Re: [PATCH net] ipv6: prevent fib6_run_gc() contention

[David Miller <davem@davemloft.net>]

From: Michal Kubecek <mkubecek@suse.cz>
Date: Tue, 11 Jun 2013 12:07:18 +0200

> On Mon, Jun 10, 2013 at 02:26:42PM -0700, David Miller wrote:
>> From: Michal Kubecek <mkubecek@suse.cz>
>> Date: Tue, 4 Jun 2013 13:08:59 +0200
>> 
>> > On a high-traffic router with many processors and many IPv6 dst
>> > entries, soft lockup in fib6_run_gc() can occur when number of
>> > entries reaches gc_thresh.
>> > 
>> > This happens because fib6_run_gc() uses fib6_gc_lock to allow
>> > only one thread to run the garbage collector but ip6_dst_gc()
>> > doesn't update net->ipv6.ip6_rt_last_gc until fib6_run_gc()
>> > returns. On a system with many entries, this can take some time
>> > so that in the meantime, other threads pass the tests in
>> > ip6_dst_gc() (ip6_rt_last_gc is still not updated) and wait for
>> > the lock. They then have to run the garbage collector one after
>> > another which blocks them for quite long.
>> > 
>> > Resolve this by replacing special value ~0UL of expire parameter
>> > to fib6_run_gc() by explicit "force" parameter to choose between
>> > spin_lock_bh() and spin_trylock_bh() and call fib6_run_gc() with
>> > force=false if gc_thresh is reached but not max_size.
>> > 
>> > Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
>> 
>> It seems to me that it would be much simpler to simply update
>> ip6_rt_last_gc first, that way the other threads would elide the GC
>> call.
> 
> That was my original idea but I was afraid that while the remaining
> window in ip6_dst_gc() would be very short and probably safe, we could
> still run into problem if fib6_gc_lock was locked by some other caller
> of fib6_run_gc() which doesn't update net->ipv6.ip6_rt_last_gc,
> especially via a timer.

Why don't you simply try it and find out?