Date:   Tue, 30 Mar 2021 15:36:55 -0500
From:   Babu Moger <babu.moger@....com>
To:     James Morse <james.morse@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        shameerali.kolothum.thodi@...wei.com,
        Jamie Iles <jamie@...iainc.com>,
        D Scott Phillips OS <scott@...amperecomputing.com>
Subject: Re: [PATCH v2 00/24] x86/resctrl: Merge the CDP resources

Hi James,
Thanks for the patches. Few comments below.

On 3/12/21 11:58 AM, James Morse wrote:
> Hi folks,
> 
> Thanks to Reinette and Jamie for the comments on v1. Major changes in v2 are
> to keep the closid in resctrl_arch_update_domains(), eliminating one patch,
> splitting another that was making two sorts of change, and to re-order the
> first few patches. See each patches changelog for more.
> ----
> 
> This series re-folds the resctrl code so the CDP resources (L3CODE et al)
> behaviour is all contained in the filesystem parts, with a minimum amount
> of arch specific code.
> 
> Arm have some CPU support for dividing caches into portions, and
> applying bandwidth limits at various points in the SoC. The collective term
> for these features is MPAM: Memory Partitioning and Monitoring.
> 
> MPAM is similar enough to Intel RDT, that it should use the defacto linux
> interface: resctrl. This filesystem currently lives under arch/x86, and is
> tightly coupled to the architecture.
> Ultimately, my plan is to split the existing resctrl code up to have an
> arch<->fs abstraction, then move all the bits out to fs/resctrl. From there
> MPAM can be wired up.
> 
> x86 might have two resources with cache controls, (L2 and L3) but has
> extra copies for CDP: L{2,3}{CODE,DATA}, which are marked as enabled
> if CDP is enabled for the corresponding cache.
> 
> MPAM has an equivalent feature to CDP, but its a property of the CPU,
> not the cache. Resctrl needs to have x86's odd/even behaviour, as that
> its the ABI, but this isn't how the MPAM hardware works. It is entirely
> possible that an in-kernel user of MPAM would not be using CDP, whereas
> resctrl is.
> Pretending L3CODE and L3DATA are entirely separate resources is a neat
> trick, but doing this is specific to x86.
> Doing this leaves the arch code in control of various parts of the
> filesystem ABI: the resources names, and the way the schemata are parsed.
> Allowing this stuff to vary between architectures is bad for user space.
> 
> This series collapses the CODE/DATA resources, moving all the user-visible
> resctrl ABI into what becomes the filesystem code. CDP becomes the type of
> configuration being applied to a cache. This is done by adding a
> struct resctrl_schema to the parts of resctrl that will move to fs. This
> holds the arch-code resource that is in use for this schema, along with
> other properties like the name, and whether the configuration being applied
> is CODE/DATA/BOTH.

I applied your patches on my AMD box. Seeing some difference in the behavior.

Before these patches.

# dmesg |grep -i resctrl
[   13.076973] resctrl: L3 allocation detected
[   13.087835] resctrl: L3DATA allocation detected
[   13.092886] resctrl: L3CODE allocation detected
[   13.097936] resctrl: MB allocation detected
[   13.102599] resctrl: L3 monitoring detected


After the patches.

# dmesg |grep -i resctrl
[   13.076973] resctrl: L3 allocation detected
[   13.097936] resctrl: MB allocation detected
[   13.102599] resctrl: L3 monitoring detected

You can see that L3DATA and L3CODE disappeared. I think we should keep the
behavior same for x86(at least).



I am still not clear why we needed resctrl_conf_type

enum resctrl_conf_type {
        CDP_BOTH,
        CDP_CODE,
        CDP_DATA,
};

Right now, I see all the resources are initialized as CDP_BOTH.

