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

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Message-ID: <7e9e0964-6532-42e6-9005-18715aaac5a6@lunn.ch>
Date: Wed, 13 Nov 2024 00:11:23 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
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 Tue, Nov 12, 2024 at 10:56:19PM +0000, Vadim Fedorenko wrote:
> 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.

I'm just thinking of Donald Knuth:

“The real problem is that programmers have spent far too much time
worrying about efficiency in the wrong places and at the wrong times;
premature optimization is the root of all evil (or at least most of
it) in programming.”

	Andrew