Re: [PATCH next 4/5] locking/osq_lock: Optimise per-cpu data accesses.
From: Waiman Long
Date: Sat Dec 30 2023 - 22:04:24 EST
On 12/30/23 06:35, David Laight wrote:
From: Ingo Molnar
Sent: 30 December 2023 11:09
* Waiman Long <longman@xxxxxxxxxx> 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@xxxxxxxxxx>
---
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