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

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Date:   Wed, 20 Oct 2021 15:51:51 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>, x86@...nel.org
Subject: Re: [tip: sched/core] sched: Add cluster scheduler level for x86

On 10/20/21 3:40 PM, Peter Zijlstra wrote:
> 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))
> 

Adding Suravee.