[PATCH]vmscan: fix a livelock in kswapd

[PATCH]vmscan: fix a livelock in kswapd

[Shaohua Li]

I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
kswapd2 are keeping running and I can't access filesystem, but most memory is
free. This looks like a regression since commit 08951e545918c159.
Node 2 and 3 have only ZONE_NORMAL, but balance_pgdat() will return 0 for
classzone_idx. The reason is end_zone in balance_pgdat() is 0 by default, if
all zones have watermark ok, end_zone will keep 0.
Later sleeping_prematurely() always returns true. Because this is an order 3
wakeup, and if classzone_idx is 0, both balanced_pages and present_pages
in pgdat_balanced() are 0.
We add a special case here. If a zone has no page, we think it's balanced. This
fixes the livelock.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5ed24b9..ad4056f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2310,7 +2310,8 @@ static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced_pages,
 	for (i = 0; i <= classzone_idx; i++)
 		present_pages += pgdat->node_zones[i].present_pages;
 
-	return balanced_pages > (present_pages >> 2);
+	/* A special case here: if zone has no page, we think it's balanced */
+	return balanced_pages >= (present_pages >> 2);
 }
 
 /* is kswapd sleeping prematurely? */


--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH]vmscan: fix a livelock in kswapd

[Mel Gorman]

On Tue, Jul 19, 2011 at 03:09:27PM +0800, Shaohua Li wrote:
> I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
> After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
> kswapd2 are keeping running and I can't access filesystem, but most memory is
> free. This looks like a regression since commit 08951e545918c159.
> Node 2 and 3 have only ZONE_NORMAL, but balance_pgdat() will return 0 for
> classzone_idx. The reason is end_zone in balance_pgdat() is 0 by default, if
> all zones have watermark ok, end_zone will keep 0.
> Later sleeping_prematurely() always returns true. Because this is an order 3
> wakeup, and if classzone_idx is 0, both balanced_pages and present_pages
> in pgdat_balanced() are 0.
> We add a special case here. If a zone has no page, we think it's balanced. This
> fixes the livelock.
> 
> Signed-off-by: Shaohua Li <shaohua.li@intel.com>
> 

Acked-by: Mel Gorman <mgorman@suse.de>

It's also needed for 3.0 and 2.6.39-stable I believe.

-- 
Mel Gorman
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH]vmscan: fix a livelock in kswapd

[Minchan Kim]

On Tue, Jul 19, 2011 at 4:09 PM, Shaohua Li <shaohua.li@intel.com> wrote:
> I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
> After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
> kswapd2 are keeping running and I can't access filesystem, but most memory is
> free. This looks like a regression since commit 08951e545918c159.

Could you tell me what is 08951e545918c159?
You mean [ebd64e21ec5a,
mm-vmscan-only-read-new_classzone_idx-from-pgdat-when-reclaiming-successfully]
?


-- 
Kind regards,
Minchan Kim

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH]vmscan: fix a livelock in kswapd

[Shaohua Li]

On Tue, 2011-07-19 at 16:45 +0800, Minchan Kim wrote:
> On Tue, Jul 19, 2011 at 4:09 PM, Shaohua Li <shaohua.li@intel.com> wrote:
> > I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
> > After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
> > kswapd2 are keeping running and I can't access filesystem, but most memory is
> > free. This looks like a regression since commit 08951e545918c159.
> 
> Could you tell me what is 08951e545918c159?
> You mean [ebd64e21ec5a,
> mm-vmscan-only-read-new_classzone_idx-from-pgdat-when-reclaiming-successfully]
> ?
ha, sorry, I should copy the commit title.
08951e545918c159(mm: vmscan: correct check for kswapd sleeping in
sleeping_prematurely)

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH]vmscan: fix a livelock in kswapd

[Minchan Kim]

