linux-kernel - Re: [PATCH 3/3] mm, vmscan: Prevent kswapd sleeping prematurely due to mismatched classzone

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Date:   Thu, 16 Feb 2017 08:10:39 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Hillf Danton <hillf.zj@...baba-inc.com>
Cc:     'Andrew Morton' <akpm@...ux-foundation.org>,
        'Shantanu Goel' <sgoel01@...oo.com>,
        'Chris Mason' <clm@...com>,
        'Johannes Weiner' <hannes@...xchg.org>,
        'Vlastimil Babka' <vbabka@...e.cz>,
        'LKML' <linux-kernel@...r.kernel.org>,
        'Linux-MM' <linux-mm@...ck.org>
Subject: Re: [PATCH 3/3] mm, vmscan: Prevent kswapd sleeping prematurely due
 to mismatched classzone_idx

On Thu, Feb 16, 2017 at 02:23:08PM +0800, Hillf Danton wrote:
> On February 15, 2017 5:23 PM Mel Gorman wrote: 
> >   */
> >  static int kswapd(void *p)
> >  {
> > -	unsigned int alloc_order, reclaim_order, classzone_idx;
> > +	unsigned int alloc_order, reclaim_order;
> > +	unsigned int classzone_idx = MAX_NR_ZONES - 1;
> >  	pg_data_t *pgdat = (pg_data_t*)p;
> >  	struct task_struct *tsk = current;
> > 
> > @@ -3447,20 +3466,23 @@ static int kswapd(void *p)
> >  	tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
> >  	set_freezable();
> > 
> > -	pgdat->kswapd_order = alloc_order = reclaim_order = 0;
> > -	pgdat->kswapd_classzone_idx = classzone_idx = 0;
> > +	pgdat->kswapd_order = 0;
> > +	pgdat->kswapd_classzone_idx = MAX_NR_ZONES;
> >  	for ( ; ; ) {
> >  		bool ret;
> > 
> > +		alloc_order = reclaim_order = pgdat->kswapd_order;
> > +		classzone_idx = kswapd_classzone_idx(pgdat, classzone_idx);
> > +
> >  kswapd_try_sleep:
> >  		kswapd_try_to_sleep(pgdat, alloc_order, reclaim_order,
> >  					classzone_idx);
> > 
> >  		/* Read the new order and classzone_idx */
> >  		alloc_order = reclaim_order = pgdat->kswapd_order;
> > -		classzone_idx = pgdat->kswapd_classzone_idx;
> > +		classzone_idx = kswapd_classzone_idx(pgdat, 0);
> >  		pgdat->kswapd_order = 0;
> > -		pgdat->kswapd_classzone_idx = 0;
> > +		pgdat->kswapd_classzone_idx = MAX_NR_ZONES;
> > 
> >  		ret = try_to_freeze();
> >  		if (kthread_should_stop())
> > @@ -3486,9 +3508,6 @@ static int kswapd(void *p)
> >  		reclaim_order = balance_pgdat(pgdat, alloc_order, classzone_idx);
> >  		if (reclaim_order < alloc_order)
> >  			goto kswapd_try_sleep;
> 
> If we fail order-5 request,  can we then give up order-5, and
> try order-3 if requested, after napping?
> 

That has no bearing upon this patch. At this point, kswapd has stopped
reclaiming at the requested order and is preparing to sleep. If there is
a parallel request for order-3 while it's sleeping, it'll wake and start
reclaiming at order-3 as requested.

-- 
Mel Gorman
SUSE Labs