linux-kernel - Re: [PATCH v4 21/24] x86/resctrl: Allow overflow/limbo handlers to be scheduled on any-but cpu

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Message-ID: <5ed8081e-ad27-7320-bea3-263e858efc63@arm.com>
Date:   Fri, 28 Jul 2023 17:29:51 +0100
From:   James Morse <james.morse@....com>
To:     Reinette Chatre <reinette.chatre@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Babu Moger <Babu.Moger@....com>,
        shameerali.kolothum.thodi@...wei.com,
        D Scott Phillips OS <scott@...amperecomputing.com>,
        carl@...amperecomputing.com, lcherian@...vell.com,
        bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com,
        xingxin.hx@...nanolis.org, baolin.wang@...ux.alibaba.com,
        Jamie Iles <quic_jiles@...cinc.com>,
        Xin Hao <xhao@...ux.alibaba.com>, peternewman@...gle.com,
        dfustini@...libre.com
Subject: Re: [PATCH v4 21/24] x86/resctrl: Allow overflow/limbo handlers to be
 scheduled on any-but cpu

Hi Reinette,

On 15/06/2023 23:25, Reinette Chatre wrote:
> On 5/25/2023 11:02 AM, James Morse wrote:
>> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
>> index 021a8956518c..9cba8fc405b9 100644
>> --- a/arch/x86/kernel/cpu/resctrl/internal.h
>> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
>> @@ -79,6 +79,37 @@ static inline unsigned int cpumask_any_housekeeping(const struct cpumask *mask)
>>  	return cpu;
>>  }
>>  
>> +/**
>> + * cpumask_any_housekeeping_but() - Chose any cpu in @mask, preferring those
>> + *			            that aren't marked nohz_full, excluding
>> + *				    the provided CPU
>> + * @mask:	The mask to pick a CPU from.
>> + * @exclude_cpu:The CPU to avoid picking.
>> + *
>> + * Returns a CPU from @mask, but not @but. If there are housekeeping CPUs that
> 
> "but not @exclude_cpu"
> 
>> + * don't use nohz_full, these are preferred.
>> + * Returns >= nr_cpu_ids if no CPUs are available.
>> + */
>> +static inline unsigned int
>> +cpumask_any_housekeeping_but(const struct cpumask *mask, int exclude_cpu)
>> +{
>> +	int cpu, hk_cpu;
> 
> Should these be unsigned int?

Yup, fixed.


>> +
>> +	cpu = cpumask_any_but(mask, exclude_cpu);
>> +	if (tick_nohz_full_cpu(cpu)) {
>> +		hk_cpu = cpumask_nth_andnot(0, mask, tick_nohz_full_mask);
>> +		if  (hk_cpu == exclude_cpu) {
>> +			hk_cpu = cpumask_nth_andnot(1, mask,
>> +						    tick_nohz_full_mask);
>> +		}
>> +
> 
> These braces are not necessary.

My C parser is pretty dumb, and is easily confused by things like that....


> If they are added to help readability then
> perhaps the indentation can be reduced by using an earlier:
> 
> 	if (!tick_nohz_full_cpu(cpu))
> 		return cpu;

Even better!


>> +		if (hk_cpu < nr_cpu_ids)
>> +			cpu = hk_cpu;
>> +	}
>> +
>> +	return cpu;
>> +}
>> +
>>  struct rdt_fs_context {
>>  	struct kernfs_fs_context	kfc;
>>  	bool				enable_cdpl2;

>> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
>> index ced933694f60..ae02185f3354 100644
>> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
>> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
>> @@ -485,7 +485,7 @@ static void add_rmid_to_limbo(struct rmid_entry *entry)
>>  		 * setup up the limbo worker.
>>  		 */
>>  		if (!has_busy_rmid(r, d))
>> -			cqm_setup_limbo_handler(d, CQM_LIMBOCHECK_INTERVAL);
>> +			cqm_setup_limbo_handler(d, CQM_LIMBOCHECK_INTERVAL, -1);
> 
> Should this -1 be RESCTRL_PICK_ANY_CPU?
> 
>>  		set_bit(idx, d->rmid_busy_llc);
>>  		entry->busy++;
>>  	}
>> @@ -810,15 +810,28 @@ void cqm_handle_limbo(struct work_struct *work)
>>  	mutex_unlock(&rdtgroup_mutex);
>>  }
>>  
>> -void cqm_setup_limbo_handler(struct rdt_domain *dom, unsigned long delay_ms)
>> +/**
>> + * cqm_setup_limbo_handler() - Schedule the limbo handler to run for this
>> + *                             domain.
>> + * @delay_ms:      How far in the future the handler should run.
>> + * @exclude_cpu:   Which CPU the handler should not run on, -1 to pick any CPU.
> 
> Should -1 be RESCTRL_PICK_ANY_CPU? 
> 
>> + */
>> +void cqm_setup_limbo_handler(struct rdt_domain *dom, unsigned long delay_ms,
>> +			     int exclude_cpu)
>>  {
>>  	unsigned long delay = msecs_to_jiffies(delay_ms);
>>  	int cpu;
>>  
>> -	cpu = cpumask_any_housekeeping(&dom->cpu_mask);
>> -	dom->cqm_work_cpu = cpu;
>> +	if (exclude_cpu == RESCTRL_PICK_ANY_CPU)
>> +		cpu = cpumask_any_housekeeping(&dom->cpu_mask);
>> +	else
>> +		cpu = cpumask_any_housekeeping_but(&dom->cpu_mask,
>> +						   exclude_cpu);
>>  
>> -	schedule_delayed_work_on(cpu, &dom->cqm_limbo, delay);
>> +	if (cpu < nr_cpu_ids) {
>> +		dom->cqm_work_cpu = cpu;
> 
> Should cqm_work_cpu not perhaps be set to nr_cpu_ids on failure? If it keeps
> pointing to CPU that ran worker previously there may be unexpected behavior. 
> 
> Note the different behavior between cqm_setup_limbo_handler() and
> mbm_setup_overflow_handler() in this regard.

Sure,


>> +		schedule_delayed_work_on(cpu, &dom->cqm_limbo, delay);
>> +	}
>>  }
>>  
>>  void mbm_handle_overflow(struct work_struct *work)

>> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
>> index ecd41762d61a..089b91133e5e 100644
>> --- a/include/linux/resctrl.h
>> +++ b/include/linux/resctrl.h
>> @@ -9,6 +9,9 @@
>>  /* CLOSID value used by the default control group */
>>  #define RESCTRL_RESERVED_CLOSID		0
>>  
>> +/* Indicates no CPU needs to be excluded */
> 
> This comment seems to just be a rewrite of the macro name.

I'm more than happy to remove it!



Thanks,

James