Subject: Re: [RFC 7/8] powerpc/book3s64/slb: Add no_slb_preload early cmdline param

From: Ritesh Harjani (IBM) <[email-masked]>
Date: Sat, 30 Aug 2025 15:41:44 +0530

Christophe Leroy <[email-masked]> writes:

> Le 30/08/2025 à 05:51, Ritesh Harjani (IBM) a écrit :
>> no_slb_preload cmdline can come useful in quickly disabling and/or
>> testing the performance impact of userspace slb preloads. Recently there
>> was a slb multi-hit issue due to slb preload cache which was very
>> difficult to triage. This cmdline option allows to quickly disable
>> preloads and verify if the issue exists in preload cache or somewhere
>> else. This can also be a useful option to see the effect of slb preloads
>> for any application workload e.g. number of slb faults with or w/o slb
>> preloads.
>> 
>> For e.g. with the next patch where we added slb_faults counter to /proc/vmstat:
>> 
>> with slb_preload:
>> slb_faults (minimal initrd boot): 15
>> slb_faults (full systemd boot): 300
>> 
>> with no_slb_preload:
>> slb_faults (minimal initrd boot): 33
>> slb_faults (full systemd boot): 138180
>> 
>> Cc: Madhavan Srinivasan <[email-masked]>
>> Cc: Michael Ellerman <[email-masked]>
>> Cc: Nicholas Piggin <[email-masked]>
>> Cc: Christophe Leroy <[email-masked]>
>> Cc: Paul Mackerras <[email-masked]>
>> Cc: "Aneesh Kumar K.V" <[email-masked]>
>> Cc: Donet Tom <[email-masked]>
>> Cc: [email-masked]
>> Signed-off-by: Ritesh Harjani (IBM) <[email-masked]>
>> ---
>>   Documentation/admin-guide/kernel-parameters.txt |  3 +++
>>   arch/powerpc/mm/book3s64/hash_utils.c           |  3 +++
>>   arch/powerpc/mm/book3s64/internal.h             |  7 +++++++
>>   arch/powerpc/mm/book3s64/slb.c                  | 15 +++++++++++++++
>>   4 files changed, 28 insertions(+)
>> 
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index 747a55abf494..9a66f255b659 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -7135,6 +7135,9 @@
>>   			them frequently to increase the rate of SLB faults
>>   			on kernel addresses.
>> 
>> +	no_slb_preload	[PPC,EARLY]
>> +			Disables slb preloading for userspace.
>> +
>>   	sunrpc.min_resvport=
>>   	sunrpc.max_resvport=
>>   			[NFS,SUNRPC]
>> diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
>> index 495b6da6f5d4..abf703563ea3 100644
>> --- a/arch/powerpc/mm/book3s64/hash_utils.c
>> +++ b/arch/powerpc/mm/book3s64/hash_utils.c
>> @@ -1319,6 +1319,9 @@ static void __init htab_initialize(void)
>>   	if (stress_slb_enabled)
>>   		static_branch_enable(&stress_slb_key);
>> 
>> +	if (no_slb_preload)
>> +		static_branch_enable(&no_slb_preload_key);
>> +
>>   	if (stress_hpt_enabled) {
>>   		unsigned long tmp;
>>   		static_branch_enable(&stress_hpt_key);
>> diff --git a/arch/powerpc/mm/book3s64/internal.h b/arch/powerpc/mm/book3s64/internal.h
>> index c26a6f0c90fc..cad08d83369c 100644
>> --- a/arch/powerpc/mm/book3s64/internal.h
>> +++ b/arch/powerpc/mm/book3s64/internal.h
>> @@ -22,6 +22,13 @@ static inline bool stress_hpt(void)
>>   	return static_branch_unlikely(&stress_hpt_key);
>>   }
>> 
>> +extern bool no_slb_preload;
>> +DECLARE_STATIC_KEY_FALSE(no_slb_preload_key);
>> +static inline bool slb_preload_disabled(void)
>> +{
>> +	return static_branch_unlikely(&no_slb_preload_key);
>> +}
>> +
>>   void hpt_do_stress(unsigned long ea, unsigned long hpte_group);
>> 
>>   void exit_lazy_flush_tlb(struct mm_struct *mm, bool always_flush);
>> diff --git a/arch/powerpc/mm/book3s64/slb.c b/arch/powerpc/mm/book3s64/slb.c
>> index 780792b9a1e5..297ab0e93c1e 100644
>> --- a/arch/powerpc/mm/book3s64/slb.c
>> +++ b/arch/powerpc/mm/book3s64/slb.c
>> @@ -42,6 +42,15 @@ early_param("stress_slb", parse_stress_slb);
>> 
>>   __ro_after_init DEFINE_STATIC_KEY_FALSE(stress_slb_key);
>> 
>> +bool no_slb_preload __initdata;
>> +static int __init parse_no_slb_preload(char *p)
>> +{
>> +	no_slb_preload = true;
>> +	return 0;
>
> Can't you call static_branch_disable() directly from here and avoid 
> doing it in hash_utils.c ?
>

parse_early_param() for cmdline options happens before the setup_feature_keys().
Hence we cannot call static_branch_disable() here in parse_no_slb_preload()

-ritesh