linux-kernel - Re: [PATCH v3 10/19] x86/resctrl: Allow resctrl_arch_rmid

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Date:   Thu, 27 Apr 2023 15:12:21 +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
Subject: Re: [PATCH v3 10/19] x86/resctrl: Allow resctrl_arch_rmid_read() to
 sleep

Hi Reinette,

On 01/04/2023 00:26, Reinette Chatre wrote:
> On 3/20/2023 10:26 AM, James Morse wrote:
> 
> ...
> 
>>  int resctrl_arch_rmid_read(struct rdt_resource *r, struct rdt_domain *d,
>>  			   u32 closid, u32 rmid, enum resctrl_event_id eventid,
>>  			   u64 *val)
>>  {
>>  	struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r);
>>  	struct rdt_hw_domain *hw_dom = resctrl_to_arch_dom(d);
>> +	struct __rmid_read_arg arg;
>>  	struct arch_mbm_state *am;
>>  	u64 msr_val, chunks;
>> -	int ret;
>> +	int err;
>>  
>> -	if (!cpumask_test_cpu(smp_processor_id(), &d->cpu_mask))
>> -		return -EINVAL;
>> +	arg.rmid = rmid;
>> +	arg.eventid = eventid;
>>  
>> -	ret = __rmid_read(rmid, eventid, &msr_val);
>> -	if (ret)
>> -		return ret;
>> +	err = smp_call_function_any(&d->cpu_mask, smp_call_rmid_read, &arg, true);
>> +	if (err)
>> +		return err;
> 
> This seems to break the assumption of expected return values. __mon_event_count()
> does: 
> 	rr->err = resctrl_arch_rmid_read()
> 
> and later rdtgroup_mondata_show() only expects -EIO or -EINVAL as errors, with
> default of success.

Yes, looks like I dithered on whether cpus_read_lock() should be held over this function,
or it should tolerate the error. This is protected by rdtgroup_mutex, which means the
hotplug callbacks can't run concurrently, so the error can't occur.

I'll change it to ignore the return value.


Thanks,

James