netdev - Re: [PATCH v2 net-next 5/5] tcp: Introduce optional per-netns ehash.

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Message-ID: <CANn89iJ55OHnWh88-pRxMt4d_4cbr5Fa+JOH2VDrT1SWq1t=ZA@mail.gmail.com>
Date:   Mon, 29 Aug 2022 15:59:04 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Kuniyuki Iwashima <kuniyu@...zon.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Kuniyuki Iwashima <kuni1840@...il.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 net-next 5/5] tcp: Introduce optional per-netns ehash.

On Mon, Aug 29, 2022 at 9:21 AM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> The more sockets we have in the hash table, the longer we spend looking
> up the socket.  While running a number of small workloads on the same
> host, they penalise each other and cause performance degradation.>

...
> +static int proc_tcp_child_ehash_entries(struct ctl_table *table, int write,
> +                                       void *buffer, size_t *lenp, loff_t *ppos)
> +{
> +       unsigned int tcp_child_ehash_entries;
> +       int ret;
> +
> +       ret = proc_douintvec_minmax(table, write, buffer, lenp, ppos);
> +       if (!write || ret)
> +               return ret;
> +
> +       tcp_child_ehash_entries = READ_ONCE(*(unsigned int *)table->data);
> +       if (tcp_child_ehash_entries)
> +               tcp_child_ehash_entries = roundup_pow_of_two(tcp_child_ehash_entries);

This is not thread safe.

You could simply perform the roundup_pow_of_two() elsewhere,
eg in tcp_set_hashinfo() (and leave the sysctl as set by the user)

> +
> +       WRITE_ONCE(*(unsigned int *)table->data, tcp_child_ehash_entries);
> +
> +       return 0;
> +}
> +
>