Message-ID: <31c04287-c81f-47e9-86a1-50099cc2bbf7@amd.com>
Date: Mon, 10 Feb 2025 12:54:53 -0600
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>, corbet@....net,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, tony.luck@...el.com, peternewman@...gle.com
Cc: x86@...nel.org, hpa@...or.com, paulmck@...nel.org,
 akpm@...ux-foundation.org, thuth@...hat.com, rostedt@...dmis.org,
 xiongwei.song@...driver.com, pawan.kumar.gupta@...ux.intel.com,
 daniel.sneddon@...ux.intel.com, jpoimboe@...nel.org, perry.yuan@....com,
 sandipan.das@....com, kai.huang@...el.com, xiaoyao.li@...el.com,
 seanjc@...gle.com, xin3.li@...el.com, andrew.cooper3@...rix.com,
 ebiggers@...gle.com, mario.limonciello@....com, james.morse@....com,
 tan.shaopeng@...itsu.com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, maciej.wieczor-retman@...el.com,
 eranian@...gle.com
Subject: Re: [PATCH v11 19/23] x86/resctrl: Introduce the interface to switch
 between monitor modes

Hi Reinette,

On 2/6/25 12:05, Reinette Chatre wrote:
> Hi Babu,
> 
> On 1/22/25 12:20 PM, Babu Moger wrote:
>> Resctrl subsystem can support two monitoring modes, 'mbm_cntr_assign' or
>> 'default'. In mbm_cntr_assign, monitoring event can only accumulate data
>> while it is backed by a hardware counter. In 'default' mode, resctrl
>> assumes there is a hardware counter for each event within every CTRL_MON
>> and MON group.
>>
>> Introduce interface to switch between mbm_cntr_assign and default modes.
>>
>> $ cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
>> [mbm_cntr_assign]
>> default
>>
>> To enable the "mbm_cntr_assign" mode:
>> $ echo "mbm_cntr_assign" > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
>>
>> To enable the default monitoring mode:
>> $ echo "default" > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
>>
>> MBM event counters are automatically reset as part of changing the mode.
>> Clear both architectural and non-architectural event states to prevent
>> overflow conditions during the next event read.
>>
>> Signed-off-by: Babu Moger <babu.moger@....com>
>> ---
> 
> 
>> ---
>>  Documentation/arch/x86/resctrl.rst     | 25 ++++++++++++-
>>  arch/x86/kernel/cpu/resctrl/rdtgroup.c | 50 +++++++++++++++++++++++++-
>>  2 files changed, 73 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
>> index 072b15550ff7..5d18c4c8bc48 100644
>> --- a/Documentation/arch/x86/resctrl.rst
>> +++ b/Documentation/arch/x86/resctrl.rst
>> @@ -259,7 +259,10 @@ with the following files:
>>  
>>  "mbm_assign_mode":
>>  	Reports the list of monitoring modes supported. The enclosed brackets
>> -	indicate which mode is enabled.
>> +	indicate which mode is enabled. The MBM events (mbm_total_bytes and/or
>> +	mbm_local_bytes) associated with counters may reset when "mbm_assign_mode"
>> +	is changed.
>> +
>>  	::
>>  
>>  	  # cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
>> @@ -275,6 +278,16 @@ with the following files:
>>  	available is described in the "num_mbm_cntrs" file. Changing the mode
>>  	may cause all counters on a resource to reset.
>>  
>> +	Moving to mbm_cntr_assign mode require users to assign the counters to
>> +	the events. Otherwise, the MBM event counters will return "Unassigned"
>> +	when read.
> 
> Again ... please be consistent in using single or double quotes for information
> returned from file.

ok. Will change it to 'Unassigned'.

