Re: [PATCH v2 00/24] x86/resctrl: Merge the CDP resources

Babu Moger
2021-03-30T20:37:55Z

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(-)
>