linux-kernel - Re: [PATCH v5 17/20] x86/resctrl: Introduce the interface switch between monitor modes

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Message-ID: <ac5595a8-b3f3-7f9a-c297-1b7d0220da06@amd.com>
Date: Tue, 16 Jul 2024 17:46:58 -0500
From: "Moger, Babu" <bmoger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>,
 Babu Moger <babu.moger@....com>, corbet@....net, fenghua.yu@...el.com,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com
Cc: x86@...nel.org, hpa@...or.com, paulmck@...nel.org, rdunlap@...radead.org,
 tj@...nel.org, peterz@...radead.org, yanjiewtw@...il.com,
 kim.phillips@....com, lukas.bulwahn@...il.com, seanjc@...gle.com,
 jmattson@...gle.com, leitao@...ian.org, jpoimboe@...nel.org,
 rick.p.edgecombe@...el.com, kirill.shutemov@...ux.intel.com,
 jithu.joseph@...el.com, kai.huang@...el.com, kan.liang@...ux.intel.com,
 daniel.sneddon@...ux.intel.com, pbonzini@...hat.com, sandipan.das@....com,
 ilpo.jarvinen@...ux.intel.com, peternewman@...gle.com,
 maciej.wieczor-retman@...el.com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, eranian@...gle.com, james.morse@....com
Subject: Re: [PATCH v5 17/20] x86/resctrl: Introduce the interface switch
 between monitor modes

Hi Reinette,

On 7/12/2024 5:14 PM, Reinette Chatre wrote:
> Hi Babu,
> 
> On 7/3/24 2:48 PM, Babu Moger wrote:
>> Introduce interface to switch between ABMC and legacy modes.
>>
>> By default ABMC is enabled on boot if the feature is available.
>> Provide the interface to go back to legacy mode if required.
>>
>> $ cat /sys/fs/resctrl/info/L3_MON/mbm_mode
>> [abmc]
>> legacy
>>
>> To enable the legacy monitoring feature:
>> $ echo "legacy" > /sys/fs/resctrl/info/L3_MON/mbm_mode
>>
>> Signed-off-by: Babu Moger <babu.moger@....com>
>> ---
>> v4: Minor commit text changes. Keep the default to ABMC when supported.
>>      Fixed comments to reflect changed interface "mbm_mode".
>>
>> v3: New patch to address the review comments from upstream.
>> ---
>>   Documentation/arch/x86/resctrl.rst     | 10 +++++++
>>   arch/x86/kernel/cpu/resctrl/rdtgroup.c | 37 +++++++++++++++++++++++++-
>>   2 files changed, 46 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/arch/x86/resctrl.rst 
>> b/Documentation/arch/x86/resctrl.rst
>> index 11b7a5f26b40..4c41c5622627 100644
>> --- a/Documentation/arch/x86/resctrl.rst
>> +++ b/Documentation/arch/x86/resctrl.rst
>> @@ -291,6 +291,16 @@ with the following files:
>>       Without ABMC enabled, monitoring will work in "legacy" mode
>>       without assignment option.
>> +    * To enable ABMC feature:
>> +      ::
>> +
>> +        # echo  "abmc" > /sys/fs/resctrl/info/L3_MON/mbm_mode
>> +
>> +    * To enable the legacy monitoring feature:
>> +      ::
>> +
>> +        # echo  "legacy" > /sys/fs/resctrl/info/L3_MON/mbm_mode
>> +
> 
> Needs details on what user can expect to happen to counters/data when
> switching between modes.

Sure. Will add the details.

> 
>>   "num_mbm_cntrs":
>>       The number of monitoring counters available for assignment.
>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c 
>> b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> index 475a0c7b2a25..531233779f8d 100644
>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> @@ -910,6 +910,40 @@ static int rdtgroup_num_mbm_cntrs_show(struct 
>> kernfs_open_file *of,
>>       return 0;
>>   }
>> +static ssize_t rdtgroup_mbm_mode_write(struct kernfs_open_file *of,
>> +                       char *buf, size_t nbytes,
>> +                       loff_t off)
>> +{
>> +    struct rdt_resource *r = of->kn->parent->priv;
>> +    int ret = 0;
>> +
>> +    if (!r->mon.abmc_capable)
>> +        return -EINVAL;
>> +
> 
> Why should a user not be able to write "legacy" into this
> file if "legacy" is the only mode supported?

Yes. Will fix it.

> 
>> +    /* Valid input requires a trailing newline */
>> +    if (nbytes == 0 || buf[nbytes - 1] != '\n')
>> +        return -EINVAL;
>> +
>> +    buf[nbytes - 1] = '\0';
>> +
>> +    cpus_read_lock();
>> +    mutex_lock(&rdtgroup_mutex);
>> +
>> +    rdt_last_cmd_clear();
>> +
>> +    if (!strcmp(buf, "legacy"))
>> +        resctrl_arch_abmc_disable();
>> +    else if (!strcmp(buf, "abmc"))
>> +        ret = resctrl_arch_abmc_enable();
>> +    else
>> +        ret = -EINVAL;
>> +
>> +    mutex_unlock(&rdtgroup_mutex);
>> +    cpus_read_unlock();
>> +
>> +    return ret ?: nbytes;
>> +}
>> +
>>   #ifdef CONFIG_PROC_CPU_RESCTRL
>>   /*
>> @@ -2103,9 +2137,10 @@ static struct rftype res_common_files[] = {
>>       },
>>       {
>>           .name        = "mbm_mode",
>> -        .mode        = 0444,
>> +        .mode        = 0644,
>>           .kf_ops        = &rdtgroup_kf_single_ops,
>>           .seq_show    = rdtgroup_mbm_mode_show,
>> +        .write        = rdtgroup_mbm_mode_write,
>>       },
>>       {
>>           .name        = "cpus",
> 
> Reinette
> 

-- 
- Babu Moger