Re: [PATCH] mm/khugepaged: make reserved memory adaptively

[Yang Shi <shy828301@gmail.com>]

On Fri, Sep 1, 2023 at 10:26 PM wolfgang huang <wolfgang9277@126.com> wrote:
>
> From: wolfgang huang <huangjinhui@kylinos.cn>
>
> In the 64k page configuration of ARM64, the size of THP is 512MB,
> which usually reserves almost 5% of memory. However, the probability
> of THP usage is not high, especially in the madvise configure. and
> THP is not usually used, but a large amount of memory is reserved
> for THP use, resulting in a lot of memory waste.
>
> So a dynamic method of adjusting reserved memory is proposed. when
> scan and trying to merge into THP, the reserved memory is pulled up,
> and when the system does not need to synthesize THP, the reserved
> memory is kept low. this method can save a lot of memory in scenarios
> without synthesizing THP, and can effectively pull up the reserved
> memory to support when THP synthesis is needed. especially in the
> case of madvise configure, the running effect will be better.
>
> Signed-off-by: wolfgang huang <huangjinhui@kylinos.cn>

Thanks for the patch. There was already a thread about this problem,
please refer to:
https://lore.kernel.org/linux-mm/20230817035155.84230-1-liusong@linux.alibaba.com/

> ---
>  mm/khugepaged.c | 36 +++++++++++++++++++++++++++++++++++-
>  1 file changed, 35 insertions(+), 1 deletion(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 88433cc25d8a..41a837e618a5 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -129,6 +129,10 @@ static struct khugepaged_scan khugepaged_scan = {
>         .mm_head = LIST_HEAD_INIT(khugepaged_scan.mm_head),
>  };
>
> +/* khugepaged should scan or not, trying to be thp, default as false */
> +static unsigned int khugepaged_thp_scan_state;
> +static void set_recommended_min_free_kbytes(void);
> +
>  #ifdef CONFIG_SYSFS
>  static ssize_t scan_sleep_millisecs_show(struct kobject *kobj,
>                                          struct kobj_attribute *attr,
> @@ -2549,6 +2553,33 @@ static void khugepaged_wait_work(void)
>                 wait_event_freezable(khugepaged_wait, khugepaged_wait_event());
>  }
>
> +static int khugepaged_threshold(void)
> +{
> +       /* thp size threshold check */
> +       if ((PAGE_SIZE << HPAGE_PMD_ORDER) >= SZ_512M)
> +               return true;
> +       return false;
> +}
> +
> +static void khugepaged_update_wmarks(void)
> +{
> +       if (!khugepaged_threshold())
> +               return;
> +
> +       /* __khugepaged_enter push khugepaged to work, raise watermark */
> +       if (khugepaged_has_work()) {
> +               /* Once set, do not repeat distrub watermark */
> +               if (!khugepaged_thp_scan_state) {
> +                       khugepaged_thp_scan_state = true;
> +                       mutex_lock(&khugepaged_mutex);
> +                       set_recommended_min_free_kbytes();
> +                       mutex_unlock(&khugepaged_mutex);
> +               }
> +       } else {
> +               khugepaged_thp_scan_state = false;
> +       }
> +}
> +
>  static int khugepaged(void *none)
>  {
>         struct khugepaged_mm_slot *mm_slot;
> @@ -2559,6 +2590,7 @@ static int khugepaged(void *none)
>         while (!kthread_should_stop()) {
>                 khugepaged_do_scan(&khugepaged_collapse_control);
>                 khugepaged_wait_work();
> +               khugepaged_update_wmarks();
>         }
>
>         spin_lock(&khugepaged_mm_lock);
> @@ -2576,7 +2608,9 @@ static void set_recommended_min_free_kbytes(void)
>         int nr_zones = 0;
>         unsigned long recommended_min;
>
> -       if (!hugepage_flags_enabled()) {
> +       if (!hugepage_flags_enabled() ||
> +               (khugepaged_threshold() &&
> +                !khugepaged_thp_scan_state)) {
>                 calculate_min_free_kbytes();
>                 goto update_wmarks;
>         }
> --
> 2.34.1
>
>