[PATCH v13 12/32] x86,fs/resctrl: Support binary fixed point event counters

Tony Luck
2025-10-29T16:21:34Z

resctrl assumes that all monitor events can be displayed as unsigned
decimal integers.

Hardware architecture counters may provide some telemetry events with
greater precision where the event is not a simple count, but is a
measurement of some sort (e.g. Joules for energy consumed).

Add a new argument to resctrl_enable_mon_event() for architecture code
to inform the file system that the value for a counter is a fixed-point
value with a specific number of binary places.
Only allow architecture to use floating point format on events that the
file system has marked with mon_evt::is_floating_point.

Display fixed point values with values rounded to an appropriate number
of decimal places for the precision of the number of binary places
provided. Add one extra decimal place for every three additional binary
places, except for low precision binary values where exact representation
is possible:

1 binary place is 0.0 or 0.5 => 1 decimal place
2 binary places is 0.0, 0.25, 0.5, 0.75 => 2 decimal places
3 binary places is 0.0, 0.125, etc. => 3 decimal places

Signed-off-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
---
include/linux/resctrl.h | 3 +-
fs/resctrl/internal.h | 8 +++
arch/x86/kernel/cpu/resctrl/core.c | 6 +--
fs/resctrl/ctrlmondata.c | 84 ++++++++++++++++++++++++++++++
fs/resctrl/monitor.c | 10 +++-
5 files changed, 105 insertions(+), 6 deletions(-)

diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
index 702205505dc9..a7e5a546152d 100644
--- a/include/linux/resctrl.h
+++ b/include/linux/resctrl.h
@@ -409,7 +409,8 @@ u32 resctrl_arch_get_num_closid(struct rdt_resource *r);
u32 resctrl_arch_system_num_rmid_idx(void);
int resctrl_arch_update_domains(struct rdt_resource *r, u32 closid);

-void resctrl_enable_mon_event(enum resctrl_event_id eventid, bool any_cpu);
+void resctrl_enable_mon_event(enum resctrl_event_id eventid, bool any_cpu,
+ unsigned int binary_bits);

bool resctrl_is_mon_event_enabled(enum resctrl_event_id eventid);

diff --git a/fs/resctrl/internal.h b/fs/resctrl/internal.h
index 40b76eaa33d0..f5189b6771a0 100644
--- a/fs/resctrl/internal.h
+++ b/fs/resctrl/internal.h
@@ -62,6 +62,9 @@ static inline struct rdt_fs_context *rdt_fc2context(struct fs_context *fc)
* Only valid if @evtid is an MBM event.
* @configurable: true if the event is configurable
* @any_cpu: true if the event can be read from any CPU
+ * @is_floating_point: event values are displayed in floating point format
+ * @binary_bits: number of fixed-point binary bits from architecture,
+ * only valid if @is_floating_point is true
* @enabled: true if the event is enabled
*/
struct mon_evt {
@@ -71,6 +74,8 @@ struct mon_evt {
u32 evt_cfg;
bool configurable;
bool any_cpu;
+ bool is_floating_point;
+ unsigned int binary_bits;
bool enabled;
};

@@ -79,6 +84,9 @@ extern struct mon_evt mon_event_all[QOS_NUM_EVENTS];
#define for_each_mon_event(mevt) for (mevt = &mon_event_all[QOS_FIRST_EVENT]; \
mevt < &mon_event_all[QOS_NUM_EVENTS]; mevt++)

+/* Limit for mon_evt::binary_bits */
+#define MAX_BINARY_BITS 27
+
/**
* struct mon_data - Monitoring details for each event file.
* @list: Member of the global @mon_data_kn_priv_list list.
diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
index 78ad493dcc01..c435319552be 100644
--- a/arch/x86/kernel/cpu/resctrl/core.c
+++ b/arch/x86/kernel/cpu/resctrl/core.c
@@ -893,15 +893,15 @@ static __init bool get_rdt_mon_resources(void)
bool ret = false;

if (rdt_cpu_has(X86_FEATURE_CQM_OCCUP_LLC)) {
- resctrl_enable_mon_event(QOS_L3_OCCUP_EVENT_ID, false);
+ resctrl_enable_mon_event(QOS_L3_OCCUP_EVENT_ID, false, 0);
ret = true;
}
if (rdt_cpu_has(X86_FEATURE_CQM_MBM_TOTAL)) {
- resctrl_enable_mon_event(QOS_L3_MBM_TOTAL_EVENT_ID, false);
+ resctrl_enable_mon_event(QOS_L3_MBM_TOTAL_EVENT_ID, false, 0);
ret = true;
}
if (rdt_cpu_has(X86_FEATURE_CQM_MBM_LOCAL)) {
- resctrl_enable_mon_event(QOS_L3_MBM_LOCAL_EVENT_ID, false);
+ resctrl_enable_mon_event(QOS_L3_MBM_LOCAL_EVENT_ID, false, 0);
ret = true;
}
if (rdt_cpu_has(X86_FEATURE_ABMC))
diff --git a/fs/resctrl/ctrlmondata.c b/fs/resctrl/ctrlmondata.c
index 883be6f0810f..290a959776de 100644
--- a/fs/resctrl/ctrlmondata.c
+++ b/fs/resctrl/ctrlmondata.c
@@ -17,6 +17,7 @@

