netdev - Re: [PATCH RFC] ipv6_fib limit spinlock hold times for /proc/net/ipv6

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Message-ID: <20140424142030.GD1960@order.stressinduktion.org>
Date:	Thu, 24 Apr 2014 16:20:30 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Chris Mason <clm@...com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH RFC] ipv6_fib limit spinlock hold times for /proc/net/ipv6_route

Hi!

On Thu, Apr 24, 2014 at 09:59:24AM -0400, Chris Mason wrote:
> The ipv6 code to dump routes in /proc/net/ipv6_route can hold
> a read lock on the table for a very long time.  This ends up blocking
> writers and triggering softlockups.
>
> This patch is a simple work around to limit the number of entries
> we'll walk while processing /proc/net/ipv6_route.  It intentionally
> slows down proc file reading to make sure we don't lock out the
> real ipv6 traffic.

I guess most time is spent in formatting and printing the rt6_info details
to the procfs file. Have you tried excluding !(rt6_info->rt6i_flags &
RTF_CACHE) routes?

Maybe this is a viable alternative. A patch could also check for
RTF_DYNAMIC and RTF_MODIFIED so we would still show redirected and
mtu-caching nodes.

> This patch is also horrible, and doesn't actually fix the entire
> problem.  We still have rcu_read_lock held the whole time we cat
> /proc/net/ipv6_route.  On an unpatched machine, I've clocked the
> time required to cat /proc/net/ipv6_route at 14 minutes.
> 
> java cats this proc file on startup to search for local routes, and the
> resulting contention on the table lock makes our boxes fall over.

Urks, does plain openjdk do that or is this something in your application?

> 
> So, I'm sending the partial fix to get discussion started.

I am planing to submit patches which reduce the caching of DST_HOST
entries in the ipv6 fib next month which will result in a much smaller
fib to walk by then.

Greetings,

  Hannes

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