Date: Mon, 30 Oct 2023 23:08:26 +0100
From: Heiner Kallweit <hkallweit1@...il.com>
To: Jacob Keller <jacob.e.keller@...el.com>,
 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>, 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 30.10.2023 22:50, Jacob Keller wrote:
> 
> 
> 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>

Cover letter is still missing, and there's a v5 already.
Good example why we have the "max one version per day" rule.

There's still some issues with the series, see my review comments
for v5. As-is I'd NAK the series.

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