llc_aggr_tolerance scales two independent gates with a single value:
the number of cores in an LLC compared against the process's active
thread count in invalid_llc_nr(), and the LLC size compared against
the process's memory footprint in exceed_llc_capacity().
The right settings are workload and platform specific. A Verilator
run is one example: its RSS is large while only a small part of it
is hot, so an RSS-based footprint estimate overstates its cache
usage and should not decide whether it is aggregated. And running
its threads on the SMT siblings of a single LLC can beat spreading
them across LLCs on some platforms (e.g. AMD EPYC Turin) but not on
others (e.g. EPYC Milan). Tuning for such a workload means a larger
thread-count tolerance while the footprint tolerance keeps its own
setting - which a single combined knob cannot express.
Split the knob into llc_aggr_tolerance_nr for the thread-count gate
and llc_aggr_tolerance_size for the footprint gate, and let
get_sched_cache_scale() take the tolerance value from the caller.
Both new debugfs files keep the semantics of the old knob (0 disables
aggregation for that gate, values >= 100 mean unlimited).
While making the tunables independently settable, also make the
percentage knobs safe for any value: llc_overaggr_pct and llc_imb_pct
are exposed via debugfs as unbounded u32 values and feed percentage
multiplications:
util * 100 < max * aggr_pct (fits_llc_capacity())
util1 * 100 > util2 * (100 + llc_imb_pct) (util_greater())
CONFIG_SCHED_CACHE only depends on SMP, so this also builds on 32-bit
where unsigned long is 32 bits: a large enough percentage makes
max * aggr_pct (or util2 * (100 + pct)) wrap and produce a garbage
comparison, and the SMT-1 bump aggr_pct * 3 / 2 can wrap the u32
itself. Rather than capping the tunables, detect the overflow at the
point of use with check_mul_overflow()/check_add_overflow() and fall
back to the saturated meaning of the comparison: an overflowing
threshold is effectively unlimited (the LLC always has room), and an
overflowing bias is effectively infinite (the destination utilization
is never considered noticeably greater).
Assisted-by: Claude:claude-fable-5
Signed-off-by: Yangyu Chen <[email protected]>
---
kernel/sched/debug.c | 6 +++--
kernel/sched/fair.c | 62 +++++++++++++++++++++++++++++++-------------
kernel/sched/sched.h | 3 ++-
3 files changed, 50 insertions(+), 21 deletions(-)
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 40584b27ea0c..b7be245b9fc2 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -672,8 +672,10 @@ static __init int sched_init_debug(void)
llc = debugfs_create_dir("llc_balancing", debugfs_sched);
debugfs_create_file("enabled", 0644, llc, NULL,
&sched_cache_enable_fops);
- debugfs_create_u32("aggr_tolerance", 0644, llc,
- &llc_aggr_tolerance);
+ debugfs_create_u32("aggr_tolerance_nr", 0644, llc,
+ &llc_aggr_tolerance_nr);
+ debugfs_create_u32("aggr_tolerance_size", 0644, llc,
+ &llc_aggr_tolerance_size);
debugfs_create_u32("epoch_period", 0644, llc,
&llc_epoch_period);
debugfs_create_u32("epoch_affinity_timeout", 0644, llc,
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d78467ec6ee1..c31acbfa3247 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1404,7 +1404,8 @@ static void set_next_buddy(struct sched_entity *se);
*/
#define EPOCH_PERIOD (HZ / 100) /* 10 ms */
#define EPOCH_LLC_AFFINITY_TIMEOUT 5 /* 50 ms */
-__read_mostly unsigned int llc_aggr_tolerance = 1;
+__read_mostly unsigned int llc_aggr_tolerance_nr = 1;
+__read_mostly unsigned int llc_aggr_tolerance_size = 1;
__read_mostly unsigned int llc_epoch_period = EPOCH_PERIOD;
__read_mostly unsigned int llc_epoch_affinity_timeout =
EPOCH_LLC_AFFINITY_TIMEOUT;
__read_mostly unsigned int llc_imb_pct = 20;
@@ -1418,10 +1419,8 @@ static int llc_id(int cpu)
return per_cpu(sd_llc_id, cpu);
}
-static inline int get_sched_cache_scale(int mul)
+static inline int get_sched_cache_scale(unsigned int tol, int mul)
{
- unsigned int tol = READ_ONCE(llc_aggr_tolerance);
-
if (!tol)
return 0;
@@ -1453,23 +1452,23 @@ static bool exceed_llc_capacity(struct mm_struct *mm,
int cpu)
footprint = READ_ONCE(mm->sc_stat.footprint);
/*
- * Scale the LLC size by 256*llc_aggr_tolerance
+ * Scale the LLC size by 256*llc_aggr_tolerance_size
* and compare it to the task's footprint.