> 
>> +
>> +	The mode is beneficial for AMD platforms that support more CTRL_MON
>> +	and MON groups than available hardware counters. By default, this
>> +	feature is enabled on AMD platforms with the ABMC (Assignable Bandwidth
>> +	Monitoring Counters) capability, ensuring counters remain assigned even
>> +	when the corresponding RMID is not actively used by any processor.
>> +
>>  	"default":
>>  
>>  	In default mode, resctrl assumes there is a hardware counter for each
>> @@ -283,6 +296,16 @@ with the following files:
>>  	"mbm_total_bytes" or "mbm_local_bytes" will report 'Unavailable' if
>>  	there is no counter associated with that event.
>>  
>> +	* To enable "mbm_cntr_assign" mode:
>> +	  ::
>> +
>> +	    # echo "mbm_cntr_assign" > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
>> +
>> +	* To enable default monitoring mode:
>> +	  ::
>> +
>> +	    # echo "default" > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
>> +
> 
> Please be consistent in the documentation.
> 
> To enable "mbm_cntr_assign" mode:
> To enable "default" mode:
> or
> To enable "mbm_cntr_assign" monitoring mode:
> To enable "default" monitoring mode:

This sounds (monitoring mode) better.

> or 
> ...?
> 
> 
> 
>>  "num_mbm_cntrs":
>>  	The number of monitoring counters available for assignment when the
>>  	system supports mbm_cntr_assign mode.
>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> index f61f0cd032ef..6922173c4f8f 100644
>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> @@ -928,6 +928,53 @@ static int resctrl_available_mbm_cntrs_show(struct kernfs_open_file *of,
>>  	return ret;
>>  }
>>  
>> +static ssize_t resctrl_mbm_assign_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;
>> +	bool enable;
>> +
>> +	/* 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, "default")) {
>> +		enable = 0;
>> +	} else if (!strcmp(buf, "mbm_cntr_assign")) {
>> +		if (r->mon.mbm_cntr_assignable) {
>> +			enable = 1;
>> +		} else {
>> +			ret = -EINVAL;
>> +			rdt_last_cmd_puts("mbm_cntr_assign mode is not supported\n");
>> +			goto write_exit;
>> +		}
>> +	} else {
>> +		ret = -EINVAL;
>> +		rdt_last_cmd_puts("Unsupported assign mode\n");
>> +		goto write_exit;
>> +	}
>> +
>> +	if (enable != resctrl_arch_mbm_cntr_assign_enabled(r)) {
>> +		ret = resctrl_arch_mbm_cntr_assign_set(r, enable);
>> +		if (!ret)
>> +			mbm_cntr_reset(r);
> 
> The following APIs interact with the MBM assignable counters:
> 
> mbm_cntr_alloc()
> mbm_cntr_get()
> mbm_cntr_free()
> 
> mbm_cntr_reset() appears to be related but does significantly more
> than interact with the MBM assignable counters and that creates a
> confusing API.
> 
> How about introducing mbm_cntr_free_all() that _only_ releases all
> MBM assignable counters and match with mbm_cntr_free() that releases
> a single MBM assignable counter? mbm_cntr_free_all() lives with the
> other functions operating on MBM assignable counters, thus not
> hiding its functionality in other parts of resctrl.
> 
> This series open codes reset of non-architectural state in two places,
> within mbm_cntr_reset() and within mbm_config_write_domain(). That
> can be turned into a new helper that only resets architectural state,
> for example resctrl_reset_rmid_all() to match existing
> resctrl_arch_reset_rmid_all().
> 
> resctrl_arch_mbm_cntr_assign_set() can also reset any architectural
> state leaving mbm_cntr_free_all() and resctrl_reset_rmid_all() to be called
> here and from within mbm_config_write_domain().
> 
> What do you think?

Sounds like a good code separation. It should be fine.
Will let you know if there are any issues.

> 
>> +	}
>> +
>> +write_exit:
>> +	mutex_unlock(&rdtgroup_mutex);
>> +	cpus_read_unlock();
>> +
>> +	return ret ?: nbytes;
>> +}
>> +
>>  #ifdef CONFIG_PROC_CPU_RESCTRL
>>  
>>  /*
>> @@ -1945,9 +1992,10 @@ static struct rftype res_common_files[] = {
>>  	},
>>  	{
>>  		.name		= "mbm_assign_mode",
>> -		.mode		= 0444,
>> +		.mode		= 0644,
>>  		.kf_ops		= &rdtgroup_kf_single_ops,
>>  		.seq_show	= resctrl_mbm_assign_mode_show,
>> +		.write		= resctrl_mbm_assign_mode_write,
>>  		.fflags		= RFTYPE_MON_INFO,
>>  	},
>>  	{
> 
> Reinette
> 

-- 
Thanks
Babu Moger

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