netdev - Re: [RFC PATCH net-next 10/10] ipv6: Create percpu rt6

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Date:	Mon, 13 Apr 2015 13:16:17 -0700
From:	Martin KaFai Lau <kafai@...com>
To:	Steffen Klassert <steffen.klassert@...unet.com>
CC:	<netdev@...r.kernel.org>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	<kernel-team@...com>
Subject: Re: [RFC PATCH net-next 10/10] ipv6: Create percpu rt6_info

On Mon, Apr 13, 2015 at 12:59:44PM +0200, Steffen Klassert wrote:
> On Fri, Apr 10, 2015 at 06:59:36PM -0700, Martin KaFai Lau wrote:
> 
> > diff --git a/include/uapi/linux/ipv6_route.h b/include/uapi/linux/ipv6_route.h
> > index 2be7bd1..f6598d1 100644
> > --- a/include/uapi/linux/ipv6_route.h
> > +++ b/include/uapi/linux/ipv6_route.h
> > @@ -34,6 +34,7 @@
> >  #define RTF_PREF(pref)	((pref) << 27)
> >  #define RTF_PREF_MASK	0x18000000
> >  
> > +#define RTF_PCPU	0x40000000
> 
> This percpu flag is something internal, should IMO not be added
> to the uapi.
Make sense.  Where may be the right place for it?

It seems 'uapi/linux/ipv6_route.h' is the one which has all IPv6 flags laid out.
It makes modification easier since it tells us which bit is still available.

How about we add a comment to 'uapi/linux/ipv6_route.h' and move the '#define'
to 'linux/ipv6_route.h'?

> 
> > @@ -1978,8 +2100,13 @@ static void ip6_rt_copy_init(struct rt6_info *rt,
> >  static struct rt6_info *ip6_rt_cache_alloc(struct rt6_info *ort,
> >  					   const struct in6_addr *dest)
> >  {
> > -	struct rt6_info *rt = ip6_dst_alloc(dev_net(ort->dst.dev), ort->dst.dev,
> > -					    0, ort->rt6i_table);
> > +	struct rt6_info *rt;
> > +
> > +	if (ort->rt6i_flags & RTF_PCPU)
> > +		ort = (struct rt6_info *)ort->dst.from;
> > +
> > +	rt = __ip6_dst_alloc(dev_net(ort->dst.dev), ort->dst.dev,
> > +			     0, ort->rt6i_table);
> 
> Why don't you allocate the percpu resources for cached routes?
> I think using ip6_dst_alloc() would be better here, cached routes
> could benefit from this too.
Not sure if we want to open up percpu cache opportunity for RTF_CACHE
route after receiving ICMPv6 too-big from remote peers and some of our machines
have a lot of CPU (like 40), so I opt not to do it.
If we do this, we probably need to modify the gc and its related counting also.

> And in particular rt6_release() tries
> to free the percpu resources unconditionally when the route is removed
> from the fib tree. This crashes if the percpu resources are not allocated.
Good catch and will fix in v2.

Thanks,
--Martin
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