linux-kernel - Re: [PATCH v5 09/20] x86/resctrl: Initialize monitor counters bitmap

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Message-ID: <77124a1f-5440-4627-955f-9fe1d2f4ae18@intel.com>
Date: Thu, 1 Aug 2024 14:05:30 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Peter Newman <peternewman@...gle.com>, Babu Moger <babu.moger@....com>
CC: <corbet@....net>, <fenghua.yu@...el.com>, <tglx@...utronix.de>,
	<mingo@...hat.com>, <bp@...en8.de>, <dave.hansen@...ux.intel.com>,
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	<rdunlap@...radead.org>, <tj@...nel.org>, <peterz@...radead.org>,
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	<linux-kernel@...r.kernel.org>, <eranian@...gle.com>, <james.morse@....com>
Subject: Re: [PATCH v5 09/20] x86/resctrl: Initialize monitor counters bitmap

Hi Peter,

On 7/26/24 3:48 PM, Peter Newman wrote:
> Hi Babu,
> 
> On Wed, Jul 3, 2024 at 2:50 PM Babu Moger <babu.moger@....com> wrote:
>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> index 4f47f52e01c2..b3d3fa048f15 100644
>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> @@ -185,6 +185,23 @@ bool closid_allocated(unsigned int closid)
>>          return !test_bit(closid, &closid_free_map);
>>   }
>>
>> +/*
>> + * Counter bitmap and its length for tracking available counters.
>> + * ABMC feature provides set of hardware counters for enabling events.
>> + * Each event takes one hardware counter. Kernel needs to keep track
>> + * of number of available counters.
>> + */
>> +static unsigned long mbm_cntrs_free_map;
>> +static unsigned int mbm_cntrs_free_map_len;
> 
> If counter assignment is supported at a per-domain granularity, then
> counter id allocation needs to be done per-domain rather than
> globally. For example, if I free a counter from one group in a

It is not obvious to me that counter assignment supported per-domain
requires allocation per-domain. I think this may get complicated when
resources are monitored with one counter when tasks run in one domain
and another counter when the same tasks run in another domain.

> particular domain, it should be available to allocate to another group
> only in that domain.
> 
> When I attempt this using the current series, the resulting behavior
> is quite interesting. I noticed Reinette also commented on this later
> in the series, noticing that counters are only allocated permanently
> to groups and never move as a result of writing to mbm_control.

As I understand this is separate from how the counter is allocated, but instead
just a gap in current implementation of intended interface.

> 
> # grep 'g1[45]' info/L3_MON/mbm_control
> test/g14/0=tl;1=tl;2=tl;3=tl;4=tl;5=tl;6=tl;7=tl;8=tl;9=tl;10=tl;11=tl;12=tl;13=tl;14=tl;15=tl;16=tl;17=tl;18=tl;19=tl;20=tl;21=tl;22=tl;23=tl;24=tl;25=tl;26=tl;27=tl;28=tl;29=tl;30=tl;31=tl;
> test/g15/0=_;1=_;2=_;3=_;4=_;5=_;6=_;7=_;8=_;9=_;10=_;11=_;12=_;13=_;14=_;15=_;16=_;17=_;18=_;19=_;20=_;21=_;22=_;23=_;24=_;25=_;26=_;27=_;28=_;29=_;30=_;31=_;
> 
> [domains 2-31 omitted for clarity below]
> 
> # echo 'test/g14/1-t' > info/L3_MON/mbm_control
> # grep 'g1[45]' info/L3_MON/mbm_control
> test/g14/0=tl;1=l;
> test/g15/0=_;1=_;
> 
> # echo "test/g15/1+t" > info/L3_MON/mbm_control
> # grep 'g1[45]' info/L3_MON/mbm_control
> test/g14/0=tl;1=_;
> test/g15/0=_;1=_;

Thank you very much for trying this out.

Reinette