netdev - Re: [PATCH] reduce the spinlock conflict during massive connect

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Date:   Sun, 05 Nov 2017 22:58:27 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Liu Yu <liuyu924@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "David S. Miller <davem@...emloft.net>,Alexey Kuznetsov "
         "<kuznet@....inr.ac.ru>,Hideaki YOSHIFUJI" <yoshfuji@...ux-ipv6.org>
Subject: Re: [PATCH] reduce the spinlock conflict during massive connect

On Mon, 2017-11-06 at 14:48 +0800, Liu Yu wrote:
> On Mon, Nov 6, 2017 at 1:27 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > On Mon, 2017-11-06 at 10:28 +0800, Liu Yu wrote:
> >> From: Liu Yu <allanyuliu@...cent.com>
> >>
> >> When a mount of processes connect to the same port at the same address
> >> simultaneously, they are likely getting the same bhash and therefore
> >> conflict with each other.
> >>
> >> The more the cpu number, the worse in this case.
> >>
> >> Use spin_trylock instead for this scene, which seems doesn't matter
> >> for common case.
> >>
> >> Signed-off-by: Liu Yu <allanyuliu@...cent.com>
> >> ---
> >>  net/ipv4/inet_hashtables.c |    6 +++++-
> >>  1 files changed, 5 insertions(+), 1 deletions(-)
> >>
> >> diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> >> index e7d15fb..cc11ec7 100644
> >> --- a/net/ipv4/inet_hashtables.c
> >> +++ b/net/ipv4/inet_hashtables.c
> >> @@ -581,13 +581,17 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
> >>  other_parity_scan:
> >>       port = low + offset;
> >>       for (i = 0; i < remaining; i += 2, port += 2) {
> >> +             int ret;
> >> +
> >>               if (unlikely(port >= high))
> >>                       port -= remaining;
> >>               if (inet_is_local_reserved_port(net, port))
> >>                       continue;
> >>               head = &hinfo->bhash[inet_bhashfn(net, port,
> >>                                                 hinfo->bhash_size)];
> >> -             spin_lock_bh(&head->lock);
> >> +             ret = spin_trylock(&head->lock);
> >> +             if (unlikely(!ret))
> >> +                     continue;
> >>
> >>               /* Does not bother with rcv_saddr checks, because
> >>                * the established check is already unique enough.
> >
> > This is broken.
> >
> > I am pretty sure you have not really tested this patch properly.
> >
> > Chances are very high that a connect() will miss slots and wont succeed,
> > when table is almost full.
> 
> Thanks for your comments!
> 
> Can you explain how connect() miss slots when table is almost full?


Every time your spin_trylock() is failing, you will not examin one port.

Now go back to the loop :

for (i = 0; i < remaining; i += 2, port += 2) {

Since we no longer look all the ports, we are going to fail to find a
4-tuple.

Really a spin_trylock() is a not a good idea.

Find something else if you really have contention at connect()

> 
> >
> > Performance is nice, but we actually need to allocate a 4-tuple in a
> > more deterministic fashion.
> >
> 
> So, what's the 4th element would you suggest?

What 4th element I am suggesting ?