Re: [PATCH v4] net: use exponential backoff in netdev_wait_allrefs

[David Miller]

From: fruggeri@xxxxxxxxxx (Francesco Ruggeri)
Date: Fri, 18 Sep 2020 13:19:01 -0700

> The combination of aca_free_rcu, introduced in commit 2384d02520ff
> ("net/ipv6: Add anycast addresses to a global hashtable"), and
> fib6_info_destroy_rcu, introduced in commit 9b0a8da8c4c6 ("net/ipv6:
> respect rcu grace period before freeing fib6_info"), can result in
> an extra rcu grace period being needed when deleting an interface,
> with the result that netdev_wait_allrefs ends up hitting the msleep(250),
> which is considerably longer than the required grace period.
> This can result in long delays when deleting a large number of interfaces,
> and it can be observed with this script:
  ...
> Time with this patch on a 5.4 kernel:
> 
> real	0m7.704s
> user	0m0.385s
> sys	0m1.230s
> 
> Time without this patch:
> 
> real    0m31.522s
> user    0m0.438s
> sys     0m1.156s
> 
> v2: use exponential backoff instead of trying to wake up
>     netdev_wait_allrefs.
> v3: preserve reverse christmas tree ordering of local variables
> v4: try an extra rcu_barrier before the backoff, plus some
>     cosmetic changes.
> 
> Signed-off-by: Francesco Ruggeri <fruggeri@xxxxxxxxxx>

Applied to net-next, thanks.