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Message-ID: <6144e652-7599-4b04-a08e-9f059ff81e26@efficios.com>
Date: Wed, 6 Aug 2025 12:57:40 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Gabriele Monaco <gmonaco@...hat.com>, linux-kernel@...r.kernel.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
David Hildenbrand <david@...hat.com>, Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 3/4] sched: Compact RSEQ concurrency IDs in batches
On 2025-08-05 08:42, Gabriele Monaco wrote:
> On Wed, 2025-07-16 at 18:06 +0200, Gabriele Monaco wrote:
>> Currently, task_mm_cid_work() is called from resume_user_mode_work().
>> This can delay the execution of the corresponding thread for the
>> entire duration of the function, negatively affecting the response in
>> case of real time tasks.
>> In practice, we observe task_mm_cid_work increasing the latency of
>> 30-35us on a 128 cores system, this order of magnitude is meaningful
>> under PREEMPT_RT.
>>
>> Run the task_mm_cid_work in batches of up to
>> CONFIG_RSEQ_CID_SCAN_BATCH CPUs, this reduces the duration of the
>> delay for each scan.
>>
>> The task_mm_cid_work contains a mechanism to avoid running more
>> frequently than every 100ms. Keep this pseudo-periodicity only on
>> complete scans.
>> This means each call to task_mm_cid_work returns prematurely if the
>> period did not elapse and a scan is not ongoing (i.e. the next batch
>> to scan is not the first).
>> This way full scans are not excessively delayed while still keeping
>> each run, and introduced latency, short.
>>
>
> Mathieu, would you have some time to look at this implementation?
Hi Gabriele,
Please note that I am currently on vacation. I'll be back shortly
before the end of August, but I'm afraid there are other tasks I
need to focus on before I can get back to this. I'm adding this
review to my todo list for September.
Thanks,
Mathieu
>
> Thanks,
> Gabriele
>
>> Fixes: 223baf9d17f2 ("sched: Fix performance regression introduced by
>> mm_cid")
>> Signed-off-by: Gabriele Monaco <gmonaco@...hat.com>
>> ---
>> include/linux/mm_types.h | 15 +++++++++++++++
>> init/Kconfig | 12 ++++++++++++
>> kernel/sched/core.c | 37 ++++++++++++++++++++++++++++++++++---
>> 3 files changed, 61 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index e6d6e468e64b4..a822966a584f3 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -995,6 +995,13 @@ struct mm_struct {
>> * When the next mm_cid scan is due (in jiffies).
>> */
>> unsigned long mm_cid_next_scan;
>> + /*
>> + * @mm_cid_scan_batch: Counter for batch used in the
>> next scan.
>> + *
>> + * Scan in batches of CONFIG_RSEQ_CID_SCAN_BATCH.
>> This field
>> + * increments at each scan and reset when all
>> batches are done.
>> + */
>> + unsigned int mm_cid_scan_batch;
>> /**
>> * @nr_cpus_allowed: Number of CPUs allowed for mm.
>> *
>> @@ -1385,6 +1392,7 @@ static inline void mm_init_cid(struct mm_struct
>> *mm, struct task_struct *p)
>> raw_spin_lock_init(&mm->cpus_allowed_lock);
>> cpumask_copy(mm_cpus_allowed(mm), &p->cpus_mask);
>> cpumask_clear(mm_cidmask(mm));
>> + mm->mm_cid_scan_batch = 0;
>> }
>>
>> static inline int mm_alloc_cid_noprof(struct mm_struct *mm, struct
>> task_struct *p)
>> @@ -1423,8 +1431,15 @@ static inline void mm_set_cpus_allowed(struct
>> mm_struct *mm, const struct cpumas
>>
>> static inline bool mm_cid_needs_scan(struct mm_struct *mm)
>> {
>> + unsigned int next_batch;
>> +
>> if (!mm)
>> return false;
>> + next_batch = READ_ONCE(mm->mm_cid_scan_batch);
>> + /* Always needs scan unless it's the first batch. */
>> + if (CONFIG_RSEQ_CID_SCAN_BATCH * next_batch <
>> num_possible_cpus() &&
>> + next_batch)
>> + return true;
>> return time_after(jiffies, READ_ONCE(mm->mm_cid_next_scan));
>> }
>> #else /* CONFIG_SCHED_MM_CID */
>> diff --git a/init/Kconfig b/init/Kconfig
>> index 666783eb50abd..98d7f078cd6df 100644
>> --- a/init/Kconfig
>> +++ b/init/Kconfig
>> @@ -1860,6 +1860,18 @@ config DEBUG_RSEQ
>>
>> If unsure, say N.
