[mm/contpte v3 0/1] mm/contpte: Optimize loop to reduce redundant operations

[Xavier]

Hi Andrew,


At 2025-04-16 10:10:27, "Andrew Morton" <akpm at linux-foundation.org> wrote:
>
>Please try to avoid presentation of a [0/N] cover letter when N==1!  A
>simple singleton patch is better.

Got it, I'll keep this in mind for future submissions. Thanks for the reminder!

>
>On Tue, 15 Apr 2025 16:22:04 +0800 Xavier <xavier_qy at 163.com> wrote:
>
>> Patch V3 has changed the while loop to a for loop according to the suggestions
>> of Dev. Meanwhile, to improve efficiency, the definition of local variables has
>> been removed. This macro is only used within the current function and there
>
>which function?

It's contpte_ptep_get().

>
>> will be no additional risks. In order to verify the optimization performance of
>> Patch V3, a test function has been designed. By repeatedly calling mlock in a
>> loop, the kernel is made to call contpte_ptep_get extensively to test the
>> optimization effect of this function.
>> The function's execution time and instruction statistics have been traced using
>> perf, and the following are the operation results on a certain Qualcomm mobile
>> phone chip:
>
>All the words thus far appear to be discussing changes since v2.  For
>the permanent kernel record, this isn't interesting or useful material.
>So please present a standalone description which doesn't refer to
>previous iterations.
>
>It's great to present this what-i-changed-since-last-time material, but
>that is better placed after the "^---$" separator, after the
>Signed-off-by:, Reviewed-by: etc tags.
>

OK, I will follow this requirement for future submissions.

>>
>> ...
>>
>
>
>Below is what I came up with for a changelog.  Please check?

I've reviewed it, and it looks good. Thank you for your revisions!

>
>Optimize contpte_ptep_get() by adding early termination logic.  Check if
>the dirty and young bits of orig_pte are already set and skip redundant
>bit-setting operations during the loop.  This reduces unnecessary
>iterations and improves performance.
>
>The function's execution time and instruction statistics have been traced
>using perf, and the following are the operation results on a certain
>Qualcomm mobile phone chip:
>
>Instruction Statistics - Before Optimization
>#          count  event_name              # count / runtime
>      20,814,352  branch-load-misses      # 662.244 K/sec
>  41,894,986,323  branch-loads            # 1.333 G/sec
>       1,957,415  iTLB-load-misses        # 62.278 K/sec
>  49,872,282,100  iTLB-loads              # 1.587 G/sec
>     302,808,096  L1-icache-load-misses   # 9.634 M/sec
>  49,872,282,100  L1-icache-loads         # 1.587 G/sec
>
>Total test time: 31.485237 seconds.
>
>Instruction Statistics - After Optimization
>#          count  event_name              # count / runtime
>      19,340,524  branch-load-misses      # 688.753 K/sec
>  38,510,185,183  branch-loads            # 1.371 G/sec
>       1,812,716  iTLB-load-misses        # 64.554 K/sec
>  47,673,923,151  iTLB-loads              # 1.698 G/sec
>     675,853,661  L1-icache-load-misses   # 24.068 M/sec
>  47,673,923,151  L1-icache-loads         # 1.698 G/sec
>
>Total test time: 28.108048 seconds.
>
>Function Statistics - Before Optimization
>Arch: arm64
>Event: cpu-cycles (type 0, config 0)
>Samples: 1419716
>Event count: 99618088900
>
>Overhead   Symbol
>21.42%     lock_release
>21.26%     lock_acquire
>20.88%     arch_counter_get_cntvct
>14.32%     _raw_spin_unlock_irq
>6.79%      contpte_ptep_get
>2.20%      test_contpte_perf
>1.82%      follow_page_pte
>0.97%      lock_acquired
>0.97%      rcu_is_watching
>0.89%      mlock_pte_range
>0.84%      sched_clock_noinstr
>0.70%      handle_softirqs.llvm.8218488130471452153
>0.58%      test_preempt_disable_long
>0.57%      _raw_spin_unlock_irqrestore
>0.54%      arch_stack_walk
>0.51%      vm_normal_folio
>0.48%      check_preemption_disabled
>0.47%      stackinfo_get_task
>0.36%      try_grab_folio
>0.34%      preempt_count
>0.32%      trace_preempt_on
>0.29%      trace_preempt_off
>0.24%      debug_smp_processor_id
>
>Function Statistics - After Optimization
>Arch: arm64
>Event: cpu-cycles (type 0, config 0)
>Samples: 1431006
>Event count: 118856425042
>
>Overhead   Symbol
>22.59%     lock_release
>22.13%     arch_counter_get_cntvct
>22.08%     lock_acquire
>15.32%     _raw_spin_unlock_irq
>2.26%      test_contpte_perf
>1.50%      follow_page_pte
>1.49%      arch_stack_walk
>1.30%      rcu_is_watching
>1.09%      lock_acquired
>1.07%      sched_clock_noinstr
>0.88%      handle_softirqs.llvm.12507768597002095717
>0.88%      trace_preempt_off
>0.76%      _raw_spin_unlock_irqrestore
>0.61%      check_preemption_disabled
>0.52%      trace_preempt_on
>0.50%      mlock_pte_range
>0.43%      try_grab_folio
>0.41%      folio_mark_accessed
>0.40%      vm_normal_folio
>0.38%      test_preempt_disable_long
>0.28%      contpte_ptep_get
>0.27%      __traceiter_android_rvh_preempt_disable
>0.26%      debug_smp_processor_id
>0.24%      return_address
>0.20%      __pte_offset_map_lock
>0.19%      unwind_next_frame_record
>
>If there is no problem with my test program, it can be seen that there is a
>significant performance improvement both in the overall number of instructions
>and the execution time of contpte_ptep_get.
>
>If any reviewers have time, you can also test it on your machines for comparison.
>I have enabled THP and hugepages-64kB.
>
>Test function:
>
>#define PAGE_SIZE 4096
>#define CONT_PTES 16
>#define TEST_SIZE (4096* CONT_PTES * PAGE_SIZE)
>
>void rwdata(char *buf)
>{
>	for (size_t i = 0; i < TEST_SIZE; i += PAGE_SIZE) {
>		buf[i] = 'a';
>		volatile char c = buf[i];
>	}
>}
>void test_contpte_perf()
>{
>	char *buf;
>	int ret = posix_memalign((void **)&buf, PAGE_SIZE, TEST_SIZE);
>	if (ret != 0) {
>		perror("posix_memalign failed");
>		exit(EXIT_FAILURE);
>	}
>
>	rwdata(buf);
>
>	for (int j = 0; j < 500; j++) {
>		mlock(buf, TEST_SIZE);
>
>		rwdata(buf);
>
>		munlock(buf, TEST_SIZE);
>	}
>	
>	free(buf);
>}


--

Thanks,
Xavier