Re: [PATCH v2 1/6] powerpc/smp: Cache CPU has Asymmetric SMP

Srikar Dronamraju
2023-10-20T09:09:40Z

* Michael Ellerman <mpe@ellerman.id.au> [2023-10-19 15:33:16]:

> Srikar Dronamraju <srikar@linux.vnet.ibm.com> writes:
> > Currently cpu feature flag is checked whenever powerpc_smt_flags gets
> > called. This is an unnecessary overhead. CPU_FTR_ASYM_SMT is set based
> > on the processor and all processors will either have this set or will
> > have it unset.
>
> The cpu_has_feature() test is implemented with a static key.
>
> So AFAICS this is just replacing one static key with another?
>

> I see that you use the new static key in subsequent patches. But
> couldn't those just use the existing cpu feature test?
>

Yes, we can use the existing cpu feature test itself.

> Anyway I'd be interested to see how the generated code differs
> before/after this.
>
---------------------------->8----------------------------------------------8<------------
Before this change
0000000000000500 <powerpc_smt_flags>:
{
500: 00 00 4c 3c addis r2,r12,0
504: 00 00 42 38 addi r2,r2,0
508: a6 02 08 7c mflr r0
50c: 01 00 00 48 bl 50c <powerpc_smt_flags+0xc>
510: f8 ff e1 fb std r31,-8(r1)
514: 91 ff 21 f8 stdu r1,-112(r1)
#define JUMP_ENTRY_TYPE stringify_in_c(FTR_ENTRY_LONG)
#define JUMP_LABEL_NOP_SIZE 4

static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
{
asm_volatile_goto("1:\n\t"
518: 00 00 00 60 nop
printk_once(KERN_INFO "Enabling Asymmetric SMT scheduling\n");
51c: 00 00 22 3d addis r9,r2,0
flags |= SD_ASYM_PACKING;
520: 80 05 e0 3b li r31,1408
printk_once(KERN_INFO "Enabling Asymmetric SMT scheduling\n");
524: 00 00 29 89 lbz r9,0(r9)
528: 00 00 09 2c cmpwi r9,0
52c: 28 00 82 41 beq 554 <powerpc_smt_flags+0x54>
}
530: 70 00 21 38 addi r1,r1,112
534: b4 07 e3 7f extsw r3,r31
538: f8 ff e1 eb ld r31,-8(r1)
53c: 20 00 80 4e blr
int flags = SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES;
540: 80 01 e0 3b li r31,384
}
544: 70 00 21 38 addi r1,r1,112
548: b4 07 e3 7f extsw r3,r31
54c: f8 ff e1 eb ld r31,-8(r1)
550: 20 00 80 4e blr
printk_once(KERN_INFO "Enabling Asymmetric SMT scheduling\n");
554: a6 02 08 7c mflr r0
558: 00 00 62 3c addis r3,r2,0
55c: 01 00 20 39 li r9,1
560: 00 00 42 3d addis r10,r2,0
564: 00 00 63 38 addi r3,r3,0
568: 00 00 2a 99 stb r9,0(r10)
56c: 80 00 01 f8 std r0,128(r1)
570: 01 00 00 48 bl 570 <powerpc_smt_flags+0x70>
574: 00 00 00 60 nop
578: 80 00 01 e8 ld r0,128(r1)
57c: a6 03 08 7c mtlr r0
580: b0 ff ff 4b b 530 <powerpc_smt_flags+0x30>
584: 00 00 00 60 nop
588: 00 00 00 60 nop
58c: 00 00 00 60 nop

post this change.
0000000000000340 <powerpc_smt_flags>:
{
340: a6 02 08 7c mflr r0
344: 01 00 00 48 bl 344 <powerpc_smt_flags+0x4>
#define JUMP_ENTRY_TYPE stringify_in_c(FTR_ENTRY_LONG)
#define JUMP_LABEL_NOP_SIZE 4

static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
{
asm_volatile_goto("1:\n\t"
348: 00 00 00 60 nop
return SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES;
34c: 80 01 60 38 li r3,384
}
350: b4 07 63 7c extsw r3,r3
354: 20 00 80 4e blr
358: 00 00 00 60 nop
35c: 00 00 00 60 nop
return SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES | SD_ASYM_PACKING;
360: 80 05 60 38 li r3,1408
}
364: b4 07 63 7c extsw r3,r3
368: 20 00 80 4e blr
36c: 00 00 00 60 nop

---------------------------->8----------------------------------------------8<------------

I think the most of the difference is due to moving pr_info_once to
fixup_topology. Does it make sense to move the pr_info_once to
fixup_topology (which is called less often) from powerpc_smt_flags?

Even though the pr_info_once would probably translate to load + cmp + branch
we could avoid that for each smt_flag call.

So something like

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 5826f5108a12..bc22f775425b 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -993,13 +993,10 @@ static bool shared_caches;
/* cpumask of CPUs with asymmetric SMT dependency */
static int powerpc_smt_flags(void)
{
- int flags = SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES;
+ if (cpu_has_feature(CPU_FTR_ASYM_SMT)) {
+ return SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES | SD_ASYM_PACKING;

- if (cpu_has_feature(CPU_FTR_ASYM_SMT)) {
- printk_once(KERN_INFO "Enabling Asymmetric SMT scheduling\n");
- flags |= SD_ASYM_PACKING;
- }
- return flags;
+ return SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES;
}
#endif

@@ -1687,6 +1684,9 @@ static void __init fixup_topology(void)
int i;

#ifdef CONFIG_SCHED_SMT
+ if (cpu_has_feature(CPU_FTR_ASYM_SMT))
+ pr_info_once("Enabling Asymmetric SMT scheduling\n");
+
if (has_big_cores) {
pr_info("Big cores detected but using small core scheduling\n");
powerpc_topology[smt_idx].mask = smallcore_smt_mask;
--
Thanks and Regards
Srikar Dronamraju