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    Subject[PATCH 5.9 047/757] nexthop: Fix performance regression in nexthop deletion
    Date
    From: Ido Schimmel <idosch@nvidia.com>

    [ Upstream commit df6afe2f7c19349de2ee560dc62ea4d9ad3ff889 ]

    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@nvidia.com>
    Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
    Reviewed-by: David Ahern <dsahern@gmail.com>
    Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
    Link: https://lore.kernel.org/r/20201016172914.643282-1-idosch@idosch.org
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/ipv4/nexthop.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/net/ipv4/nexthop.c
    +++ b/net/ipv4/nexthop.c
    @@ -842,7 +842,7 @@ static void remove_nexthop_from_groups(s
    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)

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