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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 >> --- >>  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