linux-kernel - Re: [PATCH 1/2] mm: swap: make page

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Date:   Fri, 13 Mar 2020 12:33:50 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm: swap: make page_evictable() inline

On Fri, Mar 13, 2020 at 11:34 AM Yang Shi <yang.shi@...ux.alibaba.com> wrote:
>
> When backporting commit 9c4e6b1a7027 ("mm, mlock, vmscan: no more
> skipping pagevecs") to our 4.9 kernel, our test bench noticed around 10%
> down with a couple of vm-scalability's test cases (lru-file-readonce,
> lru-file-readtwice and lru-file-mmap-read).  I didn't see that much down
> on my VM (32c-64g-2nodes).  It might be caused by the test configuration,
> which is 32c-256g with NUMA disabled and the tests were run in root memcg,
> so the tests actually stress only one inactive and active lru.  It
> sounds not very usual in mordern production environment.
>
> That commit did two major changes:
> 1. Call page_evictable()
> 2. Use smp_mb to force the PG_lru set visible
>
> It looks they contribute the most overhead.  The page_evictable() is a
> function which does function prologue and epilogue, and that was used by
> page reclaim path only.  However, lru add is a very hot path, so it
> sounds better to make it inline.  However, it calls page_mapping() which
> is not inlined either, but the disassemble shows it doesn't do push and
> pop operations and it sounds not very straightforward to inline it.
>
> Other than this, it sounds smp_mb() is not necessary for x86 since
> SetPageLRU is atomic which enforces memory barrier already, replace it
> with smp_mb__after_atomic() in the following patch.
>
> With the two fixes applied, the tests can get back around 5% on that
> test bench and get back normal on my VM.  Since the test bench
> configuration is not that usual and I also saw around 6% up on the
> latest upstream, so it sounds good enough IMHO.
>
> The below is test data (lru-file-readtwice throughput) against the v5.6-rc4:
>         mainline        w/ inline fix
>           150MB            154MB
>

What is the test setup for the above experiment? I would like to get a repro.

> With this patch the throughput gets 2.67% up.  The data with using
> smp_mb__after_atomic() is showed in the following patch.
>
> Fixes: 9c4e6b1a7027 ("mm, mlock, vmscan: no more skipping pagevecs")
> Cc: Shakeel Butt <shakeelb@...gle.com>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
> ---
>  include/linux/swap.h | 24 +++++++++++++++++++++++-
>  mm/vmscan.c          | 23 -----------------------
>  2 files changed, 23 insertions(+), 24 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 1e99f7a..297eb66 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -374,7 +374,29 @@ extern unsigned long mem_cgroup_shrink_node(struct mem_cgroup *mem,
>  #define node_reclaim_mode 0
>  #endif
>
> -extern int page_evictable(struct page *page);
> +/*
> + * page_evictable - test whether a page is evictable
> + * @page: the page to test
> + *
> + * Test whether page is evictable--i.e., should be placed on active/inactive
> + * lists vs unevictable list.
> + *
> + * Reasons page might not be evictable:
> + * (1) page's mapping marked unevictable
> + * (2) page is part of an mlocked VMA
> + *
> + */
> +static inline int page_evictable(struct page *page)
> +{
> +       int ret;
> +
> +       /* Prevent address_space of inode and swap cache from being freed */
> +       rcu_read_lock();
> +       ret = !mapping_unevictable(page_mapping(page)) && !PageMlocked(page);
> +       rcu_read_unlock();
> +       return ret;
> +}
> +
>  extern void check_move_unevictable_pages(struct pagevec *pvec);
>
>  extern int kswapd_run(int nid);
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 8763705..855c395 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4277,29 +4277,6 @@ int node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned int order)
>  }
>  #endif
>
> -/*
> - * page_evictable - test whether a page is evictable
> - * @page: the page to test
> - *
> - * Test whether page is evictable--i.e., should be placed on active/inactive
> - * lists vs unevictable list.
> - *
> - * Reasons page might not be evictable:
> - * (1) page's mapping marked unevictable
> - * (2) page is part of an mlocked VMA
> - *
> - */
> -int page_evictable(struct page *page)
> -{
> -       int ret;
> -
> -       /* Prevent address_space of inode and swap cache from being freed */
> -       rcu_read_lock();
> -       ret = !mapping_unevictable(page_mapping(page)) && !PageMlocked(page);
> -       rcu_read_unlock();
> -       return ret;
> -}
> -
>  /**
>   * check_move_unevictable_pages - check pages for evictability and move to
>   * appropriate zone lru list
> --
> 1.8.3.1
>