netdev - Re: [PATCH net-next v4 05/10] pie: rearrange structure members and their initializations

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Message-ID: <87ftg8bxw6.fsf@toke.dk>
Date:   Tue, 21 Jan 2020 17:02:49 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Gautam Ramakrishnan <gautamramk@...il.com>,
        David Miller <davem@...emloft.net>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "Mohit P. Tahiliani" <tahiliani@...k.edu.in>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Dave Taht <dave.taht@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Leslie Monis <lesliemonis@...il.com>
Subject: Re: [PATCH net-next v4 05/10] pie: rearrange structure members and their initializations

Gautam Ramakrishnan <gautamramk@...il.com> writes:

> On Tue, Jan 21, 2020 at 8:05 PM David Miller <davem@...emloft.net> wrote:
>>
>> From: gautamramk@...il.com
>> Date: Tue, 21 Jan 2020 19:42:44 +0530
>>
>> > From: "Mohit P. Tahiliani" <tahiliani@...k.edu.in>
>> >
>> > Rearrange the members of the structures such that they appear in
>> > order of their types. Also, change the order of their
>> > initializations to match the order in which they appear in the
>> > structures. This improves the code's readability and consistency.
>> >
>> > Signed-off-by: Mohit P. Tahiliani <tahiliani@...k.edu.in>
>> > Signed-off-by: Leslie Monis <lesliemonis@...il.com>
>> > Signed-off-by: Gautam Ramakrishnan <gautamramk@...il.com>
>>
>> What matters for structure member ordering is dense packing and
>> grouping commonly-used-together elements for performance.
>>
> We shall reorder the variables as per their appearance in the
> structure and re-submit. Could you elaborate a bit on dense packing?

The compiler will align struct member offsets according to their size,
adding padding as necessary to achieve this.
So this struct:

struct s1 {
       u32 mem32_1;
       u64 mem64_1;
       u32 mem32_2;
       u64 mem64_2;
};

will have 4 bytes of padding after both mem32_1 and mem32_2, whereas
this struct:

struct s2 {
       u64 mem64_1;
       u32 mem32_1;
       u32 mem32_2;
       u64 mem64_2;
};

won't. So sizeof(struct s1) == 32, and sizeof(struct s2) == 24, and we
say that s2 is densely packed, whereas s1 has holes in it.

The pahole tool is useful to see the layout of compiled structures
(pahole -C). It will also point out any holes.

-Toke