netdev - Re: [PATCH net-next v3 1/5] net: phy: microchip_ptp : Add header file for Microchip ptp library

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Message-ID: <955cb079-b58d-4c32-8925-74f596312b21@linux.dev>
Date: Tue, 12 Nov 2024 22:56:19 +0000
From: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
To: Andrew Lunn <andrew@...n.ch>
Cc: Divya Koppera <divya.koppera@...rochip.com>, arun.ramadoss@...rochip.com,
 UNGLinuxDriver@...rochip.com, hkallweit1@...il.com, linux@...linux.org.uk,
 davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 richardcochran@...il.com
Subject: Re: [PATCH net-next v3 1/5] net: phy: microchip_ptp : Add header file
 for Microchip ptp library

On 12/11/2024 22:26, Andrew Lunn wrote:
>> I believe, the current design of mchp_ptp_clock has some issues:
>>
>> struct mchp_ptp_clock {
>>          struct mii_timestamper     mii_ts;             /*     0    48 */
>>          struct phy_device *        phydev;             /*    48     8 */
>>          struct sk_buff_head        tx_queue;           /*    56    24 */
>>          /* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */
>>          struct sk_buff_head        rx_queue;           /*    80    24 */
>>          struct list_head           rx_ts_list;         /*   104    16 */
>>          spinlock_t                 rx_ts_lock          /*   120     4 */
>>          int                        hwts_tx_type;       /*   124     4 */
>>          /* --- cacheline 2 boundary (128 bytes) --- */
>>          enum hwtstamp_rx_filters   rx_filter;          /*   128     4 */
>>          int                        layer;              /*   132     4 */
>>          int                        version;            /*   136     4 */
>>
>>          /* XXX 4 bytes hole, try to pack */
>>
>>          struct ptp_clock *         ptp_clock;          /*   144     8 */
>>          struct ptp_clock_info      caps;               /*   152   184 */
>>          /* --- cacheline 5 boundary (320 bytes) was 16 bytes ago --- */
>>          struct mutex               ptp_lock;           /*   336    32 */
>>          u16                        port_base_addr;     /*   368     2 */
>>          u16                        clk_base_addr;      /*   370     2 */
>>          u8                         mmd;                /*   372     1 */
>>
>>          /* size: 376, cachelines: 6, members: 16 */
>>          /* sum members: 369, holes: 1, sum holes: 4 */
>>          /* padding: 3 */
>>          /* last cacheline: 56 bytes */
>> };
>>
>> tx_queue will be splitted across 2 cache lines and will have spinlock on the
>> cache line next to `struct sk_buff * next`. That means 2 cachelines
>> will have to fetched to have an access to it - may lead to performance
>> issues.
>>
>> Another issue is that locks in tx_queue and rx_queue, and rx_ts_lock
>> share the same cache line which, again, can have performance issues on
>> systems which can potentially have several rx/tx queues/irqs.
>>
>> It would be great to try to reorder the struct a bit.
> 
> Dumb question: How much of this is in the hot patch? If this is only
> used for a couple of PTP packets per second, do we care about a couple
> of cache misses per second? Or will every single packet the PHY
> processes be affected by this?

Even with PTP packets timestamped only - imagine someone trying to run
PTP server part with some proper amount of clients? And it's valid to
configure more than 1 sync packet per second. It may become quite hot.