 [RDT_RESOURCE_L3] =
        {
                .conf_type                      = CDP_BOTH,
 [RDT_RESOURCE_L2] =
        {
                .conf_type                      = CDP_BOTH,
 [RDT_RESOURCE_MBA] =
        {
                .conf_type                      = CDP_BOTH,

If all the resources are CDP_BOTH, then why we need separate CDP_CODE and
CDP_DATA? Are these going to be different for ARM?

Also initializing RDT_RESOURCE_MBA as CDP_BOTH does not seem right. I dont
think there will CDP support in MBA in future.



> 
> This lets us fold the extra resources out of the arch code so that they
> don't need to be duplicated if the equivalent feature to CDP is missing, or
> implemented in a different way.
> 
> 
> The first two patches split the resource and domain structs to have an
> arch specific 'hw' portion, and the rest that is visible to resctrl.
> Future series massage the resctrl code so there are no accesses to 'hw'
> structures in the parts of resctrl that will move to fs, providing helpers
> where necessary.
> 
> This series adds temporary scaffolding, which it removes a few patches
> later. This is to allow things like the ctrlval arrays and resources to be
> merged separately, which should make is easier to bisect. These things
> are marked temporary, and should all be gone by the end of the series.
> 
> This series is a little rough around the monitors, would a fake
> struct resctrl_schema for the monitors simplify things, or be a source
> of bugs?
> 
> This series is based on v5.12-rc2, and can be retrieved from:
> git://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git mpam/resctrl_merge_cdp/v2
> 
> v1 was posted here:
> https://lore.kernel.org/lkml/20201030161120.227225-1-james.morse@arm.com/
> 
> Parts were previously posted as an RFC here:
> https://lore.kernel.org/lkml/20200214182947.39194-1-james.morse@arm.com/
> 
> 
> Thanks,
> 
> James Morse (24):
>   x86/resctrl: Split struct rdt_resource
>   x86/resctrl: Split struct rdt_domain
>   x86/resctrl: Add a separate schema list for resctrl
>   x86/resctrl: Pass the schema in info dir's private pointer
>   x86/resctrl: Label the resources with their configuration type
>   x86/resctrl: Walk the resctrl schema list instead of an arch list
>   x86/resctrl: Store the effective num_closid in the schema
>   x86/resctrl: Add resctrl_arch_get_num_closid()
>   x86/resctrl: Pass the schema to resctrl filesystem functions
>   x86/resctrl: Swizzle rdt_resource and resctrl_schema in
>     pseudo_lock_region
>   x86/resctrl: Move the schemata names into struct resctrl_schema
>   x86/resctrl: Group staged configuration into a separate struct
>   x86/resctrl: Allow different CODE/DATA configurations to be staged
>   x86/resctrl: Rename update_domains() resctrl_arch_update_domains()
>   x86/resctrl: Add a helper to read a closid's configuration
>   x86/resctrl: Add a helper to read/set the CDP configuration
>   x86/resctrl: Use cdp_enabled in rdt_domain_reconfigure_cdp()
>   x86/resctrl: Pass configuration type to resctrl_arch_get_config()
>   x86/resctrl: Make ctrlval arrays the same size
>   x86/resctrl: Apply offset correction when config is staged
>   x86/resctrl: Calculate the index from the configuration type
>   x86/resctrl: Merge the ctrl_val arrays
>   x86/resctrl: Remove rdt_cdp_peer_get()
>   x86/resctrl: Merge the CDP resources
> 
>  arch/x86/kernel/cpu/resctrl/core.c        | 272 +++++--------
>  arch/x86/kernel/cpu/resctrl/ctrlmondata.c | 164 +++++---
>  arch/x86/kernel/cpu/resctrl/internal.h    | 218 +++--------
>  arch/x86/kernel/cpu/resctrl/monitor.c     |  44 ++-
>  arch/x86/kernel/cpu/resctrl/pseudo_lock.c |  12 +-
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c    | 447 ++++++++++++----------
>  include/linux/resctrl.h                   | 176 +++++++++
>  7 files changed, 741 insertions(+), 592 deletions(-)
> 

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