Re: [PATCH 1/5] page allocator: Always wake kswapd when restarting an allocation attempt after direct reclaim failed

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> On Mon, 26 Oct 2009, KOSAKI Motohiro wrote:
> 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index bf72055..5a27896 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1899,6 +1899,12 @@ rebalance:
> >  	if (should_alloc_retry(gfp_mask, order, pages_reclaimed)) {
> >  		/* Wait for some write requests to complete then retry */
> >  		congestion_wait(BLK_RW_ASYNC, HZ/50);
> > +
> > +		/*
> > +		 * While we wait congestion wait, Amount of free memory can
> > +		 * be changed dramatically. Thus, we kick kswapd again.
> > +		 */
> > +		wake_all_kswapd(order, zonelist, high_zoneidx);
> >  		goto rebalance;
> >  	}
> >  
> 
> We're blocking to finish writeback of the directly reclaimed memory, why 
> do we need to wake kswapd afterwards?

the same reason of "goto restart" case. that's my intention.
if following scenario occur, it is equivalent that we didn't call wake_all_kswapd().

  1. call congestion_wait()
  2. kswapd reclaimed lots memory and sleep
  3. another task consume lots memory
  4. wakeup from congestion_wait()

IOW, if we falled into __alloc_pages_slowpath(), we naturally expect
next page_alloc() don't fall into slowpath. however if kswapd end to
its work too early, this assumption isn't true.

Is this too pessimistic assumption?





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