Re: [PATCH v9 2/9] x86/resctrl: Prepare for per-ctrl_mon group mba_MBps control

Reinette Chatre
2024-11-20T01:08:51Z

Hi Tony,

On 11/13/24 4:17 PM, Tony Luck wrote:
> Resctrl uses local memory bandwidth event as input to the feedback
> loop when the mba_MBps mount option is used. This means that this
> mount option cannot be used on systems that only support monitoring
> of total bandwidth.
>
> Prepare to allow users to choose the input event independently for
> each ctrl_mon group.

The lack of detail on design and implementation leaves a lot for the
reader to decipher. For example,
* the change appears to create a contract that rdtgroup.mba_mbps_event
is only valid if mba_sc is enabled, this is "documented" in the
structure member comment but not connected to the rest of implementation, not
here nor later in series.
* the patch uses *three* different checks to manage new variables:
is_mbm_local_enabled(), is_mba_sc(), and supports_mba_mbps(). Reader is
left to decipher that all checks are built on is_mbm_local_enabled()
and thus it is ok to use these checks before using the value that is only
assigned when is_mbm_local_enabled().
* clearly mba_mbps_default_event cannot always have a value making reader wonder
if enum resctrl_event_id needs a "0", takes some deciphering to get confidence
that its assignment when is_mbm_local_enabled() fits under the contract
that values are only value when is_mba_sc() and thus any code following contract by
first checking for mba_sc should never encounter a 0.
* based on premise of this work reader may consider what happens if
system does not support local MBM. more deciphering needed to get confidence
that while mba_mbps_default_event will not be set, since is_mba_sc() still
depends on local MBM this still fits under contract that mba_mbps_default_event
cannot be used in this case.

Of course, it may just me that needs more help to understand what a patch is doing
while having little insight into what it intends to do. I thought by sharing some of
the questions I felt needed to investigated may give some insight into the difficulty
a cryptic changelog creates. Review could be helped significantly if the changelog
provides insight into the design decisions.

...

> @@ -3611,6 +3613,8 @@ static int rdtgroup_mkdir_ctrl_mon(struct kernfs_node *parent_kn,
> rdt_last_cmd_puts("kernfs subdir error\n");
> goto out_del_list;
> }
> + if (is_mba_sc(NULL))
> + rdtgrp->mba_mbps_event = mba_mbps_default_event;
> }
>
> goto out_unlock;
> @@ -3970,6 +3974,8 @@ static void __init rdtgroup_setup_default(void)
> rdtgroup_default.closid = RESCTRL_RESERVED_CLOSID;
> rdtgroup_default.mon.rmid = RESCTRL_RESERVED_RMID;
> rdtgroup_default.type = RDTCTRL_GROUP;
> + if (supports_mba_mbps())
> + rdtgroup_default.mba_mbps_event = mba_mbps_default_event;
> INIT_LIST_HEAD(&rdtgroup_default.mon.crdtgrp_list);
>
> list_add(&rdtgroup_default.rdtgroup_list, &rdt_all_groups);

I do not see the default resource group's mba_mbps_event ever being reset. This means
that if the user mounts resctrl, changes mba_mbps_event, umount resctrl, remount
resctrl, then the default resource group will not have the default mba_mbps_event
but whatever was set on previous mount. Is this intended? No mention of this behavior in
changelog.

Reinette