>>
>> +config RSEQ_CID_SCAN_BATCH
>> + int "Number of CPUs to scan at every mm_cid compaction
>> attempt"
>> + range 1 NR_CPUS
>> + default 8
>> + depends on SCHED_MM_CID
>> + help
>> + CPUs are scanned pseudo-periodically to compact the CID of
>> each task,
>> + this operation can take a longer amount of time on systems
>> with many
>> + CPUs, resulting in higher scheduling latency for the
>> current task.
>> + A higher value means the CID is compacted faster, but
>> results in
>> + higher scheduling latency.
>> +
>> config CACHESTAT_SYSCALL
>> bool "Enable cachestat() system call" if EXPERT
>> default y
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 27b856a1cb0a9..eae4c8faf980b 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -10591,11 +10591,26 @@ static void
>> sched_mm_cid_remote_clear_weight(struct mm_struct *mm, int cpu,
>>
>> void task_mm_cid_work(struct task_struct *t)
>> {
>> + int weight, cpu, from_cpu, this_batch, next_batch, idx;
>> unsigned long now = jiffies, old_scan, next_scan;
>> struct cpumask *cidmask;
>> - int weight, cpu;
>> struct mm_struct *mm = t->mm;
>>
>> + /*
>> + * This function is called from __rseq_handle_notify_resume,
>> which
>> + * makes sure t is a user thread and is not exiting.
>> + */
>> + this_batch = READ_ONCE(mm->mm_cid_scan_batch);
>> + next_batch = this_batch + 1;
>> + from_cpu = cpumask_nth(this_batch *
>> CONFIG_RSEQ_CID_SCAN_BATCH,
>> + cpu_possible_mask);
>> + if (from_cpu >= nr_cpu_ids) {
>> + from_cpu = 0;
>> + next_batch = 1;
>> + }
>> + /* Delay scan only if we are done with all cpus. */
>> + if (from_cpu != 0)
>> + goto cid_compact;
>> old_scan = READ_ONCE(mm->mm_cid_next_scan);
>> next_scan = now + msecs_to_jiffies(MM_CID_SCAN_DELAY);
>> if (!old_scan) {
>> @@ -10611,17 +10626,33 @@ void task_mm_cid_work(struct task_struct
>> *t)
>> return;
>> if (!try_cmpxchg(&mm->mm_cid_next_scan, &old_scan,
>> next_scan))
>> return;
>> +
>> +cid_compact:
>> + if (!try_cmpxchg(&mm->mm_cid_scan_batch, &this_batch,
>> next_batch))
>> + return;
>> cidmask = mm_cidmask(mm);
>> /* Clear cids that were not recently used. */
>> - for_each_possible_cpu(cpu)
>> + idx = 0;
>> + cpu = from_cpu;
>> + for_each_cpu_from(cpu, cpu_possible_mask) {
>> + if (idx == CONFIG_RSEQ_CID_SCAN_BATCH)
>> + break;
>> sched_mm_cid_remote_clear_old(mm, cpu);
>> + ++idx;
>> + }
>> weight = cpumask_weight(cidmask);
>> /*
>> * Clear cids that are greater or equal to the cidmask
>> weight to
>> * recompact it.
>> */
>> - for_each_possible_cpu(cpu)
>> + idx = 0;
>> + cpu = from_cpu;
>> + for_each_cpu_from(cpu, cpu_possible_mask) {
>> + if (idx == CONFIG_RSEQ_CID_SCAN_BATCH)
>> + break;
>> sched_mm_cid_remote_clear_weight(mm, cpu, weight);
>> + ++idx;
>> + }
>> }
>>
>> void init_sched_mm_cid(struct task_struct *t)
>
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
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