linux-kernel - Re: [PATCH 2/3] can: raw: use bitfields to store flags in struct raw

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Message-ID: <f0a34514-19da-4c73-9cd4-ae220fed6447@kernel.org>
Date: Mon, 15 Sep 2025 19:47:00 +0900
From: Vincent Mailhol <mailhol@...nel.org>
To: Oliver Hartkopp <socketcan@...tkopp.net>,
 Marc Kleine-Budde <mkl@...gutronix.de>
Cc: linux-can@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] can: raw: use bitfields to store flags in struct
 raw_sock

On 15/09/2025 at 19:16, Oliver Hartkopp wrote:
> On 15.09.25 11:23, Vincent Mailhol wrote:
>> The loopback, recv_own_msgs, fd_frames and xl_frames fields of struct
>> raw_sock just need to store one bit of information.
>>
>> Declare all those members as a bitfields of type unsigned int and
>> width one bit.
>>
>> Add a temporary variable to raw_setsockopt() and raw_getsockopt() to
>> make the conversion between the stored bits and the socket interface.
>>
>> This reduces struct raw_sock by eight bytes.
>>
>> Statistics before:
>>
>>    $ pahole --class_name=raw_sock net/can/raw.o
>>    struct raw_sock {
>>        struct sock                sk __attribute__((__aligned__(8))); /*    
>> 0   776 */
>>
>>        /* XXX last struct has 1 bit hole */
>>
>>        /* --- cacheline 12 boundary (768 bytes) was 8 bytes ago --- */
>>        int                        bound;                /*   776     4 */
>>        int                        ifindex;              /*   780     4 */
>>        struct net_device *        dev;                  /*   784     8 */
>>        netdevice_tracker          dev_tracker;          /*   792     0 */
>>        struct list_head           notifier;             /*   792    16 */
>>        int                        loopback;             /*   808     4 */
>>        int                        recv_own_msgs;        /*   812     4 */
>>        int                        fd_frames;            /*   816     4 */
>>        int                        xl_frames;            /*   820     4 */
>>        struct can_raw_vcid_options raw_vcid_opts;       /*   824     4 */
>>        canid_t                    tx_vcid_shifted;      /*   828     4 */
>>        /* --- cacheline 13 boundary (832 bytes) --- */
>>        canid_t                    rx_vcid_shifted;      /*   832     4 */
>>        canid_t                    rx_vcid_mask_shifted; /*   836     4 */
>>        int                        join_filters;         /*   840     4 */
>>        int                        count;                /*   844     4 */
>>        struct can_filter          dfilter;              /*   848     8 */
>>        struct can_filter *        filter;               /*   856     8 */
>>        can_err_mask_t             err_mask;             /*   864     4 */
>>
>>        /* XXX 4 bytes hole, try to pack */
>>
>>        struct uniqframe *         uniq;                 /*   872     8 */
>>
>>        /* size: 880, cachelines: 14, members: 20 */
>>        /* sum members: 876, holes: 1, sum holes: 4 */
>>        /* member types with bit holes: 1, total: 1 */
>>        /* forced alignments: 1 */
>>        /* last cacheline: 48 bytes */
>>    } __attribute__((__aligned__(8)));
>>
>> ...and after:
>>
>>    $ pahole --class_name=raw_sock net/can/raw.o
>>    struct raw_sock {
>>        struct sock                sk __attribute__((__aligned__(8))); /*    
>> 0   776 */
>>
>>        /* XXX last struct has 1 bit hole */
>>
>>        /* --- cacheline 12 boundary (768 bytes) was 8 bytes ago --- */
>>        int                        bound;                /*   776     4 */
>>        int                        ifindex;              /*   780     4 */
>>        struct net_device *        dev;                  /*   784     8 */
>>        netdevice_tracker          dev_tracker;          /*   792     0 */
>>        struct list_head           notifier;             /*   792    16 */
>>        unsigned int               loopback:1;           /*   808: 0  4 */
>>        unsigned int               recv_own_msgs:1;      /*   808: 1  4 */
>>        unsigned int               fd_frames:1;          /*   808: 2  4 */
>>        unsigned int               xl_frames:1;          /*   808: 3  4 */
> 
> This means that the former data structures (int) are not copied but bits are set
> (shifted, ANDed, ORed, etc) right?
> 
> So what's the difference in the code the CPU has to process for this
> improvement? Is implementing this bitmap more efficient or similar to copy the
> (unsigned ints) as-is?

It will indeed have to add a couple assembly instructions. But this is peanuts.
In the best case, the out of order execution might very well optimize this so
that not even a CPU tick is wasted. In the worst case, it is a couple CPU ticks.

On the other hands, reducing the size by 16 bytes lowers the risk to have a
cache miss. And removing one cache miss outperforms by an order of magnitude the
penalty of adding a couple assembly instructions.

Well, I did not benchmark it, but this is a commonly accepted trade off.


Yours sincerely,
Vincent Mailhol