On Tue, Jul 19, 2011 at 04:53:04PM +0800, Shaohua Li wrote:
> On Tue, 2011-07-19 at 16:45 +0800, Minchan Kim wrote:
> > On Tue, Jul 19, 2011 at 4:09 PM, Shaohua Li <shaohua.li@intel.com> wrote:
> > > I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
> > > After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
> > > kswapd2 are keeping running and I can't access filesystem, but most memory is
> > > free. This looks like a regression since commit 08951e545918c159.
> > 
> > Could you tell me what is 08951e545918c159?
> > You mean [ebd64e21ec5a,
> > mm-vmscan-only-read-new_classzone_idx-from-pgdat-when-reclaiming-successfully]
> > ?
> ha, sorry, I should copy the commit title.
> 08951e545918c159(mm: vmscan: correct check for kswapd sleeping in
> sleeping_prematurely)
> 

I don't mean it. In my bogus git tree, I can't find it but I can look at it in repaired git tree. :)
Anyway, I have a comment. Please look at below.

On Tue, Jul 19, 2011 at 03:09:27PM +0800, Shaohua Li wrote:
> I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
> After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
> kswapd2 are keeping running and I can't access filesystem, but most memory is
> free. This looks like a regression since commit 08951e545918c159.
> Node 2 and 3 have only ZONE_NORMAL, but balance_pgdat() will return 0 for
> classzone_idx. The reason is end_zone in balance_pgdat() is 0 by default, if
> all zones have watermark ok, end_zone will keep 0.
> Later sleeping_prematurely() always returns true. Because this is an order 3
> wakeup, and if classzone_idx is 0, both balanced_pages and present_pages
> in pgdat_balanced() are 0.

Sigh. Yes.

> We add a special case here. If a zone has no page, we think it's balanced. This
> fixes the livelock.

Yes. Your patch can fix it but I don't like that it adds handling special case.
(Although Andrew merged quickly).

The problem is to return 0-classzone_idx if all zones was okay.
So how about this?

This can change old behavior slightly.
For example, if balance_pgdat calls with order-3 and all zones are okay about order-3,
it will recheck order-0 as end_zone isn't 0 any more.
But I think it's desriable side effect we have missed.

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5ed24b9..cfef52b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2389,7 +2389,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
        unsigned long balanced;
        int priority;
        int i;
-       int end_zone = 0;       /* Inclusive.  0 = ZONE_DMA */
+       int end_zone = *classzone_idx;
        unsigned long total_scanned;
        struct reclaim_state *reclaim_state = current->reclaim_state;
        unsigned long nr_soft_reclaimed;

-- 
Kinds regards,
Minchan Kim

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH]vmscan: fix a livelock in kswapd

[Shaohua Li]

On Wed, 2011-07-20 at 00:51 +0800, Minchan Kim wrote:
> On Tue, Jul 19, 2011 at 04:53:04PM +0800, Shaohua Li wrote:
> > On Tue, 2011-07-19 at 16:45 +0800, Minchan Kim wrote:
> > > On Tue, Jul 19, 2011 at 4:09 PM, Shaohua Li <shaohua.li@intel.com> wrote:
> > > > I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
> > > > After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
> > > > kswapd2 are keeping running and I can't access filesystem, but most memory is
> > > > free. This looks like a regression since commit 08951e545918c159.
> > > 
> > > Could you tell me what is 08951e545918c159?
> > > You mean [ebd64e21ec5a,
> > > mm-vmscan-only-read-new_classzone_idx-from-pgdat-when-reclaiming-successfully]
> > > ?
> > ha, sorry, I should copy the commit title.
> > 08951e545918c159(mm: vmscan: correct check for kswapd sleeping in
> > sleeping_prematurely)
> > 
> 
> I don't mean it. In my bogus git tree, I can't find it but I can look at it in repaired git tree. :)
> Anyway, I have a comment. Please look at below.
> 
> On Tue, Jul 19, 2011 at 03:09:27PM +0800, Shaohua Li wrote:
> > I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
> > After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
> > kswapd2 are keeping running and I can't access filesystem, but most memory is
> > free. This looks like a regression since commit 08951e545918c159.
> > Node 2 and 3 have only ZONE_NORMAL, but balance_pgdat() will return 0 for
> > classzone_idx. The reason is end_zone in balance_pgdat() is 0 by default, if
> > all zones have watermark ok, end_zone will keep 0.
> > Later sleeping_prematurely() always returns true. Because this is an order 3
> > wakeup, and if classzone_idx is 0, both balanced_pages and present_pages
> > in pgdat_balanced() are 0.
> 
> Sigh. Yes.
> 
> > We add a special case here. If a zone has no page, we think it's balanced. This
> > fixes the livelock.
> 
> Yes. Your patch can fix it but I don't like that it adds handling special case.
> (Although Andrew merged quickly).
The special case is reasonable, because if a zone has no page, it should
be considered balanced.

