netdev - [PATCH net] nexthop: Fix performance regression in nexthop deletion

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Message-Id: <20201016172914.643282-1-idosch@idosch.org>
Date:   Fri, 16 Oct 2020 20:29:14 +0300
From:   Ido Schimmel <idosch@...sch.org>
To:     netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuba@...nel.org, dsahern@...il.com,
        nikolay@...dia.com, mlxsw@...dia.com,
        Ido Schimmel <idosch@...dia.com>
Subject: [PATCH net] nexthop: Fix performance regression in nexthop deletion

From: Ido Schimmel <idosch@...dia.com>

While insertion of 16k nexthops all using the same netdev ('dummy10')
takes less than a second, deletion takes about 130 seconds:

# time -p ip -b nexthop.batch
real 0.29
user 0.01
sys 0.15

# time -p ip link set dev dummy10 down
real 131.03
user 0.06
sys 0.52

This is because of repeated calls to synchronize_rcu() whenever a
nexthop is removed from a nexthop group:

# /usr/share/bcc/tools/offcputime -p `pgrep -nx ip` -K
...
    b'finish_task_switch'
    b'schedule'
    b'schedule_timeout'
    b'wait_for_completion'
    b'__wait_rcu_gp'
    b'synchronize_rcu.part.0'
    b'synchronize_rcu'
    b'__remove_nexthop'
    b'remove_nexthop'
    b'nexthop_flush_dev'
    b'nh_netdev_event'
    b'raw_notifier_call_chain'
    b'call_netdevice_notifiers_info'
    b'__dev_notify_flags'
    b'dev_change_flags'
    b'do_setlink'
    b'__rtnl_newlink'
    b'rtnl_newlink'
    b'rtnetlink_rcv_msg'
    b'netlink_rcv_skb'
    b'rtnetlink_rcv'
    b'netlink_unicast'
    b'netlink_sendmsg'
    b'____sys_sendmsg'
    b'___sys_sendmsg'
    b'__sys_sendmsg'
    b'__x64_sys_sendmsg'
    b'do_syscall_64'
    b'entry_SYSCALL_64_after_hwframe'
    -                ip (277)
        126554955

Since nexthops are always deleted under RTNL, synchronize_net() can be
used instead. It will call synchronize_rcu_expedited() which only blocks
for several microseconds as opposed to multiple milliseconds like
synchronize_rcu().

With this patch deletion of 16k nexthops takes less than a second:

# time -p ip link set dev dummy10 down
real 0.12
user 0.00
sys 0.04

Tested with fib_nexthops.sh which includes torture tests that prompted
the initial change:

# ./fib_nexthops.sh
...
Tests passed: 134
Tests failed:   0

Fixes: 90f33bffa382 ("nexthops: don't modify published nexthop groups")
Signed-off-by: Ido Schimmel <idosch@...dia.com>
---
 net/ipv4/nexthop.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
index 8c0f17c6863c..0dc43ad28eb9 100644
--- a/net/ipv4/nexthop.c
+++ b/net/ipv4/nexthop.c
@@ -845,7 +845,7 @@ static void remove_nexthop_from_groups(struct net *net, struct nexthop *nh,
 		remove_nh_grp_entry(net, nhge, nlinfo);
 
 	/* make sure all see the newly published array before releasing rtnl */
-	synchronize_rcu();
+	synchronize_net();
 }
 
 static void remove_nexthop_group(struct nexthop *nh, struct nl_info *nlinfo)
-- 
2.26.2