Re: [PATCH v4 1/5] r8169: Coalesce r8169_mac_ocp_write/modify calls to reduce spinlock stalls
From: Mirsad Todorovac
Date: Mon Oct 30 2023 - 23:51:26 EST
On 10/31/23 00:14, Jacob Keller wrote:
On 10/30/2023 3:08 PM, Heiner Kallweit wrote:
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@xxxxxxxxx>
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.
I realise we need to keep the development process coherent. I am sorry that
my inexperience in the patch submission process made the whole series look bad.
As I previously stated to Mr. Kallweit, I will do the required number of iterations
to ensure the quality of the patches (I saw some go up to over 20 versions).
Heh, ya. A v5 was sent without there being a single (public) comment on
the list prior to my reviewing. I didn't notice the v5, and my mail
scripts pointed out this series didn't have anyone who'd looked at it
yet.. I guess I could have searched for and noticed a newer version.
Well, dear Sir,
I see I owe you an apology for I did not know about the "max one version per day"
rule. I was warned however not to overwhelm the maintainers by Guillaume Nault in
January and somehow I hypomanicaly OCD'd on this. My fault entirely.
I hope we can mend this.
I guess this is my time to take a break, do some homework and return to the drawing
board.
Besides, now we are in the merge window anyway, so I should thank Mr. Kallweit for
the special attention and for making an exception.
Am I allowed to keep Mr. Keller's Reviewed-by: tags on the reviewed diffs provided
that I fix the cover letter issue and objections?
Have a nice day.
Regards,
Mirsad