linux-kernel - Re: [PATCH 1/6] mm: Add batch size for free page reporting

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Date:   Wed, 6 Jan 2021 08:59:03 -0800
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Michal Hocko <mhocko@...e.com>,
        Liang Li <liliangleo@...iglobal.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org
Cc:     Liang Li <liliang324@...il.com>
Subject: Re: [PATCH 1/6] mm: Add batch size for free page reporting

On Tue, Jan 5, 2021 at 7:47 PM Liang Li <liliang324@...il.com> wrote:
>
> Use the page order as the only threshold for page reporting
> is not flexible and has some flaws. Because scan a long free
> list is not cheap, it's better to wake up the page reporting
> worker when there are more pages, wake it up for a sigle page
> may not worth.
> This patch add a batch size as another threshold to control the
> waking up of reporting worker.
>
> Cc: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> Cc: Mel Gorman <mgorman@...hsingularity.net>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Dave Hansen <dave.hansen@...el.com>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Alex Williamson <alex.williamson@...hat.com>
> Cc: Michael S. Tsirkin <mst@...hat.com>
> Cc: Liang Li <liliang324@...il.com>
> Signed-off-by: Liang Li <liliangleo@...iglobal.com>

So you are going to need a lot more explanation for this. Page
reporting already had the concept of batching as you could only scan
once every 2 seconds as I recall. Thus the "PAGE_REPORTING_DELAY". The
change you are making doesn't make any sense without additional
context.

> ---
>  mm/page_reporting.c |  1 +
>  mm/page_reporting.h | 12 ++++++++++--
>  2 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page_reporting.c b/mm/page_reporting.c
> index cd8e13d41df4..694df981ddd2 100644
> --- a/mm/page_reporting.c
> +++ b/mm/page_reporting.c
> @@ -12,6 +12,7 @@
>
>  #define PAGE_REPORTING_DELAY   (2 * HZ)
>  static struct page_reporting_dev_info __rcu *pr_dev_info __read_mostly;
> +unsigned long page_report_batch_size  __read_mostly = 16 * 1024 * 1024UL;
>
>  enum {
>         PAGE_REPORTING_IDLE = 0,
> diff --git a/mm/page_reporting.h b/mm/page_reporting.h
> index 2c385dd4ddbd..b8fb3bbb345f 100644
> --- a/mm/page_reporting.h
> +++ b/mm/page_reporting.h
> @@ -12,6 +12,8 @@
>
>  #define PAGE_REPORTING_MIN_ORDER       pageblock_order
>
> +extern unsigned long page_report_batch_size;
> +
>  #ifdef CONFIG_PAGE_REPORTING
>  DECLARE_STATIC_KEY_FALSE(page_reporting_enabled);
>  void __page_reporting_notify(void);
> @@ -33,6 +35,8 @@ static inline bool page_reported(struct page *page)
>   */
>  static inline void page_reporting_notify_free(unsigned int order)
>  {
> +       static long batch_size;
> +

I'm not sure this makes a tone of sense to place the value in an
inline function. It might make more sense to put this new code in
__page_reporting_notify so that all callers would be referring to the
same batch_size value and you don't have to bother with the export of
the page_report_batch_size value.

>         /* Called from hot path in __free_one_page() */
>         if (!static_branch_unlikely(&page_reporting_enabled))
>                 return;
> @@ -41,8 +45,12 @@ static inline void page_reporting_notify_free(unsigned int order)
>         if (order < PAGE_REPORTING_MIN_ORDER)
>                 return;
>
> -       /* This will add a few cycles, but should be called infrequently */
> -       __page_reporting_notify();
> +       batch_size += (1 << order) << PAGE_SHIFT;
> +       if (batch_size >= page_report_batch_size) {
> +               batch_size = 0;

I would probably run this in the opposite direction. Rather than
running batch_size to zero I would look at adding a "batch_remaining"
and then when it is < 0 you could then reset it back to
page_report_batch_size. Doing that you only have to read one variable
most of the time instead of doing a comparison against two.

> +               /* This add a few cycles, but should be called infrequently */
> +               __page_reporting_notify();
> +       }
>  }
>  #else /* CONFIG_PAGE_REPORTING */
>  #define page_reported(_page)   false
> --
> 2.18.2
>
>