netdev - Re: Why do we need tasklet in IFB?

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Message-ID: <CAAmHdhx3bTi7G=WFHZkv5C4FA4REascj+QyRQHJ9i9fAPSFgSw@mail.gmail.com>
Date:   Mon, 31 Oct 2016 11:02:54 -0700
From:   Michael Ma <make0818@...il.com>
To:     Stephen Hemminger <stephen@...workplumber.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Why do we need tasklet in IFB?

2016-10-31 11:02 GMT-07:00 Michael Ma <make0818@...il.com>:
> 2016-10-28 14:52 GMT-07:00 Michael Ma <make0818@...il.com>:
>> 2016-10-28 14:48 GMT-07:00 Stephen Hemminger <stephen@...workplumber.org>:
>>> On Fri, 28 Oct 2016 14:45:07 -0700
>>> Michael Ma <make0818@...il.com> wrote:
>>>
>>>> 2016-10-28 14:38 GMT-07:00 Stephen Hemminger <stephen@...workplumber.org>:
>>>> > On Fri, 28 Oct 2016 14:36:27 -0700
>>>> > Michael Ma <make0818@...il.com> wrote:
>>>> >
>>>> >> Hi -
>>>> >>
>>>> >> Currently IFB uses tasklet to process tx/rx on the interface that
>>>> >> forwarded the packet to IFB. My understanding on why we're doing this
>>>> >> is that since dev_queue_xmit() can be invoked in interrupt, we want to
>>>> >> defer the processing of original tx/rx in case ifb_xmit() is called
>>>> >> from interrupt.
>>>> >
>>>> > dev_queue_xmit is only called from interrupt if doing netconsole.
>>>>
>
> In fact this doesn't seem to explain since if the original path is tx
> and the context is interrupt, IFB will call dev_queue_xmit as well so
> the context can be interrupt in that case.
>
> Then tasklet is still unnecessary.
>
>>>> OK - so the reason is that netif_receive_skb() can only be invoked
>>>> from softirq and we have to use tasklet in IFB to guarantee this.
>>>>
>>>> Then if the original path is rx, tasklet is unnecessary because
>>>> ifb_xmit() is invoked from netif_receive_skb() which is always in the
>>>> softirq context, right?
>>>
>>> The other reason is to avoid deep kernel callstacks

I see - this seems to be the ultimate reason but in case we know the
actual IFB configuration is simple then tasklet can still be avoided,
right?