Re: [PATCH 3/3] mm, vmscan: Prevent kswapd sleeping prematurely due to mismatched classzone_idx

[Vlastimil Babka]

On 02/16/2017 09:21 AM, Hillf Danton wrote:
> 
> 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.

"Giving up on order-5" means it will set sc.order to 0, go to sleep (assuming
order-0 watermarks are OK) and wakeup kcompactd for order-5. There's no way how
kswapd could help an order-3 allocation at that point - it's up to kcompactd.

> thanks
> Hillf
>