linux-kernel - Re: [ovs-dev] [PATCH net-next v2] openvswitch: reduce stack usage in do_execute

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Message-ID: <f7t34yzpdo2.fsf@redhat.com>
Date:   Wed, 27 Sep 2023 16:24:45 -0400
From:   Aaron Conole <aconole@...hat.com>
To:     Ilya Maximets <i.maximets@....org>
Cc:     netdev@...r.kernel.org, dev@...nvswitch.org,
        linux-kernel@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [ovs-dev] [PATCH net-next v2] openvswitch: reduce stack usage
 in do_execute_actions

Ilya Maximets <i.maximets@....org> writes:

> do_execute_actions() function can be called recursively multiple
> times while executing actions that require pipeline forking or
> recirculations.  It may also be re-entered multiple times if the packet
> leaves openvswitch module and re-enters it through a different port.
>
> Currently, there is a 256-byte array allocated on stack in this
> function that is supposed to hold NSH header.  Compilers tend to
> pre-allocate that space right at the beginning of the function:
>
>      a88:       48 81 ec b0 01 00 00    sub    $0x1b0,%rsp
>
> NSH is not a very common protocol, but the space is allocated on every
> recursive call or re-entry multiplying the wasted stack space.
>
> Move the stack allocation to push_nsh() function that is only used
> if NSH actions are actually present.  push_nsh() is also a simple
> function without a possibility for re-entry, so the stack is returned
> right away.
>
> With this change the preallocated space is reduced by 256 B per call:
>
>      b18:       48 81 ec b0 00 00 00    sub    $0xb0,%rsp
>
> Signed-off-by: Ilya Maximets <i.maximets@....org>
> ---

Thanks, Ilya.

Reviewed-by: Aaron Conole <aconole@...hat.com>