Re: [PATCH v3 11/19] x86/resctrl: Allow arch to allocate memory needed in resctrl_arch_rmid_read()

[Reinette Chatre <reinette.chatre@intel.com>]

Hi James,

On 3/20/2023 10:26 AM, James Morse wrote:
> Depending on the number of monitors available, Arm's MPAM may need to
> allocate a monitor prior to reading the counter value. Allocating a
> contended resource may involve sleeping.
> 
> All callers of resctrl_arch_rmid_read() read the counter on more than
> one domain. If the monitor is allocated globally, there is no need to

This does not seem accurate considering the __check_limbo() call that
is called for a single domain.

> allocate and free it for each call to resctrl_arch_rmid_read().
> 
> Add arch hooks for this allocation, which need calling before
> resctrl_arch_rmid_read(). The allocated monitor is passed to
> resctrl_arch_rmid_read(), then freed again afterwards. The helper
> can be called on any CPU, and can sleep.
> 
> Tested-by: Shaopeng Tan <tan.shaopeng@fujitsu.com>
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
>  arch/x86/include/asm/resctrl.h         | 11 +++++++
>  arch/x86/kernel/cpu/resctrl/internal.h |  1 +
>  arch/x86/kernel/cpu/resctrl/monitor.c  | 40 +++++++++++++++++++++++---
>  include/linux/resctrl.h                |  4 +--
>  4 files changed, 50 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/resctrl.h b/arch/x86/include/asm/resctrl.h
> index 752123b0ce40..1c87f1626456 100644
> --- a/arch/x86/include/asm/resctrl.h
> +++ b/arch/x86/include/asm/resctrl.h
> @@ -136,6 +136,17 @@ static inline u32 resctrl_arch_rmid_idx_encode(u32 ignored, u32 rmid)
>  	return rmid;
>  }
>  
> +/* x86 can always read an rmid, nothing needs allocating */
> +struct rdt_resource;
> +static inline int resctrl_arch_mon_ctx_alloc(struct rdt_resource *r, int evtid)
> +{
> +	might_sleep();
> +	return 0;
> +};
> +
> +static inline void resctrl_arch_mon_ctx_free(struct rdt_resource *r, int evtid,
> +					     int ctx) { };
> +
>  void resctrl_cpu_detect(struct cpuinfo_x86 *c);
>  
>  #else
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index a07557390895..7262b355e128 100644
> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> @@ -135,6 +135,7 @@ struct rmid_read {
>  	bool			first;
>  	int			err;
>  	u64			val;
> +	int			arch_mon_ctx;
>  };
>  
>  extern bool rdt_alloc_capable;
> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> index de72df06b37b..f38cd2f12285 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -15,6 +15,7 @@
>   * Software Developer Manual June 2016, volume 3, section 17.17.
>   */
>  
> +#include <linux/cpu.h>

Why is this needed?

>  #include <linux/module.h>
>  #include <linux/sizes.h>
>  #include <linux/slab.h>
> @@ -271,7 +272,7 @@ static void smp_call_rmid_read(void *_arg)
>  
>  int resctrl_arch_rmid_read(struct rdt_resource *r, struct rdt_domain *d,
>  			   u32 closid, u32 rmid, enum resctrl_event_id eventid,
> -			   u64 *val)
> +			   u64 *val, int ignored)
>  {
>  	struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r);
>  	struct rdt_hw_domain *hw_dom = resctrl_to_arch_dom(d);
> @@ -317,9 +318,14 @@ void __check_limbo(struct rdt_domain *d, bool force_free)
>  	u32 idx_limit = resctrl_arch_system_num_rmid_idx();
>  	struct rmid_entry *entry;
>  	u32 idx, cur_idx = 1;
> +	int arch_mon_ctx;
>  	bool rmid_dirty;
>  	u64 val = 0;
>  
> +	arch_mon_ctx = resctrl_arch_mon_ctx_alloc(r, QOS_L3_OCCUP_EVENT_ID);
> +	if (arch_mon_ctx < 0)
> +		return;
> +

The vision for this is not clear to me. When I read that context needs to be allocated
I expect it to return a pointer to some new context, not an int. What would the
"context" consist of?


...

> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
> index ff7452f644e4..03e4f41cd336 100644
> --- a/include/linux/resctrl.h
> +++ b/include/linux/resctrl.h
> @@ -233,6 +233,7 @@ void resctrl_offline_domain(struct rdt_resource *r, struct rdt_domain *d);
>   * @rmid:		rmid of the counter to read.
>   * @eventid:		eventid to read, e.g. L3 occupancy.
>   * @val:		result of the counter read in bytes.
> + * @arch_mon_ctx:	An allocated context from resctrl_arch_mon_ctx_alloc().
>   *

Could this description be expanded to indicate what this context is used for?

>   * Call from process context on a CPU that belongs to domain @d.
>   *


Reinette