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

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Date:   Thu, 7 Jan 2021 10:59:55 +0800
From:   Liang Li <liliang324@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     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
Subject: Re: [PATCH 1/6] mm: Add batch size for free page reporting

> 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.

The reason for adding a batch is mainly for page prezero, I just want to make it
configurable to control the 'cache pollution', for that case, the
reporting thread
should not be woken up too frequently.

> > ---
> >  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.

you are right, will change.

> >         /* 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.

You are right again.

Thanks
Liang