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

[Andrea Righi <andrea.righi@canonical.com>]

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.

Thanks,
-Andrea