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Message-ID: <9d6dbc7e-19bd-4b41-896f-1307470adb7a@intel.com>
Date: Wed, 17 Dec 2025 21:01:05 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Aaron Lu <ziqianlu@...edance.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>, Tim Chen
<tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH v2 23/23] -- DO NOT APPLY!!! -- sched/cache/debug: Display
the per LLC occupancy for each process via proc fs
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@...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.
>
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
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