> The problem is to return 0-classzone_idx if all zones was okay.
> So how about this?
My original implementation is like this (I return a populated zone with
minimum zone index). But I changed my mind later. the end_zone is zone
we work, so return 0 is reasonable, because all zones are ok. Maybe we
should return -1 if all zones are ok, but this is another story.

> This can change old behavior slightly.
> For example, if balance_pgdat calls with order-3 and all zones are okay about order-3,
> it will recheck order-0 as end_zone isn't 0 any more.
> But I think it's desriable side effect we have missed.
if order-3 is ok, order-0 is ok too I think, so the check is
unnecessary.

Thanks,
Shaohua

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH]vmscan: fix a livelock in kswapd

[Minchan Kim]

On Wed, Jul 20, 2011 at 9:43 AM, Shaohua Li <shaohua.li@intel.com> wrote:
> On Wed, 2011-07-20 at 00:51 +0800, Minchan Kim wrote:
>> On Tue, Jul 19, 2011 at 04:53:04PM +0800, Shaohua Li wrote:
>> > On Tue, 2011-07-19 at 16:45 +0800, Minchan Kim wrote:
>> > > On Tue, Jul 19, 2011 at 4:09 PM, Shaohua Li <shaohua.li@intel.com> wrote:
>> > > > I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
>> > > > After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
>> > > > kswapd2 are keeping running and I can't access filesystem, but most memory is
>> > > > free. This looks like a regression since commit 08951e545918c159.
>> > >
>> > > Could you tell me what is 08951e545918c159?
>> > > You mean [ebd64e21ec5a,
>> > > mm-vmscan-only-read-new_classzone_idx-from-pgdat-when-reclaiming-successfully]
>> > > ?
>> > ha, sorry, I should copy the commit title.
>> > 08951e545918c159(mm: vmscan: correct check for kswapd sleeping in
>> > sleeping_prematurely)
>> >
>>
>> I don't mean it. In my bogus git tree, I can't find it but I can look at it in repaired git tree. :)
>> Anyway, I have a comment. Please look at below.
>>
>> On Tue, Jul 19, 2011 at 03:09:27PM +0800, Shaohua Li wrote:
>> > I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
>> > After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
>> > kswapd2 are keeping running and I can't access filesystem, but most memory is
>> > free. This looks like a regression since commit 08951e545918c159.
>> > Node 2 and 3 have only ZONE_NORMAL, but balance_pgdat() will return 0 for
>> > classzone_idx. The reason is end_zone in balance_pgdat() is 0 by default, if
>> > all zones have watermark ok, end_zone will keep 0.
>> > Later sleeping_prematurely() always returns true. Because this is an order 3
>> > wakeup, and if classzone_idx is 0, both balanced_pages and present_pages
>> > in pgdat_balanced() are 0.
>>
>> Sigh. Yes.
>>
>> > We add a special case here. If a zone has no page, we think it's balanced. This
>> > fixes the livelock.
>>
>> Yes. Your patch can fix it but I don't like that it adds handling special case.
>> (Although Andrew merged quickly).
> The special case is reasonable, because if a zone has no page, it should
> be considered balanced.

Yes. It's not bad and even simple but my concern is that at the moment
kswapd code is very complicated and it's not hot path so I would like
to add more readable code.

