linux-kernel - Re: [PATCH v10 12/14] x86/sgx: Turn on per-cgroup EPC reclamation

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Message-ID: <d25dbe76d48a0b6c74fa09b06f1ca3fdf234a190.camel@intel.com>
Date: Mon, 8 Apr 2024 12:20:23 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "hpa@...or.com" <hpa@...or.com>, "tim.c.chen@...ux.intel.com"
	<tim.c.chen@...ux.intel.com>, "linux-sgx@...r.kernel.org"
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Subject: Re: [PATCH v10 12/14] x86/sgx: Turn on per-cgroup EPC reclamation


> --- a/arch/x86/kernel/cpu/sgx/epc_cgroup.h
> +++ b/arch/x86/kernel/cpu/sgx/epc_cgroup.h
> @@ -28,6 +28,10 @@ static inline int sgx_cgroup_try_charge(struct sgx_cgroup *sgx_cg, enum sgx_recl
>  static inline void sgx_cgroup_uncharge(struct sgx_cgroup *sgx_cg) { }
>  
>  static inline void sgx_cgroup_init(void) { }
> +
> +static inline void sgx_cgroup_reclaim_pages(struct misc_cg *root, struct mm_struct *charge_mm)
> +{
> +}
>  #else
>  struct sgx_cgroup {
>  	struct misc_cg *cg;
> @@ -65,6 +69,9 @@ static inline void sgx_put_cg(struct sgx_cgroup *sgx_cg)
>  
>  int sgx_cgroup_try_charge(struct sgx_cgroup *sgx_cg, enum sgx_reclaim reclaim);
>  void sgx_cgroup_uncharge(struct sgx_cgroup *sgx_cg);
> +bool sgx_cgroup_lru_empty(struct misc_cg *root);
> +bool sgx_cgroup_should_reclaim(struct sgx_cgroup *sgx_cg);
> +void sgx_cgroup_reclaim_pages(struct misc_cg *root,  struct mm_struct *charge_mm);

Seems the decision to choose whether to implement a stub function for some
function is quite random to me.

My impression is people in general don't like #ifdef in the C file but prefer to
implementing it in the header in some helper function.

I guess you might want to just implement a stub function for each of the 3
functions exposed, so that we can eliminate some #ifdefs in the sgx/main.c (see
below).

>  void sgx_cgroup_init(void);
>  
>  #endif
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index 7f92455d957d..68f28ff2d5ef 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -34,6 +34,16 @@ static struct sgx_epc_lru_list sgx_global_lru;
>  
>  static inline struct sgx_epc_lru_list *sgx_lru_list(struct sgx_epc_page *epc_page)
>  {
> +#ifdef CONFIG_CGROUP_SGX_EPC
> +	if (epc_page->sgx_cg)
> +		return &epc_page->sgx_cg->lru;
> +
> +	/*
> +	 * This should not happen when cgroup is enabled: Every page belongs
> +	 * to a cgroup, or the root by default.
> +	 */
> +	WARN_ON_ONCE(1);

In the case MISC cgroup is enabled in Kconfig but disabled by command line, I
think this becomes legal now?

> +#endif
>  	return &sgx_global_lru;
>  }
>  
> @@ -42,7 +52,11 @@ static inline struct sgx_epc_lru_list *sgx_lru_list(struct sgx_epc_page *epc_pag
>   */
>  static inline bool sgx_can_reclaim(void)
>  {
> +#ifdef CONFIG_CGROUP_SGX_EPC
> +	return !sgx_cgroup_lru_empty(misc_cg_root());
> +#else
>  	return !list_empty(&sgx_global_lru.reclaimable);
> +#endif
>  }
>  

Here you are using #ifdef  CONFIG_CGRUP_SGX_EPC, but ...

>  static atomic_long_t sgx_nr_free_pages = ATOMIC_LONG_INIT(0);
> @@ -404,7 +418,10 @@ static bool sgx_should_reclaim(unsigned long watermark)
>  
>  static void sgx_reclaim_pages_global(struct mm_struct *charge_mm)
>  {
> -	sgx_reclaim_pages(&sgx_global_lru, charge_mm);
> +	if (IS_ENABLED(CONFIG_CGROUP_SGX_EPC))
> +		sgx_cgroup_reclaim_pages(misc_cg_root(), charge_mm);
> +	else
> +		sgx_reclaim_pages(&sgx_global_lru, charge_mm);
>  }

... here you are using IS_ENABLED(CONFIG_CGROUP_SGX_EPC).

Any reason they are not consistent?

Also, in the case where MISC cgroup is disabled via commandline, I think it
won't work, because misc_cg_root() should be NULL in this case while
IS_ENABLED(CONFIG_CGROUP_SGX_EPC) is true.

>  
>  /*
> @@ -414,6 +431,16 @@ static void sgx_reclaim_pages_global(struct mm_struct *charge_mm)
>   */
>  void sgx_reclaim_direct(void)
>  {
> +#ifdef CONFIG_CGROUP_SGX_EPC
> +	struct sgx_cgroup *sgx_cg = sgx_get_current_cg();
> +
> +	/* Make sure there are some free pages at cgroup level */
> +	if (sgx_cg && sgx_cgroup_should_reclaim(sgx_cg)) {
> +		sgx_cgroup_reclaim_pages(sgx_cg->cg, current->mm);
> +		sgx_put_cg(sgx_cg);
> +	}
> +#endif

This #ifdef CONFIG_CGROUP_SGX_EPC can be removed if we implement a stub function
for sgx_cgroup_should_reclaim().

> +	/* Make sure there are some free pages at global level */
>  	if (sgx_should_reclaim(SGX_NR_LOW_PAGES))
>  		sgx_reclaim_pages_global(current->mm);
>  }