[PATCH v9 07/12] x86/resctrl: Add a global ERDT monitor scale

[Chen Yu]

The limbo handler polls the LLC occupancy of stale RMIDs and frees an RMID
once its occupancy drops below max_threshold_occupancy. User space writes
that threshold in bytes, but the hardware counts occupancy in units of the
occupancy scale, so the threshold is rounded down to a multiple of that
scale. The scale is the number of bytes one counter unit represents, i.e.
the granularity of the occupancy counter.

ERDT may provide per-domain monitoring scale factors, but resctrl rounds
max_threshold_occupancy through a single global
resctrl_rmid_realloc_threshold value that is visible to user space.

Prepare for that by adding erdt_scale/erdt_get_scale() and teaching
resctrl_arch_round_mon_val() to consult the ERDT scale when firmware has
populated one.

When CMRC parsing provides per-domain up_scale values, erdt_scale will be
updated with max(). The reason to use max is because rounding is a floor:
a larger scale yields a slightly lower threshold, i.e. an RMID has to
drop to a slightly lower occupancy before it is reused.

For now erdt_scale is intentionally left at its default value of 0.

Suggested-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
---
Changes since v8:
- New patch. erdt_scale and erdt_get_scale() were introduced inside
  "x86/resctrl: Parse ACPI CMRC table" without any mention in its
  changelog. Split them out and describe why a single global scale is
  used. (Reinette Chatre)
- Reword the erdt_scale comment: it is not used by the limbo handler.
  (Reinette Chatre)

 arch/x86/kernel/cpu/resctrl/erdt.c     | 16 ++++++++++++++++
 arch/x86/kernel/cpu/resctrl/internal.h |  1 +
 arch/x86/kernel/cpu/resctrl/monitor.c  |  5 ++++-
 3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/resctrl/erdt.c b/arch/x86/kernel/cpu/resctrl/erdt.c
index d4b630358d6d..09072e478bde 100644
--- a/arch/x86/kernel/cpu/resctrl/erdt.c
+++ b/arch/x86/kernel/cpu/resctrl/erdt.c
@@ -38,6 +38,22 @@ static u16 first_rmdd_domain_id;
  */
 static unsigned int erdt_max_rmid;
 
+/*
+ * Used only to round the global resctrl_rmid_realloc_threshold for the limbo
+ * handler. resctrl_rmid_realloc_threshold is exposed to user space as a single
+ * global value, so ERDT uses one scale derived from the per-domain
+ * cmrc->up_scale values. max() keeps this single threshold conservative across
+ * domains because the rounding is a floor: a larger scale yields a slightly
+ * lower threshold, i.e. an RMID has to drop to a slightly lower occupancy
+ * before it is reused.
+ */
+static unsigned int erdt_scale;
+
+unsigned int erdt_get_scale(void)
+{
+	return erdt_scale;
+}
+
 static void __iomem *erdt_ioremap(resource_size_t base, u32 num_pages, const char *desc)
 {
 	void __iomem *addr;
diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
index 48aa64c1ff96..75dd4736937d 100644
--- a/arch/x86/kernel/cpu/resctrl/internal.h
+++ b/arch/x86/kernel/cpu/resctrl/internal.h
@@ -284,6 +284,7 @@ static inline void intel_aet_mon_domain_setup(int cpu, int id, struct rdt_resour
 static inline bool intel_handle_aet_option(bool force_off, char *tok) { return false; }
 #endif
 
+unsigned int erdt_get_scale(void);
 void erdt_init(void);
 void erdt_exit(void);
 bool erdt_try_bind_cpu(int cpu);
diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
index e42ee2769a39..f1a28a3c81bc 100644
--- a/arch/x86/kernel/cpu/resctrl/monitor.c
+++ b/arch/x86/kernel/cpu/resctrl/monitor.c
@@ -423,7 +423,10 @@ __init void resctrl_arch_update_snc(void)
 
 unsigned int resctrl_arch_round_mon_val(unsigned int val)
 {
-	unsigned int scale = boot_cpu_data.x86_cache_occ_scale;
+	unsigned int scale = erdt_get_scale();
+
+	if (!scale)
+		scale = boot_cpu_data.x86_cache_occ_scale;
 
 	/* h/w works in units of "boot_cpu_data.x86_cache_occ_scale" */
 	val /= scale;
-- 
2.43.0