Re: [PATCH v8 14/24] x86/resctrl: Allow resctrl_arch_rmid_read() to sleep

[Moger, Babu]

On 12/15/23 11:43, James Morse wrote:
> MPAM's cache occupancy counters can take a little while to settle once
> the monitor has been configured. The maximum settling time is described
> to the driver via a firmware table. The value could be large enough
> that it makes sense to sleep. To avoid exposing this to resctrl, it
> should be hidden behind MPAM's resctrl_arch_rmid_read().
> 
> resctrl_arch_rmid_read() may be called via IPI meaning it is unable
> to sleep. In this case resctrl_arch_rmid_read() should return an error
> if it needs to sleep. This will only affect MPAM platforms where
> the cache occupancy counter isn't available immediately, nohz_full is
> in use, and there are no housekeeping CPUs in the necessary domain.
> 
> There are three callers of resctrl_arch_rmid_read():
> __mon_event_count() and __check_limbo() are both called from a
> non-migrateable context. mon_event_read() invokes __mon_event_count()
> using smp_call_on_cpu(), which adds work to the target CPUs workqueue.
> rdtgroup_mutex() is held, meaning this cannot race with the resctrl
> cpuhp callback. __check_limbo() is invoked via schedule_delayed_work_on()
> also adds work to a per-cpu workqueue.
> 
> The remaining call is add_rmid_to_limbo() which is called in response
> to a user-space syscall that frees an RMID. This opportunistically
> reads the LLC occupancy counter on the current domain to see if the
> RMID is over the dirty threshold. This has to disable preemption to
> avoid reading the wrong domain's value. Disabling pre-emption here
> prevents resctrl_arch_rmid_read() from sleeping.
> 
> add_rmid_to_limbo() walks each domain, but only reads the counter
> on one domain. If the system has more than one domain, the RMID will
> always be added to the limbo list. If the RMIDs usage was not over the
> threshold, it will be removed from the list when __check_limbo() runs.
> Make this the default behaviour. Free RMIDs are always added to the
> limbo list for each domain.
> 
> The user visible effect of this is that a clean RMID is not available
> for re-allocation immediately after 'rmdir()' completes, this behaviour
> was never portable as it never happened on a machine with multiple
> domains.
> 
> Removing this path allows resctrl_arch_rmid_read() to sleep if its called
> with interrupts unmasked. Document this is the expected behaviour, and
> add a might_sleep() annotation to catch changes that won't work on arm64.
> 
> Signed-off-by: James Morse <james.morse@xxxxxxx>
> Tested-by: Shaopeng Tan <tan.shaopeng@xxxxxxxxxxx>
> Tested-by: Peter Newman <peternewman@xxxxxxxxxx>
> Tested-by: Babu Moger <babu.moger@xxxxxxx>
> Reviewed-by: Shaopeng Tan <tan.shaopeng@xxxxxxxxxxx>
> Reviewed-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>

