linux-kernel - Re: [PATCH v4 08/21] x86/resctrl: Switch over to the resctrl mbps

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Date:   Tue, 7 Jun 2022 13:07:24 +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>,
        lcherian@...vell.com, bobo.shaobowang@...wei.com,
        tan.shaopeng@...itsu.com, Jamie Iles <quic_jiles@...cinc.com>,
        Cristian Marussi <cristian.marussi@....com>,
        Xin Hao <xhao@...ux.alibaba.com>, xingxin.hx@...nanolis.org,
        baolin.wang@...ux.alibaba.com
Subject: Re: [PATCH v4 08/21] x86/resctrl: Switch over to the resctrl mbps_val
 list

Hi Reinette,

On 17/05/2022 17:19, Reinette Chatre wrote:
> On 4/12/2022 5:44 AM, James Morse wrote:
>> Updates to resctrl's software controller follow the same path as
>> other configuration updates, but they don't modify the hardware state.
>> rdtgroup_schemata_write() uses parse_line() and the resource's
>> parse_ctrlval() function to stage the configuration.
>> resctrl_arch_update_domains() then updates the mbps_val[] array
>> instead, and resctrl_arch_update_domains() skips the rdt_ctrl_update()
>> call that would update hardware.
>>
>> This complicates the interface between resctrl's filesystem parts
>> and architecture specific code. It should be possible for mba_sc
>> to be completely implemented by the filesystem parts of resctrl. This
>> would allow it to work on a second architecture with no additional code.
>> resctrl_arch_update_domains() using the mbps_val[] array prevents this.
>>
>> Change parse_bw() to write the configuration value directly to the
>> mbps_val[] array in the domain structure. Change rdtgroup_schemata_write()
>> to skip the call to resctrl_arch_update_domains(), meaning all the
>> mba_sc specific code in resctrl_arch_update_domains() can be removed.
>> On the read-side, show_doms() and update_mba_bw() are changed to read
>> the mbps_val[] array from the domain structure. With this,
>> resctrl_arch_get_config() no longer needs to consider mba_sc resources.

> This sounds like a good cleanup and I understand it to not intend 
> functional change, so a bit more information is needed on the change
> in rdtgroup_init_alloc(). More below.

>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> index 9d5be6a73644..07904308245c 100644
>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> @@ -1932,11 +1937,6 @@ static int set_mba_sc(bool mba_sc)
>>  
>>  	r->membw.mba_sc = mba_sc;
>>  
>> -	list_for_each_entry(d, &r->domains, list) {
>> -		for (i = 0; i < num_closid; i++)
>> -			d->mbps_val[i] = MBA_MAX_MBPS;
>> -	}
>> -
>>  	return 0;
>>  }

> With this removed, where is rdt_domain->mbps_val reset on remount of resctrl?

Oops, this is a bug. Its a left over from when the rdt_domain->mbps_val[] array was only
allocated when mba_sc was enabled.


>> @@ -2809,15 +2809,18 @@ static int rdtgroup_init_cat(struct resctrl_schema *s, u32 closid)
>>  }
>>  
>>  /* Initialize MBA resource with default values. */
>> -static void rdtgroup_init_mba(struct rdt_resource *r)
>> +static void rdtgroup_init_mba(struct rdt_resource *r, u32 closid)
>>  {
>>  	struct resctrl_staged_config *cfg;
>>  	struct rdt_domain *d;
>>  
>>  	list_for_each_entry(d, &r->domains, list) {
>>  		cfg = &d->staged_config[CDP_NONE];
>> -		cfg->new_ctrl = is_mba_sc(r) ? MBA_MAX_MBPS : r->default_ctrl;
>> +		cfg->new_ctrl = r->default_ctrl;
>>  		cfg->have_new_ctrl = true;
>> +
>> +		if (is_mba_sc(r))
>> +			d->mbps_val[closid] = MBA_MAX_MBPS;
>>  	}
>>  }
>>  
>> @@ -2831,7 +2834,7 @@ static int rdtgroup_init_alloc(struct rdtgroup *rdtgrp)
>>  	list_for_each_entry(s, &resctrl_schema_all, list) {
>>  		r = s->res;
>>  		if (r->rid == RDT_RESOURCE_MBA) {
>> -			rdtgroup_init_mba(r);
>> +			rdtgroup_init_mba(r, rdtgrp->closid);
>>  		} else {
>>  			ret = rdtgroup_init_cat(s, rdtgrp->closid);
>>  			if (ret < 0)
> 
> What follows this hunk and continues to be called is:
> 
> 	ret = resctrl_arch_update_domains(r, rdtgrp->closid);
> 
> before this patch resctrl_arch_update_domains() would just have updated
> the mbps_val but not made any configuration changed if is_mba_sc() is true.
> Before this patch configuration changes done in
> resctrl_arch_update_domains() is omitted when is_mba_sc() is true
> but after earlier change in this patch it proceeds and will result in
> configuration change.

Yes, this will write the default ctrl_val into hardware. Previously it may have an unknown
value from a previous allocation of the closid, update_mba_bw() will eventually adjust
that to something sensible.

I think I ignored this as harmless, but I agree its better to keep the existing behaviour.
I'll add:
|		if (is_mba_sc(r))
|			continue;

to both rdtgroup_init_mba() and rdtgroup_init_alloc().


Thanks,

James