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Message-ID: <910ac70a-5503-4594-b5b2-b2ba86bf8435@redhat.com>
Date: Sat, 30 Dec 2023 22:04:10 -0500
From: Waiman Long <longman@...hat.com>
To: David Laight <David.Laight@...LAB.COM>, 'Ingo Molnar' <mingo@...nel.org>
Cc: "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
"'peterz@...radead.org'" <peterz@...radead.org>,
"'mingo@...hat.com'" <mingo@...hat.com>, "'will@...nel.org'"
<will@...nel.org>, "'boqun.feng@...il.com'" <boqun.feng@...il.com>,
'Linus Torvalds' <torvalds@...ux-foundation.org>,
"'xinhui.pan@...ux.vnet.ibm.com'" <xinhui.pan@...ux.vnet.ibm.com>,
"'virtualization@...ts.linux-foundation.org'"
<virtualization@...ts.linux-foundation.org>,
'Zeng Heng' <zengheng4@...wei.com>
Subject: Re: [PATCH next 4/5] locking/osq_lock: Optimise per-cpu data
accesses.
On 12/30/23 06:35, David Laight wrote:
> From: Ingo Molnar
>> Sent: 30 December 2023 11:09
>>
>>
>> * Waiman Long <longman@...hat.com> wrote:
>>
>>> On 12/29/23 15:57, David Laight wrote:
>>>> this_cpu_ptr() is rather more expensive than raw_cpu_read() since
>>>> the latter can use an 'offset from register' (%gs for x86-84).
>>>>
>>>> Add a 'self' field to 'struct optimistic_spin_node' that can be
>>>> read with raw_cpu_read(), initialise on first call.
>>>>
>>>> Signed-off-by: David Laight <david.laight@...lab.com>
>>>> ---
>>>> kernel/locking/osq_lock.c | 14 +++++++++-----
>>>> 1 file changed, 9 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
>>>> index 9bb3a077ba92..b60b0add0161 100644
>>>> --- a/kernel/locking/osq_lock.c
>>>> +++ b/kernel/locking/osq_lock.c
>>>> @@ -13,7 +13,7 @@
>>>> */
>>>> struct optimistic_spin_node {
>>>> - struct optimistic_spin_node *next, *prev;
>>>> + struct optimistic_spin_node *self, *next, *prev;
>>>> int locked; /* 1 if lock acquired */
>>>> int cpu; /* encoded CPU # + 1 value */
>>>> };
>>>> @@ -93,12 +93,16 @@ osq_wait_next(struct optimistic_spin_queue *lock,
>>>> bool osq_lock(struct optimistic_spin_queue *lock)
>>>> {
>>>> - struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
>>>> + struct optimistic_spin_node *node = raw_cpu_read(osq_node.self);
>>> My gcc 11 compiler produces the following x86-64 code:
>>>
>>> 92 struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
>>> 0x0000000000000029 <+25>: mov %rcx,%rdx
>>> 0x000000000000002c <+28>: add %gs:0x0(%rip),%rdx # 0x34
>>> <osq_lock+36>
>>>
>>> Which looks pretty optimized for me. Maybe older compiler may generate more
>>> complex code. However, I do have some doubt as to the benefit of this patch
>>> at the expense of making the code a bit more complex.
> My changed code is one instruction shorter!
> 18: 65 48 8b 15 00 00 00 mov %gs:0x0(%rip),%rdx # 20 <osq_lock+0x20>
> 1f: 00
> 1c: R_X86_64_PC32 .data..percpu..shared_aligned-0x4
> However is might have one less cache line miss.
>
>> GCC-11 is plenty of a look-back window in terms of compiler efficiency:
>> latest enterprise distros use GCC-11 or newer, while recent desktop
>> distros use GCC-13. Anything older won't matter, because no major
>> distribution is going to use new kernels with old compilers.
> There must be a difference in the header files as well.
> Possibly forced by the older compiler I'm using (7.5 from Ubuntu 18.04).
> But maybe based on some config option.
>
> I'm seeing this_cpu_ptr(&xxx) converted to per_cpu_ptr(&xxx, smp_processor_id())
> which necessitates an array lookup (indexed by cpu number).
> Whereas I think you are seeing it implemented as
> raw_cpu_read(per_cpu_data_base) + offset_to(xxx)
>
> So the old code generates (after the prologue):
> 10: 49 89 fd mov %rdi,%r13
> 13: 49 c7 c4 00 00 00 00 mov $0x0,%r12
> 16: R_X86_64_32S .data..percpu..shared_aligned
> 1a: e8 00 00 00 00 callq 1f <osq_lock+0x1f>
> 1b: R_X86_64_PC32 debug_smp_processor_id-0x4
> 1f: 89 c0 mov %eax,%eax
> 21: 48 8b 1c c5 00 00 00 mov 0x0(,%rax,8),%rbx
> 28: 00
> 25: R_X86_64_32S __per_cpu_offset
> 29: e8 00 00 00 00 callq 2e <osq_lock+0x2e>
> 2a: R_X86_64_PC32 debug_smp_processor_id-0x4
> 2e: 4c 01 e3 add %r12,%rbx
> 31: 83 c0 01 add $0x1,%eax
> 34: c7 43 10 00 00 00 00 movl $0x0,0x10(%rbx)
> 3b: 48 c7 03 00 00 00 00 movq $0x0,(%rbx)
> 42: 89 43 14 mov %eax,0x14(%rbx)
> 45: 41 87 45 00 xchg %eax,0x0(%r13)
>
> I was also surprised that smp_processor_id() is a real function rather
> than an offset from %gs.
I have looked up definition of this_cpu_ptr() and gotten the following
results:
this_cpu_ptr() => raw_cpu_ptr() => arch_raw_cpu_ptr()
/*
* Compared to the generic __my_cpu_offset version, the following
* saves one instruction and avoids clobbering a temp register.
*/
#define arch_raw_cpu_ptr(ptr) \
({ \
unsigned long tcp_ptr__; \
asm ("add " __percpu_arg(1) ", %0" \
: "=r" (tcp_ptr__) \
: "m" (this_cpu_off), "0" (ptr)); \
(typeof(*(ptr)) __kernel __force *)tcp_ptr__; \
})
The presence of debug_smp_processor_id in your compiled code is likely
due to the setting of CONFIG_DEBUG_PREEMPT in your kernel config.
#ifdef CONFIG_DEBUG_PREEMPT
extern unsigned int debug_smp_processor_id(void);
# define smp_processor_id() debug_smp_processor_id()
#else
# define smp_processor_id() __smp_processor_id()
#endif
I don't have that config entry in my kernel config and so I only get 2
instructions for this_cpu_ptr(). We are not going to optimize the code
specifically for CONFIG_DEBUG_PREEMPT and so this patch should be dropped.
Cheers,
Longman
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