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Message-Id: <20251217095912.GB2073381@bytedance.com>
Date: Wed, 17 Dec 2025 17:59:12 +0800
From: "Aaron Lu" <ziqianlu@...edance.com>
To: "Chen Yu" <yu.c.chen@...el.com>, "Tim Chen" <tim.c.chen@...ux.intel.com>
Cc: "Peter Zijlstra" <peterz@...radead.org>,
"Ingo Molnar" <mingo@...hat.com>,
"K Prateek Nayak" <kprateek.nayak@....com>,
"Gautham R . Shenoy" <gautham.shenoy@....com>,
"Vincent Guittot" <vincent.guittot@...aro.org>,
"Juri Lelli" <juri.lelli@...hat.com>,
"Dietmar Eggemann" <dietmar.eggemann@....com>,
"Steven Rostedt" <rostedt@...dmis.org>,
"Ben Segall" <bsegall@...gle.com>, "Mel Gorman" <mgorman@...e.de>,
"Valentin Schneider" <vschneid@...hat.com>,
"Madadi Vineeth Reddy" <vineethr@...ux.ibm.com>,
"Hillf Danton" <hdanton@...a.com>,
"Shrikanth Hegde" <sshegde@...ux.ibm.com>,
"Jianyong Wu" <jianyong.wu@...look.com>,
"Yangyu Chen" <cyy@...self.name>,
"Tingyin Duan" <tingyin.duan@...il.com>,
"Vern Hao" <vernhao@...cent.com>, "Vern Hao" <haoxing990@...il.com>,
"Len Brown" <len.brown@...el.com>, "Aubrey Li" <aubrey.li@...el.com>,
"Zhao Liu" <zhao1.liu@...el.com>, "Chen Yu" <yu.chen.surf@...il.com>,
"Adam Li" <adamli@...amperecomputing.com>,
"Tim Chen" <tim.c.chen@...el.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 23/23] -- DO NOT APPLY!!! -- sched/cache/debug: Display the per LLC occupancy for each process via proc fs
On Wed, Dec 03, 2025 at 03:07:42PM -0800, Tim Chen wrote:
> From: Chen Yu <yu.c.chen@...el.com>
>
> 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@...el.com>
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> ---
> 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.
> + 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);
+
+ 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;
+
/*
* get_cpu_cacheinfo_level() can not be used
* because it requires the cpu_hotplug_lock
@@ -1257,8 +1265,10 @@ static bool exceed_llc_capacity(struct mm_struct *mm, int cpu)
* L2 becomes the LLC.
*/
ci = _get_cpu_cacheinfo_level(cpu, 2);
- if (!ci)
+ if (!ci) {
+ mm->mm_sched_flags |= MM_SCHED_NO_CACHE_INFO;
return true;
+ }
}
llc = ci->size;
@@ -1283,13 +1293,20 @@ static bool exceed_llc_capacity(struct mm_struct *mm, int cpu)
if (scale == INT_MAX)
return false;
- return ((llc * scale) <= (rss * PAGE_SIZE));
+ if ((llc * scale) <= (rss * PAGE_SIZE)) {
+ mm->mm_sched_flags |= MM_SCHED_EXCEED_LLC_CAPACITY;
+ return true;
+ }
+
+ return false;
}
static bool exceed_llc_nr(struct mm_struct *mm, int cpu)
{
int smt_nr = 1, scale;
+ mm->mm_sched_flags &= ~MM_SCHED_EXCEED_LLC_NR;
+
#ifdef CONFIG_SCHED_SMT
if (sched_smt_active())
smt_nr = cpumask_weight(cpu_smt_mask(cpu));
@@ -1313,7 +1330,12 @@ static bool exceed_llc_nr(struct mm_struct *mm, int cpu)
if (scale == INT_MAX)
return false;
- return ((mm->nr_running_avg * smt_nr) > (scale * per_cpu(sd_llc_size, cpu)));
+ if ((mm->nr_running_avg * smt_nr) > (scale * per_cpu(sd_llc_size, cpu))) {
+ mm->mm_sched_flags |= MM_SCHED_EXCEED_LLC_NR;
+ return true;
+ }
+
+ return false;
}
static void account_llc_enqueue(struct rq *rq, struct task_struct *p)
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