netdev - Re: [PATCH 4.14.y stable] xfrm: policy: remove pcpu policy cache

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Date:   Thu, 22 Aug 2019 06:09:41 -0700
From:   Greg KH <greg@...ah.com>
To:     Florian Westphal <fw@...len.de>
Cc:     stable@...r.kernel.org, vakul.garg@....com, netdev@...r.kernel.org,
        Kristian Evensen <kristian.evensen@...il.com>,
        Steffen Klassert <steffen.klassert@...unet.com>
Subject: Re: [PATCH 4.14.y stable] xfrm: policy: remove pcpu policy cache

On Thu, Aug 22, 2019 at 01:21:09PM +0200, Florian Westphal wrote:
> commit e4db5b61c572475bbbcf63e3c8a2606bfccf2c9d upstream.
> 
> Kristian Evensen says:
>   In a project I am involved in, we are running ipsec (Strongswan) on
>   different mt7621-based routers. Each router is configured as an
>   initiator and has around ~30 tunnels to different responders (running
>   on misc. devices). Before the flow cache was removed (kernel 4.9), we
>   got a combined throughput of around 70Mbit/s for all tunnels on one
>   router. However, we recently switched to kernel 4.14 (4.14.48), and
>   the total throughput is somewhere around 57Mbit/s (best-case). I.e., a
>   drop of around 20%. Reverting the flow cache removal restores, as
>   expected, performance levels to that of kernel 4.9.
> 
> When pcpu xdst exists, it has to be validated first before it can be
> used.
> 
> A negative hit thus increases cost vs. no-cache.
> 
> As number of tunnels increases, hit rate decreases so this pcpu caching
> isn't a viable strategy.
> 
> Furthermore, the xdst cache also needs to run with BH off, so when
> removing this the bh disable/enable pairs can be removed too.
> 
> Kristian tested a 4.14.y backport of this change and reported
> increased performance:
> 
>   In our tests, the throughput reduction has been reduced from around -20%
>   to -5%. We also see that the overall throughput is independent of the
>   number of tunnels, while before the throughput was reduced as the number
>   of tunnels increased.
> 
> Reported-by: Kristian Evensen <kristian.evensen@...il.com>
> Signed-off-by: Florian Westphal <fw@...len.de>
> Signed-off-by: Steffen Klassert <steffen.klassert@...unet.com>
> ---
>  Vakul Garg reports traffic going via ipsec tunnels will cause the kernel
>  to spin in an infinite loop due to xfrm policy reference count
>  overflowing and becoming 0.
>  The refcount leak is in the pcpu cache.  Instead of fixing this, just
>  remove the pcpu cache -- its not present in any other stable release.
>  Vakul reported that this patch fixes the problem.
> 
>  There are no major deviations from the upstream revert; conflicts
>  were only due to context.

Now queued up, does 4.9.y also need this?

thanks,

greg k-h