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From: John Ousterhout <ouster@cs.stanford.edu>
To: netdev@vger.kernel.org
Subject: BUG: sk_backlog.len can overestimate
Date: Mon, 30 Sep 2019 16:58:51 -0700	[thread overview]
Message-ID: <CAGXJAmz5izfnamHA3Y_hU-AT1CX5K2MN=6BPjRXXcTCWvPeWng@mail.gmail.com> (raw)
In-Reply-To: <CAGXJAmwQw1ohc48NfAvMyNDpDgHGkdVO89Jo8B0j0TuMr7wLpA@mail.gmail.com>

As of 4.16.10, it appears to me that sk->sk_backlog_len does not
provide an accurate estimate of backlog length; this reduces the
usefulness of the "limit" argument to sk_add_backlog.

The problem is that, under heavy load, sk->sk_backlog_len can grow
arbitrarily large, even though the actual amount of data in the
backlog is small. This happens because __release_sock doesn't reset
the backlog length until it gets completely caught up. Under heavy
load, new packets can be arriving continuously  into the backlog
(which increases sk_backlog.len) while other packets are being
serviced. This can go on forever, so sk_backlog.len never gets reset
and it can become arbitrarily large.

Because of this, the "limit" argument to sk_add_backlog may not be
useful, since it could result in packets being discarded even though
the backlog is not very large.

       reply	other threads:[~2019-09-30 23:59 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CAGXJAmwQw1ohc48NfAvMyNDpDgHGkdVO89Jo8B0j0TuMr7wLpA@mail.gmail.com>
2019-09-30 23:58 ` John Ousterhout [this message]
2019-10-01  0:14   ` BUG: sk_backlog.len can overestimate Eric Dumazet
2019-10-01 15:44     ` John Ousterhout
     [not found]     ` <CAGXJAmxmJ-Vm379N4nbjXeQCAgY9ur53wmr0HZy23dQ_t++r-Q@mail.gmail.com>
     [not found]       ` <f4520c32-3133-fb3b-034e-d492d40eb066@gmail.com>
2019-10-01 15:46         ` Fwd: " John Ousterhout
2019-10-01 15:48         ` John Ousterhout
2019-10-01 16:19           ` Eric Dumazet
2019-10-01 17:25             ` John Ousterhout
2019-10-01 18:34               ` Eric Dumazet
2019-10-01 20:45                 ` John Ousterhout
2019-10-01 20:53                   ` Eric Dumazet
2019-10-01 23:01                     ` John Ousterhout

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