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 16:21:04 +0800
From:   "Hillf Danton" <hillf.zj@...baba-inc.com>
To:     "'Mel Gorman'" <mgorman@...hsingularity.net>
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 February 16, 2017 4:11 PM Mel Gorman wrote:
> 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.
> 
Right, but the order-3 request can also come up while kswapd is active and
gives up order-5.

thanks
Hillf