linux-kernel - Re: [PATCH v2] mm: swap: use fixed-size readahead during swapoff

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Date:   Tue, 14 Apr 2020 09:31:24 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Andrea Righi <andrea.righi@...onical.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>,
        Anchal Agarwal <anchalag@...zon.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mm: swap: use fixed-size readahead during swapoff

Andrea Righi <andrea.righi@...onical.com> writes:

> On Mon, Apr 13, 2020 at 09:00:34PM +0800, Huang, Ying wrote:
>> Andrea Righi <andrea.righi@...onical.com> writes:
>> 
>> [snip]
>> 
>> > diff --git a/mm/swap_state.c b/mm/swap_state.c
>> > index ebed37bbf7a3..c71abc8df304 100644
>> > --- a/mm/swap_state.c
>> > +++ b/mm/swap_state.c
>> > @@ -20,6 +20,7 @@
>> >  #include <linux/migrate.h>
>> >  #include <linux/vmalloc.h>
>> >  #include <linux/swap_slots.h>
>> > +#include <linux/oom.h>
>> >  #include <linux/huge_mm.h>
>> >  
>> >  #include <asm/pgtable.h>
>> > @@ -507,6 +508,14 @@ static unsigned long swapin_nr_pages(unsigned long offset)
>> >  	max_pages = 1 << READ_ONCE(page_cluster);
>> >  	if (max_pages <= 1)
>> >  		return 1;
>> > +	/*
>> > +	 * If current task is using too much memory or swapoff is running
>> > +	 * simply use the max readahead size. Since we likely want to load a
>> > +	 * lot of pages back into memory, using a fixed-size max readhaead can
>> > +	 * give better performance in this case.
>> > +	 */
>> > +	if (oom_task_origin(current))
>> > +		return max_pages;
>> >  
>> >  	hits = atomic_xchg(&swapin_readahead_hits, 0);
>> >  	pages = __swapin_nr_pages(prev_offset, offset, hits, max_pages,
>> 
>> Thinks this again.  If my understanding were correct, the accessing
>> pattern during swapoff is sequential, why swap readahead doesn't work?
>> If so, can you root cause that firstly?
>
> Theoretically if the pattern is sequential the current heuristic should
> already select a big readahead size, but apparently it's not doing that.
>
> I'll repeat my tests tracing the readahead size during swapoff to see
> exactly what's going on here.

I haven't verify it.  It may be helpful to call lookup_swap_cache()
before swapin_readahead() in unuse_pte_range().  The theory behind it is
to update the swap readahead statistics via lookup_swap_cache().

Best Regards,
Huang, Ying