Reviewed-by: Babu Moger <babu.moger@xxxxxxx>


> ---
> The previous version allowed resctrl_arch_rmid_read() to be called on the
> wrong CPUs, but now that this needs to take nohz_full and housekeeping into
> account, its too complex.
> 
> Changes since v3:
>  * Removed error handling for smp_call_function_any(), this can't race
>    with the cpuhp callbacks as both hold rdtgroup_mutex.
>  * Switched to the alternative of removing the counter read, this simplifies
>    things dramatically.
> 
> Changes since v4:
>  * Messed with capitalisation.
>  * Removed some dead code now that entry->busy will never be zero in
>    add_rmid_to_limbo().
>  * Rephrased the comment above resctrl_arch_rmid_read_context_check().
> 
> Changes since v5:
>  * Really rephrased the comment above resctrl_arch_rmid_read_context_check().
> ---
>  arch/x86/kernel/cpu/resctrl/monitor.c | 25 +++++--------------------
>  include/linux/resctrl.h               | 23 ++++++++++++++++++++++-
>  2 files changed, 27 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> index 7e81268137b0..2785a2a4ea33 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -277,6 +277,8 @@ int resctrl_arch_rmid_read(struct rdt_resource *r, struct rdt_domain *d,
>  	u64 msr_val, chunks;
>  	int ret;
>  
> +	resctrl_arch_rmid_read_context_check();
> +
>  	if (!cpumask_test_cpu(smp_processor_id(), &d->cpu_mask))
>  		return -EINVAL;
>  
> @@ -455,8 +457,6 @@ static void add_rmid_to_limbo(struct rmid_entry *entry)
>  {
>  	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
>  	struct rdt_domain *d;
> -	int cpu, err;
> -	u64 val = 0;
>  	u32 idx;
>  
>  	lockdep_assert_held(&rdtgroup_mutex);
> @@ -464,17 +464,7 @@ static void add_rmid_to_limbo(struct rmid_entry *entry)
>  	idx = resctrl_arch_rmid_idx_encode(entry->closid, entry->rmid);
>  
>  	entry->busy = 0;
> -	cpu = get_cpu();
>  	list_for_each_entry(d, &r->domains, list) {
> -		if (cpumask_test_cpu(cpu, &d->cpu_mask)) {
> -			err = resctrl_arch_rmid_read(r, d, entry->closid,
> -						     entry->rmid,
> -						     QOS_L3_OCCUP_EVENT_ID,
> -						     &val);
> -			if (err || val <= resctrl_rmid_realloc_threshold)
> -				continue;
> -		}
> -
>  		/*
>  		 * For the first limbo RMID in the domain,
>  		 * setup up the limbo worker.
> @@ -484,15 +474,10 @@ static void add_rmid_to_limbo(struct rmid_entry *entry)
>  		set_bit(idx, d->rmid_busy_llc);
>  		entry->busy++;
>  	}
> -	put_cpu();
>  
> -	if (entry->busy) {
> -		rmid_limbo_count++;
> -		if (IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID))
> -			closid_num_dirty_rmid[entry->closid]++;
> -	} else {
> -		list_add_tail(&entry->list, &rmid_free_lru);
> -	}
> +	rmid_limbo_count++;
> +	if (IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID))
> +		closid_num_dirty_rmid[entry->closid]++;
>  }
>  
>  void free_rmid(u32 closid, u32 rmid)
> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
> index bd4ec22b5a96..8649fc84aac2 100644
> --- a/include/linux/resctrl.h
> +++ b/include/linux/resctrl.h
> @@ -236,7 +236,12 @@ void resctrl_offline_domain(struct rdt_resource *r, struct rdt_domain *d);
>   * @eventid:		eventid to read, e.g. L3 occupancy.
>   * @val:		result of the counter read in bytes.
>   *
> - * Call from process context on a CPU that belongs to domain @d.
> + * Some architectures need to sleep when first programming some of the counters.
> + * (specifically: arm64's MPAM cache occupancy counters can return 'not ready'
> + *  for a short period of time). Call from a non-migrateable process context on
> + * a CPU that belongs to domain @d. e.g. use smp_call_on_cpu() or
> + * schedule_work_on(). This function can be called with interrupts masked,
> + * e.g. using smp_call_function_any(), but may consistently return an error.
>   *
>   * Return:
>   * 0 on success, or -EIO, -EINVAL etc on error.
> @@ -245,6 +250,22 @@ int resctrl_arch_rmid_read(struct rdt_resource *r, struct rdt_domain *d,
>  			   u32 closid, u32 rmid, enum resctrl_event_id eventid,
>  			   u64 *val);
>  
> +/**
> + * resctrl_arch_rmid_read_context_check()  - warn about invalid contexts
> + *
> + * When built with CONFIG_DEBUG_ATOMIC_SLEEP generate a warning when
> + * resctrl_arch_rmid_read() is called with preemption disabled.
> + *
> + * The contract with resctrl_arch_rmid_read() is that if interrupts
> + * are unmasked, it can sleep. This allows NOHZ_FULL systems to use an
> + * IPI, (and fail if the call needed to sleep), while most of the time
> + * the work is scheduled, allowing the call to sleep.
> + */
> +static inline void resctrl_arch_rmid_read_context_check(void)
> +{
> +	if (!irqs_disabled())
> +		might_sleep();
> +}
>  
>  /**
>   * resctrl_arch_reset_rmid() - Reset any private state associated with rmid

-- 
Thanks
Babu Moger