linux-kernel - Re: [PATCH v8 19/25] x86/resctrl: Auto assign/unassign counters when mbm_cntr

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Message-ID: <541d6c15-ed5f-8794-506c-8fa4065ca170@amd.com>
Date: Fri, 11 Oct 2024 16:17:58 -0500
From: "Moger, Babu" <bmoger@....com>
To: Tony Luck <tony.luck@...el.com>, Babu Moger <babu.moger@....com>
Cc: corbet@....net, fenghua.yu@...el.com, reinette.chatre@...el.com,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
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 kirill.shutemov@...ux.intel.com, jithu.joseph@...el.com,
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 maciej.wieczor-retman@...el.com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, eranian@...gle.com, james.morse@....com
Subject: Re: [PATCH v8 19/25] x86/resctrl: Auto assign/unassign counters when
 mbm_cntr_assign is enabled

Hi Tony,

On 10/11/2024 12:17 PM, Tony Luck wrote:
> On Wed, Oct 09, 2024 at 12:39:44PM -0500, Babu Moger wrote:
>> +/*
>> + * Called when a new group is created. If `mbm_cntr_assign` mode is enabled,
>> + * counters are automatically assigned. Each group requires two counters:
>> + * one for the total event and one for the local event. Due to the limited
>> + * number of counters, assignments may fail in some cases. However, it is
>> + * not necessary to fail the group creation. Users have the option to
>> + * modify the assignments after the group has been created.
>> + */
>> +static int rdtgroup_assign_cntrs(struct rdtgroup *rdtgrp)
>> +{
>> +	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
>> +	int ret = 0;
>> +
>> +	if (!resctrl_arch_mbm_cntr_assign_enabled(r))
>> +		return 0;
>> +
>> +	if (is_mbm_total_enabled())
>> +		ret = rdtgroup_assign_cntr_event(r, rdtgrp, NULL, QOS_L3_MBM_TOTAL_EVENT_ID);
>> +
>> +	if (!ret && is_mbm_local_enabled())
>> +		ret = rdtgroup_assign_cntr_event(r, rdtgrp, NULL, QOS_L3_MBM_LOCAL_EVENT_ID);
> 
> This overwrites the value from allocating the counter for total event.

Total event and local events have two different indexes.
Can you please elaborate?

> 
>> +
>> +	return ret;
> 
> But none of the callers check the return. Indeed it is ok (and
> expected) that counter allocation can fail.
> 
> Just make this a "void" function and delete the "ret" local variable.

Comment below.
>> +}
>> +
>> +/*
>> + * Called when a group is deleted. Counters are unassigned if it was in
>> + * assigned state.
>> + */
>> +static int rdtgroup_unassign_cntrs(struct rdtgroup *rdtgrp)
>> +{
>> +	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
>> +	int ret = 0;
>> +
>> +	if (!resctrl_arch_mbm_cntr_assign_enabled(r))
>> +		return 0;
>> +
>> +	if (is_mbm_total_enabled())
>> +		ret = rdtgroup_unassign_cntr_event(r, rdtgrp, NULL, QOS_L3_MBM_TOTAL_EVENT_ID);
>> +
>> +	if (!ret && is_mbm_local_enabled())
>> +		ret = rdtgroup_unassign_cntr_event(r, rdtgrp, NULL, QOS_L3_MBM_LOCAL_EVENT_ID);
>> +
>> +	return ret;
> 
> Ditto. No caller checks. Make this a void function. Dig down the
> call chain here. It looks like rdtgroup_unassign_cntr_event() can't
> fail, so it should be a void function too. Ditto resctrl_arch_config_cntr()

It was started a void function. In this case all the call sequence 
return 0. There is a possibility that other architectures can return 
failure(in arch calls resctrl_arch_config_cntr()). Keeping that in mind 
we added the check to handle the return values. Hope that helps.
Thanks
-- 
- Babu Moger