#include <linux/cpu.h>
#include <linux/kernfs.h>
+#include <linux/math.h>
#include <linux/seq_file.h>
#include <linux/slab.h>
#include <linux/tick.h>
@@ -597,6 +598,87 @@ void mon_event_read(struct rmid_read *rr, struct rdt_resource *r,
resctrl_arch_mon_ctx_free(r, evt->evtid, rr->arch_mon_ctx);
}

+/*
+ * Decimal place precision to use for each number of fixed-point
+ * binary bits.
+ */
+static unsigned int decplaces[MAX_BINARY_BITS + 1] = {
+ [1] = 1,
+ [2] = 2,
+ [3] = 3,
+ [4] = 3,
+ [5] = 3,
+ [6] = 3,
+ [7] = 3,
+ [8] = 3,
+ [9] = 3,
+ [10] = 4,
+ [11] = 4,
+ [12] = 4,
+ [13] = 5,
+ [14] = 5,
+ [15] = 5,
+ [16] = 6,
+ [17] = 6,
+ [18] = 6,
+ [19] = 7,
+ [20] = 7,
+ [21] = 7,
+ [22] = 8,
+ [23] = 8,
+ [24] = 8,
+ [25] = 9,
+ [26] = 9,
+ [27] = 9
+};
+
+static void print_event_value(struct seq_file *m, unsigned int binary_bits, u64 val)
+{
+ unsigned long long frac;
+ char buf[10];
+
+ if (!binary_bits) {
+ seq_printf(m, "%llu.0\n", val);
+ return;
+ }
+
+ /* Mask off the integer part of the fixed-point value. */
+ frac = val & GENMASK_ULL(binary_bits, 0);
+
+ /*
+ * Multiply by 10^{desired decimal places}. The integer part of
+ * the fixed point value is now almost what is needed.
+ */
+ frac *= int_pow(10ull, decplaces[binary_bits]);
+
+ /*
+ * Round to nearest by adding a value that would be a "1" in the
+ * binary_bits + 1 place. Integer part of fixed point value is
+ * now the needed value.
+ */
+ frac += 1ull << (binary_bits - 1);
+
+ /*
+ * Extract the integer part of the value. This is the decimal
+ * representation of the original fixed-point fractional value.
+ */
+ frac >>= binary_bits;
+
+ /*
+ * "frac" is now in the range [0 .. 10^decplaces). I.e. string
+ * representation will fit into chosen number of decimal places.
+ */
+ snprintf(buf, sizeof(buf), "%0*llu", decplaces[binary_bits], frac);
+
+ /* Trim trailing zeroes */
+ for (int i = decplaces[binary_bits] - 1; i > 0; i--) {
+ if (buf[i] != '0')
+ break;
+ buf[i] = '\0';
+ }
+ seq_printf(m, "%llu.%s\n", val >> binary_bits, buf);
+}
+
int rdtgroup_mondata_show(struct seq_file *m, void *arg)
{
struct kernfs_open_file *of = m->private;
@@ -674,6 +756,8 @@ int rdtgroup_mondata_show(struct seq_file *m, void *arg)
seq_puts(m, "Unavailable\n");
else if (rr.err == -ENOENT)
seq_puts(m, "Unassigned\n");
+ else if (evt->is_floating_point)
+ print_event_value(m, evt->binary_bits, rr.val);
else
seq_printf(m, "%llu\n", rr.val);

diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
index 6eab98b47816..7d1b65316bc8 100644
--- a/fs/resctrl/monitor.c
+++ b/fs/resctrl/monitor.c
@@ -975,16 +975,22 @@ struct mon_evt mon_event_all[QOS_NUM_EVENTS] = {
},
};

-void resctrl_enable_mon_event(enum resctrl_event_id eventid, bool any_cpu)
+void resctrl_enable_mon_event(enum resctrl_event_id eventid, bool any_cpu, unsigned int binary_bits)
{
- if (WARN_ON_ONCE(eventid < QOS_FIRST_EVENT || eventid >= QOS_NUM_EVENTS))
+ if (WARN_ON_ONCE(eventid < QOS_FIRST_EVENT || eventid >= QOS_NUM_EVENTS ||
+ binary_bits > MAX_BINARY_BITS))
return;
if (mon_event_all[eventid].enabled) {
pr_warn("Duplicate enable for event %d\n", eventid);
return;
}
+ if (binary_bits && !mon_event_all[eventid].is_floating_point) {
+ pr_warn("Event %d may not be floating point\n", eventid);
+ return;
+ }

mon_event_all[eventid].any_cpu = any_cpu;
+ mon_event_all[eventid].binary_bits = binary_bits;
mon_event_all[eventid].enabled = true;
}

--
2.51.0