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Subject[PATCH v3 2/2] x86/resctrl: Avoid redundant counter read in __mon_event_count()
From
__mon_event_count() does the per-RMID, per-domain work for
user-initiated event count reads and the initialization of new monitor
groups.

In the initialization case, after resctrl_arch_reset_rmid() calls
__rmid_read() to record an initial count for a new monitor group, it
immediately calls resctrl_arch_rmid_read(). This re-read of the hardware
counter is unnecessary and the following computations are ignored by the
caller during initialization.

Following return from resctrl_arch_reset_rmid(), just clear the
mbm_state and return. This involves moving the mbm_state lookup into the
rr->first case, as it's not needed for regular event count reads: the
QOS_L3_OCCUP_EVENT_ID case was redundant with the accumulating logic at
the end of the function.

Signed-off-by: Peter Newman <peternewman@google.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
---
v3:
- changelog clarifications suggested by Reinette
v2:
(patch introduced)

v2: https://lore.kernel.org/lkml/20221214160856.2164207-2-peternewman@google.com/
---
arch/x86/kernel/cpu/resctrl/monitor.c | 43 ++++++++++++---------------
1 file changed, 19 insertions(+), 24 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
index 77538abeb72a..e708df478077 100644
--- a/arch/x86/kernel/cpu/resctrl/monitor.c
+++ b/arch/x86/kernel/cpu/resctrl/monitor.c
@@ -366,41 +366,36 @@ void free_rmid(u32 rmid)
list_add_tail(&entry->list, &rmid_free_lru);
}

+static struct mbm_state *get_mbm_state(struct rdt_domain *d, u32 rmid,
+ enum resctrl_event_id evtid)
+{
+ switch (evtid) {
+ case QOS_L3_MBM_TOTAL_EVENT_ID:
+ return &d->mbm_total[rmid];
+ case QOS_L3_MBM_LOCAL_EVENT_ID:
+ return &d->mbm_local[rmid];
+ default:
+ return NULL;
+ }
+}
+
static int __mon_event_count(u32 rmid, struct rmid_read *rr)
{
struct mbm_state *m;
u64 tval = 0;

- if (rr->first)
+ if (rr->first) {
resctrl_arch_reset_rmid(rr->r, rr->d, rmid, rr->evtid);
+ m = get_mbm_state(rr->d, rmid, rr->evtid);
+ if (m)
+ memset(m, 0, sizeof(struct mbm_state));
+ return 0;
+ }

rr->err = resctrl_arch_rmid_read(rr->r, rr->d, rmid, rr->evtid, &tval);
if (rr->err)
return rr->err;

- switch (rr->evtid) {
- case QOS_L3_OCCUP_EVENT_ID:
- rr->val += tval;
- return 0;
- case QOS_L3_MBM_TOTAL_EVENT_ID:
- m = &rr->d->mbm_total[rmid];
- break;
- case QOS_L3_MBM_LOCAL_EVENT_ID:
- m = &rr->d->mbm_local[rmid];
- break;
- default:
- /*
- * Code would never reach here because an invalid
- * event id would fail in resctrl_arch_rmid_read().
- */
- return -EINVAL;
- }
-
- if (rr->first) {
- memset(m, 0, sizeof(struct mbm_state));
- return 0;
- }
-
rr->val += tval;

return 0;
--
2.39.0.314.g84b9a713c41-goog
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