linux-kernel - Re: [PATCH v5 21/24] x86/resctrl: Allow overflow/limbo handlers to be scheduled on any-but cpu

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Message-ID: <9fca4876-01f4-6a84-2dc6-d92458d3b72c@arm.com>
Date:   Thu, 24 Aug 2023 17:57:51 +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 v5 21/24] x86/resctrl: Allow overflow/limbo handlers to be
 scheduled on any-but cpu

Hi Reinette,

On 09/08/2023 23:38, Reinette Chatre wrote:
> On 7/28/2023 9:42 AM, James Morse wrote:
>> When a CPU is taken offline resctrl may need to move the overflow or
>> limbo handlers to run on a different CPU.
>>
>> Once the offline callbacks have been split, cqm_setup_limbo_handler()
>> will be called while the CPU that is going offline is still present
>> in the cpu_mask.
>>
>> Pass the CPU to exclude to cqm_setup_limbo_handler() and
>> mbm_setup_overflow_handler(). These functions can use a variant of
>> cpumask_any_but() when selecting the CPU. -1 is used to indicate no CPUs
>> need excluding.
>>
>> A subsequent patch moves these calls to be before CPUs have been removed,
>> so this exclude_cpus behaviour is temporary.

>> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
>> index c0b1ad8d8f6d..471cdc4e4eae 100644
>> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
>> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c

>> @@ -816,15 +817,28 @@ 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,
>> + *		   RESCTRL_PICK_ANY_CPU to 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);
>> +	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);
> 
> Having callers need to do this checking seems unnecessary and makes the
> code complicated. Can cpumask_any_housekeeping_but() instead be made
> slightly smarter to handle the case where exclude_cpu == RESCTRL_PICK_ANY_CPU ?
> 
> Looks like there is a bit of duplication between 
> cpumask_any_housekeeping() and cpumask_any_housekeeping_but().

Yup, this was because I was originally going to add them to cpumask.h, but figured it
would be easier to leave them here - in a shape that could be moved to cpumask.h if anyone
else needs them.

Using one helper for both would simplify things for resctrl, I'll do that.


Thanks,

James