>
>> The problem is to return 0-classzone_idx if all zones was okay.
>> So how about this?
> My original implementation is like this (I return a populated zone with
> minimum zone index). But I changed my mind later. the end_zone is zone
> we work, so return 0 is reasonable, because all zones are ok. Maybe we

If it is reasonable, did you work on ZONE_DMA(zone index: 0)?

> should return -1 if all zones are ok, but this is another story.

I think that return classzone_id(-1) and handle such case is more readable.

>
>> This can change old behavior slightly.
>> For example, if balance_pgdat calls with order-3 and all zones are okay about order-3,
>> it will recheck order-0 as end_zone isn't 0 any more.
>> But I think it's desriable side effect we have missed.
> if order-3 is ok, order-0 is ok too I think, so the check is
> unnecessary.

No. It's not for the zone but *zones.
In case of reclaiming higher order zone, it can sleep without all
zones being balanced so that precious order-0 of some zone would be
not balanced.
Even we can lost chance of clearing congestion flag of the zone.
It would be a another patch.

In conclusion, I would like to avoid complicated thing but I am going
to be not against you strongly if other doesn't agree on me.
I might need a time to clean kswapd's spagetti up.

-- 
Kind regards,
Minchan Kim

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH]vmscan: fix a livelock in kswapd

[Shaohua Li]

On Wed, 2011-07-20 at 12:09 +0800, Minchan Kim wrote:
> On Wed, Jul 20, 2011 at 9:43 AM, Shaohua Li <shaohua.li@intel.com> wrote:
> > On Wed, 2011-07-20 at 00:51 +0800, Minchan Kim wrote:
> >> On Tue, Jul 19, 2011 at 04:53:04PM +0800, Shaohua Li wrote:
> >> > On Tue, 2011-07-19 at 16:45 +0800, Minchan Kim wrote:
> >> > > On Tue, Jul 19, 2011 at 4:09 PM, Shaohua Li <shaohua.li@intel.com> wrote:
> >> > > > I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
> >> > > > After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
> >> > > > kswapd2 are keeping running and I can't access filesystem, but most memory is
> >> > > > free. This looks like a regression since commit 08951e545918c159.
> >> > >
> >> > > Could you tell me what is 08951e545918c159?
> >> > > You mean [ebd64e21ec5a,
> >> > > mm-vmscan-only-read-new_classzone_idx-from-pgdat-when-reclaiming-successfully]
> >> > > ?
> >> > ha, sorry, I should copy the commit title.
> >> > 08951e545918c159(mm: vmscan: correct check for kswapd sleeping in
> >> > sleeping_prematurely)
> >> >
> >>
> >> I don't mean it. In my bogus git tree, I can't find it but I can look at it in repaired git tree. :)
> >> Anyway, I have a comment. Please look at below.
> >>
> >> On Tue, Jul 19, 2011 at 03:09:27PM +0800, Shaohua Li wrote:
> >> > I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
> >> > After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
> >> > kswapd2 are keeping running and I can't access filesystem, but most memory is
> >> > free. This looks like a regression since commit 08951e545918c159.
> >> > Node 2 and 3 have only ZONE_NORMAL, but balance_pgdat() will return 0 for
> >> > classzone_idx. The reason is end_zone in balance_pgdat() is 0 by default, if
> >> > all zones have watermark ok, end_zone will keep 0.
> >> > Later sleeping_prematurely() always returns true. Because this is an order 3
> >> > wakeup, and if classzone_idx is 0, both balanced_pages and present_pages
> >> > in pgdat_balanced() are 0.
> >>
> >> Sigh. Yes.
> >>
> >> > We add a special case here. If a zone has no page, we think it's balanced. This
> >> > fixes the livelock.
> >>
> >> Yes. Your patch can fix it but I don't like that it adds handling special case.
> >> (Although Andrew merged quickly).
> > The special case is reasonable, because if a zone has no page, it should
> > be considered balanced.
> 
> Yes. It's not bad and even simple but my concern is that at the moment
> kswapd code is very complicated and it's not hot path so I would like
> to add more readable code.
> 
> >
> >> The problem is to return 0-classzone_idx if all zones was okay.
> >> So how about this?
> > My original implementation is like this (I return a populated zone with
> > minimum zone index). But I changed my mind later. the end_zone is zone
> > we work, so return 0 is reasonable, because all zones are ok. Maybe we
> 
> If it is reasonable, did you work on ZONE_DMA(zone index: 0)?
return -1 can help.

