Re: [PATCH v2 23/23] -- DO NOT APPLY!!! -- sched/cache/debug: Display the per LLC occupancy for each process via proc fs
From: Chen, Yu C
Date: Wed Dec 17 2025 - 10:55:26 EST
On 12/17/2025 5:59 PM, Aaron Lu wrote:
On Wed, Dec 03, 2025 at 03:07:42PM -0800, Tim Chen wrote:
From: Chen Yu <yu.c.chen@xxxxxxxxx>
Debug patch only.
Show the per-LLC occupancy in /proc/{PID}/schedstat, with each column
corresponding to one LLC. This can be used to verify if the cache-aware
load balancer works as expected by aggregating threads onto dedicated LLCs.
Suppose there are 2 LLCs and the sampling duration is 10 seconds:
Enable the cache aware load balance:
0 12281 <--- LLC0 residency delta is 0, LLC1 is 12 seconds
0 18881
0 16217
disable the cache aware load balance:
6497 15802
9299 5435
17811 8278
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
fs/proc/base.c | 22 ++++++++++++++++++++++
include/linux/mm_types.h | 19 +++++++++++++++++--
include/linux/sched.h | 3 +++
kernel/sched/fair.c | 40 ++++++++++++++++++++++++++++++++++++++--
4 files changed, 80 insertions(+), 4 deletions(-)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 6299878e3d97..f4be96f4bd01 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -518,6 +518,28 @@ static int proc_pid_schedstat(struct seq_file *m, struct pid_namespace *ns,
(unsigned long long)task->se.sum_exec_runtime,
(unsigned long long)task->sched_info.run_delay,
task->sched_info.pcount);
+#ifdef CONFIG_SCHED_CACHE
+ if (sched_cache_enabled()) {
+ struct mm_struct *mm = task->mm;
+ u64 *llc_runtime;
+
+ if (!mm)
+ return 0;
+
+ llc_runtime = kcalloc(max_llcs, sizeof(u64), GFP_KERNEL);
+ if (!llc_runtime)
+ return 0;
+
+ if (get_mm_per_llc_runtime(task, llc_runtime))
+ goto out;
+
+ for (int i = 0; i < max_llcs; i++)
+ seq_printf(m, "%llu ", llc_runtime[i]);
I feel it is better to also mark the current preferred LLC of this
process so that I can know how well it works.
Sure.
+ seq_puts(m, "\n");
+out:
+ kfree(llc_runtime);
+ }
+#endif
return 0;
}
BTW, is there a way to tell if a process is being taken care of by
'cache aware scheduling' or it's blocked due to its huge rss or having
too many threads?
I used below debug code to get these info through schedstat, but maybe I
missed something and there is a simpler method?
diff --git a/fs/proc/base.c b/fs/proc/base.c
index f4be96f4bd015..c709a1a1bd867 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -505,6 +505,7 @@ static int proc_pid_stack(struct seq_file *m, struct pid_namespace *ns,
#endif
#ifdef CONFIG_SCHED_INFO
+DECLARE_PER_CPU(int, sd_llc_id);
/*
* Provides /proc/PID/schedstat
*/
@@ -522,6 +523,7 @@ static int proc_pid_schedstat(struct seq_file *m, struct pid_namespace *ns,
if (sched_cache_enabled()) {
struct mm_struct *mm = task->mm;
u64 *llc_runtime;
+ int mm_sched_llc;
if (!mm)
return 0;
@@ -533,8 +535,17 @@ static int proc_pid_schedstat(struct seq_file *m, struct pid_namespace *ns,
if (get_mm_per_llc_runtime(task, llc_runtime))
goto out;
+ if (mm->mm_sched_cpu == -1)
+ mm_sched_llc = -1;
+ else
+ mm_sched_llc = per_cpu(sd_llc_id, mm->mm_sched_cpu);
We can use llc_id(mm->mm_sched_cpu).
+
+ seq_printf(m, "%llu 0x%x\n", mm->nr_running_avg, mm->mm_sched_flags);
for (int i = 0; i < max_llcs; i++)
- seq_printf(m, "%llu ", llc_runtime[i]);
+ seq_printf(m, "%s%s%llu ",
+ i == task->preferred_llc ? "*" : "",
+ i == mm_sched_llc ? "?" : "",
+ llc_runtime[i]);
seq_puts(m, "\n");
out:
kfree(llc_runtime);
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 255c22be7312f..06bb106d1b724 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1048,6 +1048,7 @@ struct mm_struct {
raw_spinlock_t mm_sched_lock;
unsigned long mm_sched_epoch;
int mm_sched_cpu;
+ int mm_sched_flags;
u64 nr_running_avg ____cacheline_aligned_in_smp;
#endif
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 205208f061bb3..ab1cdba65d389 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1237,12 +1237,20 @@ static inline int get_sched_cache_scale(int mul)
return (1 + (llc_aggr_tolerance - 1) * mul);
}
+#define MM_SCHED_EXCEED_LLC_CAPACITY 1
+#define MM_SCHED_NO_CACHE_INFO 2
+#define MM_SCHED_EXCEED_LLC_NR 4
+#define MM_SCHED_NR_THREADS 8
+
static bool exceed_llc_capacity(struct mm_struct *mm, int cpu)
{
unsigned int llc, scale;
struct cacheinfo *ci;
unsigned long rss;
+ mm->mm_sched_flags &= ~MM_SCHED_NO_CACHE_INFO;
+ mm->mm_sched_flags &= ~MM_SCHED_EXCEED_LLC_CAPACITY;
+
Maybe we can do some read-comparison before writing the flags, previously
we found that writing the per-process mm struct is very expensive, so
maybe avoid writing to it as much as possible.
I'll fold your changes and do the test. Thanks!
Thanks,
Chenyu