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net: use exponential backoff in netdev_wait_allrefs

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The combination of aca_free_rcu, introduced in commit 2384d02520
("net/ipv6: Add anycast addresses to a global hashtable"), and
fib6_info_destroy_rcu, introduced in commit 9b0a8da8c4 ("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:

ns=dummy-ns
NIFS=100

ip netns add $ns
ip netns exec $ns ip link set lo up
ip netns exec $ns sysctl net.ipv6.conf.default.disable_ipv6=0
ip netns exec $ns sysctl net.ipv6.conf.default.forwarding=1

for ((i=0; i<$NIFS; i++))
do
        if=eth$i
        ip netns exec $ns ip link add $if type dummy
        ip netns exec $ns ip link set $if up
        ip netns exec $ns ip -6 addr add 2021:$i::1/120 dev $if
done

for ((i=0; i<$NIFS; i++))
do
        if=eth$i
        ip netns exec $ns ip link del $if
done

ip netns del $ns

Instead of using a fixed msleep(250), this patch tries an extra
rcu_barrier() followed by an exponential backoff.

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@arista.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Francesco Ruggeri 2020-09-18 13:19:01 -07:00 committed by David S. Miller
parent 1d39cd8cf7
commit 0e4be9e57e
1 changed files with 10 additions and 2 deletions
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12
net/core/dev.c
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@ -10028,10 +10028,12 @@ EXPORT_SYMBOL(netdev_refcnt_read);
* We can get stuck here if buggy protocols don't correctly
* call dev_put.
*/
#define WAIT_REFS_MIN_MSECS 1
#define WAIT_REFS_MAX_MSECS 250
static void netdev_wait_allrefs(struct net_device *dev)
{
unsigned long rebroadcast_time, warning_time;
int refcnt;
int wait = 0, refcnt;
linkwatch_forget_dev(dev);
@ -10065,7 +10067,13 @@ static void netdev_wait_allrefs(struct net_device *dev)
rebroadcast_time = jiffies;
}
msleep(250);
if (!wait) {
rcu_barrier();
wait = WAIT_REFS_MIN_MSECS;
} else {
msleep(wait);
wait = min(wait << 1, WAIT_REFS_MAX_MSECS);
}
refcnt = netdev_refcnt_read(dev);