netdev - Re: [PATCH v4 1/5] r8169: Coalesce r8169_mac_ocp_write/modify calls to reduce spinlock stalls

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Date: Mon, 30 Oct 2023 14:50:39 -0700
From: Jacob Keller <jacob.e.keller@...el.com>
To: Mirsad Goran Todorovac <mirsad.todorovac@....unizg.hr>, Jason Gunthorpe
	<jgg@...pe.ca>, Joerg Roedel <jroedel@...e.de>, Lu Baolu
	<baolu.lu@...ux.intel.com>, <iommu@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>
CC: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>, "Robin
 Murphy" <robin.murphy@....com>, Heiner Kallweit <hkallweit1@...il.com>,
	<nic_swsd@...ltek.com>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Marco Elver <elver@...gle.com>
Subject: Re: [PATCH v4 1/5] r8169: Coalesce r8169_mac_ocp_write/modify calls
 to reduce spinlock stalls



On 10/29/2023 4:04 AM, Mirsad Goran Todorovac wrote:> A pair of new
helpers r8168_mac_ocp_write_seq() and r8168_mac_ocp_modify_seq()
> are introduced.
> 
> The motivation for these helpers was the locking overhead of 130 consecutive
> r8168_mac_ocp_write() calls in the RTL8411b reset after the NIC gets confused
> if the PHY is powered-down.
> 
> To quote Heiner:
> 
>     On RTL8411b the RX unit gets confused if the PHY is powered-down.
>     This was reported in [0] and confirmed by Realtek. Realtek provided
>     a sequence to fix the RX unit after PHY wakeup.
> 
> A series of about 130 r8168_mac_ocp_write() calls is performed to program the
> RTL registers for recovery, each doing an expensive spin_lock_irqsave() and
> spin_unlock_irqrestore().
> 
> Each mac ocp write is made of:
> 
>     static void __r8168_mac_ocp_write(struct rtl8169_private *tp, u32 reg,
>                       u32 data)
>     {
>         if (rtl_ocp_reg_failure(reg))
>             return;
> 
>         RTL_W32(tp, OCPDR, OCPAR_FLAG | (reg << 15) | data);
>     }
> 
>     static void r8168_mac_ocp_write(struct rtl8169_private *tp, u32 reg,
>                     u32 data)
>     {
>         unsigned long flags;
> 
>         raw_spin_lock_irqsave(&tp->mac_ocp_lock, flags);
>         __r8168_mac_ocp_write(tp, reg, data);
>         raw_spin_unlock_irqrestore(&tp->mac_ocp_lock, flags);
>     }
> 
> Register programming is done through RTL_W32() macro which expands into
> 
>     #define RTL_W32(tp, reg, val32) writel((val32), tp->mmio_addr + (reg))
> 
> which is further (on Alpha):
> 
>     extern inline void writel(u32 b, volatile void __iomem *addr)
>     {
>         mb();
>         __raw_writel(b, addr);
>     }
> 
> or on i386/x86_64:
> 
>     #define build_mmio_write(name, size, type, reg, barrier) \
>     static inline void name(type val, volatile void __iomem *addr) \
>     { asm volatile("mov" size " %0,%1": :reg (val), \
>     "m" (*(volatile type __force *)addr) barrier); }
> 
>     build_mmio_write(writel, "l", unsigned int, "r", :"memory")
> 
> This obviously involves iat least a compiler barrier.
> 
> mb() expands into something like this i.e. on x86_64:
> 
>     #define mb()    asm volatile("lock; addl $0,0(%%esp)" ::: "memory")
> 
> This means a whole lot of memory bus stalls: for spin_lock_irqsave(),
> memory barrier, writel(), and spin_unlock_irqrestore().
> 
> With about 130 of these sequential calls to r8168_mac_ocp_write() this looks like
> a lock storm that will stall all of the cores and CPUs on the same memory controller
> for certain time I/O takes to finish.
> 
> In a sequential case of RTL register programming, the writes to RTL registers
> can be coalesced under a same raw spinlock. This can dramatically decrease the
> number of bus stalls in a multicore or multi-CPU system.
> 
> Macro helpers r8168_mac_ocp_write_seq() and r8168_mac_ocp_modify_seq() are
> provided to reduce lock contention:
> 
>     static void rtl_hw_start_8411_2(struct rtl8169_private *tp)
>     {
> 
>         ...
> 
>         /* The following Realtek-provided magic fixes an issue with the RX unit
>          * getting confused after the PHY having been powered-down.
>          */
> 
>         static const struct recover_8411b_info init_zero_seq[] = {
>             { 0xFC28, 0x0000 }, { 0xFC2A, 0x0000 }, { 0xFC2C, 0x0000 },
>             ...
>         };
> 
>         ...
> 
>         r8168_mac_ocp_write_seq(tp, init_zero_seq);
> 
>         ...
> 
>     }
> 
> The hex data is preserved intact through s/r8168_mac_ocp_write[(]tp,/{ / and s/[)];/ },/
> functions that only changed the function names and the ending of the line, so the actual
> hex data is unchanged.
> 
> To repeat, the reason for the introduction of the original commit
> was to enable recovery of the RX unit on the RTL8411b which was confused by the
> powered-down PHY. This sequence of r8168_mac_ocp_write() calls amplifies the problem
> into a series of about 500+ memory bus locks, most waiting for the main memory read,
> modify and write under a LOCK. The memory barrier in RTL_W32 should suffice for
> the programming sequence to reach RTL NIC registers.
> 
> [0] https://bugzilla.redhat.com/show_bug.cgi?id=1692075
> 


I might have chosen to send some of this information as the cover letter
for the series instead of just as part of the commit message for [1/5],
but either way:

Reviewed-by: Jacob Keller <jacob.e.keller@...el.com>