>>> is that ok? how bad is it if the wrong id is used?
>>>
>>
>> If zero is passed, which is a non-valid NAPI_ID, busy-polling will never
>> be performed.
>>
>> Depending on the structure of the driver, napi might or might not be
>> initialized (napi_id != 0) or even available. When it wasn't obvious, I
>> simply set it to zero.
>>
>> So, short; No harm if zero, but busy-polling cannot be used in an XDP
>> context.
>>
>>
>> [...]
>>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>>> index 21b71148c532..d71fe41595b7 100644
>>>> --- a/drivers/net/virtio_net.c
>>>> +++ b/drivers/net/virtio_net.c
>>>> @@ -1485,7 +1485,7 @@ static int virtnet_open(struct net_device *dev)
>>>>    			if (!try_fill_recv(vi, &vi->rq[i], GFP_KERNEL))
>>>>    				schedule_delayed_work(&vi->refill, 0);
>>>> -		err = xdp_rxq_info_reg(&vi->rq[i].xdp_rxq, dev, i);
>>>> +		err = xdp_rxq_info_reg(&vi->rq[i].xdp_rxq, dev, i, 0);
>>>>    		if (err < 0)
>>>>    			return err;
>>>
>>> Should this be rq.napi.napi_id ?
>>>
>>
>> Yes, if rq.napi.napi_id is valid here! Is it?
> 
> What initializes it? netif_napi_add? Then I think yes, it's
> initialized for all queues ...
> Needs to be tested naturally.
> 

Yeah, netid_napi_add does the id generation.

My idea was that driver would gradually move to a correct NAPI id (given
that it's hard to test w/o HW. Virtio however is simpler to test. :-)


Björn