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

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Message-ID: <CALPaoChGp+JHeyxsqDvix=t=ik5eTqc3bf+ihUL=RXx5p9r5Cw@mail.gmail.com>
Date: Fri, 26 Jul 2024 15:48:05 -0700
From: Peter Newman <peternewman@...gle.com>
To: 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, 
	paulmck@...nel.org, rdunlap@...radead.org, tj@...nel.org, 
	peterz@...radead.org, yanjiewtw@...il.com, kim.phillips@....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 v5 09/20] x86/resctrl: Initialize monitor counters bitmap

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
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.

# 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=_;


-Peter