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

[Christophe Leroy]

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 <maddy at linux.ibm.com>
> Cc: Michael Ellerman <mpe at ellerman.id.au>
> Cc: Nicholas Piggin <npiggin at gmail.com>
> Cc: Christophe Leroy <christophe.leroy at csgroup.eu>
> Cc: Paul Mackerras <paulus at ozlabs.org>
> Cc: "Aneesh Kumar K.V" <aneesh.kumar at kernel.org>
> Cc: Donet Tom <donettom at linux.ibm.com>
> Cc: linuxppc-dev at lists.ozlabs.org
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list at gmail.com>
> ---
>   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 ?

> +}
> +early_param("no_slb_preload", parse_no_slb_preload);
> +__ro_after_init DEFINE_STATIC_KEY_FALSE(no_slb_preload_key);
> +
>   static void assert_slb_presence(bool present, unsigned long ea)
>   {
>   #ifdef CONFIG_DEBUG_VM
> @@ -299,6 +308,9 @@ static void preload_add(struct thread_info *ti, unsigned long ea)
>   	unsigned char idx;
>   	unsigned long esid;
> 
> +	if (slb_preload_disabled())
> +		return;
> +
>   	if (mmu_has_feature(MMU_FTR_1T_SEGMENT)) {
>   		/* EAs are stored >> 28 so 256MB segments don't need clearing */
>   		if (ea & ESID_MASK_1T)
> @@ -414,6 +426,9 @@ void switch_slb(struct task_struct *tsk, struct mm_struct *mm)
> 
>   	copy_mm_to_paca(mm);
> 
> +	if (slb_preload_disabled())
> +		return;
> +
>   	/*
>   	 * We gradually age out SLBs after a number of context switches to
>   	 * reduce reload overhead of unused entries (like we do with FP/VEC
> --
> 2.50.1
>