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

Huang, Ying
2020-04-14T01:31:34Z

> On Mon, Apr 13, 2020 at 09:00:34PM +0800, Huang, Ying wrote:
>> Andrea Righi <andrea.righi@canonical.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