*
* Suppose the L3 size is 32MB. If the
- * llc_aggr_tolerance is 1:
+ * llc_aggr_tolerance_size is 1:
* When the footprint is larger than 32MB, the
* process is regarded as exceeding the LLC
- * capacity. If the llc_aggr_tolerance is 99:
+ * capacity. If the llc_aggr_tolerance_size is 99:
* When the footprint is larger than 784GB, the
* process is regarded as exceeding the LLC
* capacity:
* 784GB = (1 + (99 - 1) * 256) * 32MB
- * If the llc_aggr_tolerance is 100:
+ * If the llc_aggr_tolerance_size is 100:
* ignore the footprint and do the aggregation
* anyway.
*/
- scale = get_sched_cache_scale(256);
+ scale =
get_sched_cache_scale(READ_ONCE(llc_aggr_tolerance_size), 256);
if (scale == INT_MAX)
return false;
@@ -1488,10 +1487,10 @@ static bool invalid_llc_nr(struct mm_struct *mm, struct
task_struct *p,
return true;
/*
- * Scale the number of 'cores' in a LLC by llc_aggr_tolerance
+ * Scale the number of 'cores' in a LLC by llc_aggr_tolerance_nr
* and compare it to the task's active threads.
*/
- scale = get_sched_cache_scale(1);
+ scale = get_sched_cache_scale(READ_ONCE(llc_aggr_tolerance_nr), 1);
if (scale == INT_MAX)
return false;
@@ -10418,20 +10417,34 @@ static inline int
task_is_ineligible_on_dst_cpu(struct task_struct *p, int dest_
* done.
* Derived from fits_capacity().
*
+ * llc_overaggr_pct is an unbounded debugfs u32 and CONFIG_SCHED_CACHE
+ * only depends on SMP, so max * aggr_pct can wrap on 32-bit. A
+ * threshold large enough to overflow is effectively unlimited: the
+ * LLC always has room, so treat that as "fits" rather than letting
+ * the multiplication wrap.
+ *
* (default: ~50%, tunable via debugfs)
*/
static bool fits_llc_capacity(unsigned long util, unsigned long max)
{
- u32 aggr_pct = llc_overaggr_pct;
+ u32 aggr_pct = READ_ONCE(llc_overaggr_pct);
+ unsigned long thresh;
+ u32 bumped;
/*
* For single core systems, raise the aggregation
* threshold to accommodate more tasks.
*/
- if (cpu_smt_num_threads == 1)
- aggr_pct = (aggr_pct * 3 / 2);
+ if (cpu_smt_num_threads == 1) {
+ if (check_mul_overflow(aggr_pct, 3U, &bumped))
+ return true;
+ aggr_pct = bumped / 2;
+ }
+
+ if (check_mul_overflow(max, (unsigned long)aggr_pct, &thresh))
+ return true;
- return util * 100 < max * aggr_pct;
+ return util * 100 < thresh;
}
/*
@@ -10439,10 +10452,23 @@ static bool fits_llc_capacity(unsigned long util,
unsigned long max)
* is 'util1' noticeably greater than 'util2'
* Derived from capacity_greater().
* Bias is in perentage.
+ *
+ * Allows dst util to be bigger than src util by up to bias percent.
+ * llc_imb_pct is an unbounded debugfs u32; a bias large enough to
+ * overflow util2 * (100 + llc_imb_pct) is effectively infinite, so
+ * util1 is never noticeably greater - treat that as "not greater"
+ * rather than letting the multiplication wrap.
*/
-/* Allows dst util to be bigger than src util by up to bias percent */
-#define util_greater(util1, util2) \
- ((util1) * 100 > (util2) * (100 + llc_imb_pct))
+static bool util_greater(unsigned long util1, unsigned long util2)
+{
+ unsigned long bias, rhs;
+
+ if (check_add_overflow(100UL, (unsigned long)READ_ONCE(llc_imb_pct),
&bias) ||
+ check_mul_overflow(util2, bias, &rhs))
+ return false;
+
+ return util1 * 100 > rhs;
+}
static __maybe_unused bool get_llc_stats(int cpu, unsigned long *util,
unsigned long *cap)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 56acf502ba26..0ebafdcf2c7f 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -4095,7 +4095,8 @@ static inline void mm_cid_switch_to(struct task_struct
*prev, struct task_struct
DECLARE_STATIC_KEY_FALSE(sched_cache_present);
DECLARE_STATIC_KEY_FALSE(sched_cache_active);
extern int sysctl_sched_cache_user;
-extern unsigned int llc_aggr_tolerance;
+extern unsigned int llc_aggr_tolerance_nr;
+extern unsigned int llc_aggr_tolerance_size;
extern unsigned int llc_epoch_period;
extern unsigned int llc_epoch_affinity_timeout;
extern unsigned int llc_imb_pct;
--
2.47.3