> > should return -1 if all zones are ok, but this is another story.
> 
> I think that return classzone_id(-1) and handle such case is more readable.
sure, we need another patch to clean up it.

> >
> >> This can change old behavior slightly.
> >> For example, if balance_pgdat calls with order-3 and all zones are okay about order-3,
> >> it will recheck order-0 as end_zone isn't 0 any more.
> >> But I think it's desriable side effect we have missed.
> > if order-3 is ok, order-0 is ok too I think, so the check is
> > unnecessary.
> 
> No. It's not for the zone but *zones.
> In case of reclaiming higher order zone, it can sleep without all
> zones being balanced so that precious order-0 of some zone would be
> not balanced.
when balance_pgdat() skips the loop for higher order zone, it already
sets end_zone, so I thought this isn't a problem.

> Even we can lost chance of clearing congestion flag of the zone.
> It would be a another patch.
yep, the congestion flag clearing is a bit confusing. I don't even know
why just do it in high order allocation. If all zones are ok, we should
clear the flag regardless the order. 

> In conclusion, I would like to avoid complicated thing but I am going
> to be not against you strongly if other doesn't agree on me.
> I might need a time to clean kswapd's spagetti up.
ok, understand it. I have similar concerns actually. I thought my patch
is simple enough to solve the livelock. But we do have space to cleanup
balance_pgdat().

Thanks,
Shaohua

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH]vmscan: fix a livelock in kswapd

[Minchan Kim]

