Re: [PATCH v9 3/9] x86/resctrl: Modify update_mba_bw() to use per ctrl_mon group event

Reinette Chatre
2024-11-20T03:39:33Z

Hi Tony,

On 11/13/24 4:17 PM, Tony Luck wrote:
> Instead of hard-coding the memory bandwidth local event as the
> input to the mba_sc feedback look, use the event that the user

"feedback look" -> "feedback loop"

> configured for each ctrl_mon group.

From "Changelog" in Documentation/process/maintainer-tip.rst:
"It's also useful to structure the changelog into several paragraphs and not
lump everything together into a single one. A good structure is to explain
the context, the problem and the solution in separate paragraphs and this
order."

I do not find there to be a context nor problem description in the
changelog. Do you believe this changelog is appropriate for tip? Am I missing
something?

>
> Signed-off-by: Tony Luck <tony.luck@intel.com>
> ---
> arch/x86/kernel/cpu/resctrl/monitor.c | 23 ++++++++++++++++++-----
> 1 file changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> index 7ef1a293cc13..2176e355e864 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -752,20 +752,31 @@ static void update_mba_bw(struct rdtgroup *rgrp, struct rdt_mon_domain *dom_mbm)
> u32 closid, rmid, cur_msr_val, new_msr_val;
> struct mbm_state *pmbm_data, *cmbm_data;
> struct rdt_ctrl_domain *dom_mba;
> + enum resctrl_event_id evt_id;
> struct rdt_resource *r_mba;
> - u32 cur_bw, user_bw, idx;
> struct list_head *head;
> struct rdtgroup *entry;
> + u32 cur_bw, user_bw;
>
> - if (!is_mbm_local_enabled())
> + if (!is_mbm_enabled())

This change in the check is unexpected because at this point the event is still enforced to be
local MBM. This change is also undocumented so difficult to reason about.

> return;
>
> r_mba = &rdt_resources_all[RDT_RESOURCE_MBA].r_resctrl;
> + evt_id = rgrp->mba_mbps_event;

(To also answer the question in https://lore.kernel.org/all/Zzvtj8n1_ukhnRWT@agluck-desk3/ )

One key point from previous patch is that there is a new "contract" that rgrp->mba_mbps_event
is valid if mba_sc is enabled. If that contract is respected with appropriate initialization
and change of rgrp->mba_mbps_event then I do not believe that the three checks below
nor the "is_mbm_enabled()" above in reader of rgrp->mba_mbps_event are necessary since the
access of rgrp->mba_mbps_event is within "contract".
Note that caller does the checking if mba_sc is enabled:
if (is_mba_sc(NULL))
update_mba_bw(prgrp, d);

Thus doing same check within update_mba_bw() should not be necessary.

It does take a lot of digging to understand so it would really be helpful to document these types
of design decisions and reinforce them through the series.

> +
> + if (WARN_ON_ONCE(!is_mbm_event(evt_id)))
> + return;
> + if (WARN_ON_ONCE(evt_id == QOS_L3_MBM_LOCAL_EVENT_ID && !is_mbm_local_enabled()))
> + return;
> + if (WARN_ON_ONCE(evt_id == QOS_L3_MBM_TOTAL_EVENT_ID && !is_mbm_total_enabled()))
> + return;
> +
>
> closid = rgrp->closid;
> rmid = rgrp->mon.rmid;
> - idx = resctrl_arch_rmid_idx_encode(closid, rmid);
> - pmbm_data = &dom_mbm->mbm_local[idx];
> + pmbm_data = get_mbm_state(dom_mbm, closid, rmid, evt_id);
> + if (WARN_ON_ONCE(!pmbm_data))
> + return;
>
> dom_mba = get_ctrl_domain_from_cpu(smp_processor_id(), r_mba);
> if (!dom_mba) {
> @@ -784,7 +795,9 @@ static void update_mba_bw(struct rdtgroup *rgrp, struct rdt_mon_domain *dom_mbm)
> */
> head = &rgrp->mon.crdtgrp_list;
> list_for_each_entry(entry, head, mon.crdtgrp_list) {
> - cmbm_data = &dom_mbm->mbm_local[entry->mon.rmid];
> + cmbm_data = get_mbm_state(dom_mbm, entry->closid, entry->mon.rmid, evt_id);
> + if (WARN_ON_ONCE(!cmbm_data))
> + return;
> cur_bw += cmbm_data->prev_bw;
> }
>

Reinette