netdev - Re: [net-next PATCH 2/7] mlx4: use napi_consume

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Date:	Wed, 9 Mar 2016 14:07:40 -0800
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>, eugenia@...lanox.com,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Saeed Mahameed <saeedm@...lanox.com>,
	Or Gerlitz <gerlitz.or@...il.com>
Subject: Re: [net-next PATCH 2/7] mlx4: use napi_consume_skb API to get bulk
 free operations

On Wed, Mar 9, 2016 at 1:47 PM, Jesper Dangaard Brouer
<brouer@...hat.com> wrote:
> On Wed, 9 Mar 2016 13:43:59 -0800
> Alexander Duyck <alexander.duyck@...il.com> wrote:
>
>> On Wed, Mar 9, 2016 at 1:36 PM, Jesper Dangaard Brouer
>> <brouer@...hat.com> wrote:
>> > On Wed, 09 Mar 2016 16:03:20 -0500 (EST)
>> > David Miller <davem@...emloft.net> wrote:
>> >
>> >> From: Alexander Duyck <alexander.duyck@...il.com>
>> >> Date: Wed, 9 Mar 2016 08:47:58 -0800
>> >>
>> >> > On Wed, Mar 9, 2016 at 3:00 AM, Jesper Dangaard Brouer
>> >> > <brouer@...hat.com> wrote:
>> >> >> Passing the budget down was Alex'es design.  Axel any thoughts?
>> >> >
>> >> > I'd say just use dev_consume_skb_any in the bulk free instead of
>> >> > dev_consume_skb_irq.  This is slow path, as you said, so it shouldn't
>> >> > come up often.
>> >>
>> >> Agreed.
>> >>
>> >> >> I do wonder how expensive this check is... as it goes into a code
>> >> >> hotpath, which is very unlikely.  The good thing would be, that we
>> >> >> handle if buggy drivers call this function from a none softirq context
>> >> >> (as these bugs could be hard to catch).
>> >> >>
>> >> >> Can netpoll ever be called from softirq or with BH disabled? (It
>> >> >> disables IRQs, which would break calling kmem_cache_free_bulk).
>> >> >
>> >> > It is better for us to switch things out so that the napi_consume_skb
>> >> > is the fast path with dev_consume_skb_any as the slow.  There are too
>> >> > many scenarios where we could be invoking something that makes use of
>> >> > this within the Tx path so it is probably easiest to just solve it
>> >> > that way so we don't have to deal with it again in the future.
>> >>
>> >> Indeed.
>> >
>> > So, if I understand you correctly, then we drop the budget parameter
>> > and check for in_softirq(), like:
>> >
>> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>> > index 7af7ec635d90..a3c61a9b65d2 100644
>> > --- a/net/core/skbuff.c
>> > +++ b/net/core/skbuff.c
>> > @@ -796,14 +796,14 @@ void __kfree_skb_defer(struct sk_buff *skb)
>> >         _kfree_skb_defer(skb);
>> >  }
>> >
>> > -void napi_consume_skb(struct sk_buff *skb, int budget)
>> > +void napi_consume_skb(struct sk_buff *skb)
>> >  {
>> >         if (unlikely(!skb))
>> >                 return;
>> >
>> > -       /* if budget is 0 assume netpoll w/ IRQs disabled */
>> > -       if (unlikely(!budget)) {
>> > -               dev_consume_skb_irq(skb);
>> > +       /* Handle if not called from NAPI context, and netpoll invocation */
>> > +       if (unlikely(!in_softirq())) {
>> > +               dev_consume_skb_any(skb);
>> >                 return;
>> >         }
>> >
>>
>> No.  We still need to have the budget value.  What we do though is
>> have that feed into dev_consume_skb_any.
>>
>> The problem with using in_softirq is that it will trigger if softirqs
>> are just disabled so there are more possible paths where it is
>> possible.  For example the transmit path has bottom halves disabled so
>> I am pretty sure it might trigger this as well.  We want this to only
>> execute when we are running from a NAPI polling routine with a
>> non-zero budget.
>
> What about using in_serving_softirq() instead of in_softirq() ?
> (would that allow us to drop the budget parameter?)

The problem is there are multiple softirq handlers you could be
referring to.  NAPI is just one.  What if for example someone sets up
a tasklet that has to perform some sort of reset with RTNL held.  I am
pretty sure we don't want the tasklet using the NAPI free context
since there will be nothing to actually free the buffers at the end of
it.  We want to avoid that.  That is why I was using the budget value.

- Alex