On Wed, Jul 20, 2011 at 2:18 PM, Shaohua Li <shaohua.li@intel.com> wrote:
> On Wed, 2011-07-20 at 12:09 +0800, Minchan Kim wrote:
>> On Wed, Jul 20, 2011 at 9:43 AM, Shaohua Li <shaohua.li@intel.com> wrote:
>> > On Wed, 2011-07-20 at 00:51 +0800, Minchan Kim wrote:
>> >> On Tue, Jul 19, 2011 at 04:53:04PM +0800, Shaohua Li wrote:
>> >> > On Tue, 2011-07-19 at 16:45 +0800, Minchan Kim wrote:
>> >> > > On Tue, Jul 19, 2011 at 4:09 PM, Shaohua Li <shaohua.li@intel.com> wrote:
>> >> > > > I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
>> >> > > > After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
>> >> > > > kswapd2 are keeping running and I can't access filesystem, but most memory is
>> >> > > > free. This looks like a regression since commit 08951e545918c159.
>> >> > >
>> >> > > Could you tell me what is 08951e545918c159?
>> >> > > You mean [ebd64e21ec5a,
>> >> > > mm-vmscan-only-read-new_classzone_idx-from-pgdat-when-reclaiming-successfully]
>> >> > > ?
>> >> > ha, sorry, I should copy the commit title.
>> >> > 08951e545918c159(mm: vmscan: correct check for kswapd sleeping in
>> >> > sleeping_prematurely)
>> >> >
>> >>
>> >> I don't mean it. In my bogus git tree, I can't find it but I can look at it in repaired git tree. :)
>> >> Anyway, I have a comment. Please look at below.
>> >>
>> >> On Tue, Jul 19, 2011 at 03:09:27PM +0800, Shaohua Li wrote:
>> >> > I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
>> >> > After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
>> >> > kswapd2 are keeping running and I can't access filesystem, but most memory is
>> >> > free. This looks like a regression since commit 08951e545918c159.
>> >> > Node 2 and 3 have only ZONE_NORMAL, but balance_pgdat() will return 0 for
>> >> > classzone_idx. The reason is end_zone in balance_pgdat() is 0 by default, if
>> >> > all zones have watermark ok, end_zone will keep 0.
>> >> > Later sleeping_prematurely() always returns true. Because this is an order 3
>> >> > wakeup, and if classzone_idx is 0, both balanced_pages and present_pages
>> >> > in pgdat_balanced() are 0.
>> >>
>> >> Sigh. Yes.
>> >>
>> >> > We add a special case here. If a zone has no page, we think it's balanced. This
>> >> > fixes the livelock.
>> >>
>> >> Yes. Your patch can fix it but I don't like that it adds handling special case.
>> >> (Although Andrew merged quickly).
>> > The special case is reasonable, because if a zone has no page, it should
>> > be considered balanced.
>>
>> Yes. It's not bad and even simple but my concern is that at the moment
>> kswapd code is very complicated and it's not hot path so I would like
>> to add more readable code.
>>
>> >
>> >> The problem is to return 0-classzone_idx if all zones was okay.
>> >> So how about this?
>> > My original implementation is like this (I return a populated zone with
>> > minimum zone index). But I changed my mind later. the end_zone is zone
>> > we work, so return 0 is reasonable, because all zones are ok. Maybe we
>>
>> If it is reasonable, did you work on ZONE_DMA(zone index: 0)?
> return -1 can help.
>
>> > should return -1 if all zones are ok, but this is another story.
>>
>> I think that return classzone_id(-1) and handle such case is more readable.
> sure, we need another patch to clean up it.
>
>> >
>> >> This can change old behavior slightly.
>> >> For example, if balance_pgdat calls with order-3 and all zones are okay about order-3,
>> >> it will recheck order-0 as end_zone isn't 0 any more.
>> >> But I think it's desriable side effect we have missed.
>> > if order-3 is ok, order-0 is ok too I think, so the check is
>> > unnecessary.
>>
>> No. It's not for the zone but *zones.
>> In case of reclaiming higher order zone, it can sleep without all
>> zones being balanced so that precious order-0 of some zone would be
>> not balanced.
> when balance_pgdat() skips the loop for higher order zone, it already
> sets end_zone, so I thought this isn't a problem.

In case of higher order zone reclaiming, we just make sure 25% of
zones are balanced but not for order-0 on *all* zones.

>
>> Even we can lost chance of clearing congestion flag of the zone.
>> It would be a another patch.
> yep, the congestion flag clearing is a bit confusing. I don't even know
> why just do it in high order allocation. If all zones are ok, we should
> clear the flag regardless the order.

We do in not-high-order allocation as well as high order allocation if
the zone is watermark_okay.


>
>> In conclusion, I would like to avoid complicated thing but I am going
>> to be not against you strongly if other doesn't agree on me.
>> I might need a time to clean kswapd's spagetti up.
> ok, understand it. I have similar concerns actually. I thought my patch
> is simple enough to solve the livelock. But we do have space to cleanup
> balance_pgdat().

Okay. I will put it in my TODO.
Thanks, Shaohua.

-- 
Kind regards,
Minchan Kim

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH]vmscan: fix a livelock in kswapd

[Minchan Kim]

On Tue, Jul 19, 2011 at 4:09 PM, Shaohua Li <shaohua.li@intel.com> wrote:
> I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
> After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
> kswapd2 are keeping running and I can't access filesystem, but most memory is
> free. This looks like a regression since commit 08951e545918c159.
> Node 2 and 3 have only ZONE_NORMAL, but balance_pgdat() will return 0 for
> classzone_idx. The reason is end_zone in balance_pgdat() is 0 by default, if
> all zones have watermark ok, end_zone will keep 0.
> Later sleeping_prematurely() always returns true. Because this is an order 3
> wakeup, and if classzone_idx is 0, both balanced_pages and present_pages
> in pgdat_balanced() are 0.
> We add a special case here. If a zone has no page, we think it's balanced. This
> fixes the livelock.
>
> Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>


-- 
Kind regards,
Minchan Kim

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>