Re: [tip: sched/core] sched: Add cluster scheduler level for x86

[Peter Zijlstra]

On Wed, Oct 20, 2021 at 10:36:19PM +0200, Peter Zijlstra wrote:

> OK, I think I see what's happening.
> 
> AFAICT cacheinfo.c does *NOT* set l2c_id on AMD/Hygon hardware, this
> means it's set to BAD_APICID.
> 
> This then results in match_l2c() to never match. And as a direct
> consequence set_cpu_sibling_map() will generate cpu_l2c_shared_mask with
> just the one CPU set.
> 
> And we have the above result and things come unstuck if we assume:
>   SMT <= L2 <= LLC
> 
> Now, the big question, how to fix this... Does AMD have means of
> actually setting l2c_id or should we fall back to using match_smt() for
> l2c_id == BAD_APICID ?

The latter looks something like the below and ought to make EPYC at
least function as it did before.


---
 arch/x86/kernel/smpboot.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 849159797101..c2671b2333d1 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -472,7 +472,7 @@ static bool match_l2c(struct cpuinfo_x86 *c, struct cpuinfo_x86 *o)
 
 	/* Do not match if we do not have a valid APICID for cpu: */
 	if (per_cpu(cpu_l2c_id, cpu1) == BAD_APICID)
-		return false;
+		return match_smt(c, o); /* assume at least SMT shares L2 */
 
 	/* Do not match if L2 cache id does not match: */
 	if (per_cpu(cpu_l2c_id, cpu1) != per_cpu(cpu_l2c_id, cpu2))