On 03/31/2015 11:48 AM, Michal Hocko wrote:
> On Fri 27-03-15 15:23:50, Nishanth Aravamudan wrote:
>> On 27.03.2015 [13:17:59 -0700], Dave Hansen wrote:
>>> On 03/27/2015 12:28 PM, Nishanth Aravamudan wrote:
>>>> @@ -2585,7 +2585,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
>>>>
>>>> for (i = 0; i <= ZONE_NORMAL; i++) {
>>>> zone = &pgdat->node_zones[i];
>>>> - if (!populated_zone(zone))
>>>> + if (!populated_zone(zone) || !zone_reclaimable(zone))
>>>> continue;
>>>>
>>>> pfmemalloc_reserve += min_wmark_pages(zone);
>>>
>>> Do you really want zone_reclaimable()? Or do you want something more
>>> direct like "zone_reclaimable_pages(zone) == 0"?
>>
>> Yeah, I guess in my testing this worked out to be the same, since
>> zone_reclaimable_pages(zone) is 0 and so zone_reclaimable(zone) will
>> always be false. Thanks!
>>
>> Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc
>> reserves if node has no ZONE_NORMAL") from Mel.
>>
>> We have a system with the following topology:
>>
>> # numactl -H
>> available: 3 nodes (0,2-3)
>> node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
>> 23 24 25 26 27 28 29 30 31
>> node 0 size: 28273 MB
>> node 0 free: 27323 MB
>> node 2 cpus:
>> node 2 size: 16384 MB
>> node 2 free: 0 MB
>> node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
>> node 3 size: 30533 MB
>> node 3 free: 13273 MB
>> node distances:
>> node 0 2 3
>> 0: 10 20 20
>> 2: 20 10 20
>> 3: 20 20 10
>>
>> Node 2 has no free memory, because:
>> # cat /sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages
>> 1
>>
>> This leads to the following zoneinfo:
>>
>> Node 2, zone DMA
>> pages free 0
>> min 1840
>> low 2300
>> high 2760
>> scanned 0
>> spanned 262144
>> present 262144
>> managed 262144
>> ...
>> all_unreclaimable: 1
>
> Blee, this is a weird configuration.
>
>> If one then attempts to allocate some normal 16M hugepages via
>>
>> echo 37 > /proc/sys/vm/nr_hugepages
>>
>> The echo never returns and kswapd2 consumes CPU cycles.
>>
>> This is because throttle_direct_reclaim ends up calling
>> wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...).
>> pfmemalloc_watermark_ok() in turn checks all zones on the node if there
>> are any reserves, and if so, then indicates the watermarks are ok, by
>> seeing if there are sufficient free pages.
>>
>> 675becce15 added a condition already for memoryless nodes. In this case,
>> though, the node has memory, it is just all consumed (and not
>> reclaimable). Effectively, though, the result is the same on this call
>> to pfmemalloc_watermark_ok() and thus seems like a reasonable additional
>> condition.
>>
>> With this change, the afore-mentioned 16M hugepage allocation attempt
>> succeeds and correctly round-robins between Nodes 1 and 3.
>
> I am just wondering whether this is the right/complete fix. Don't we
> need a similar treatment at more places?
> I would expect kswapd would be looping endlessly because the zone
> wouldn't be balanced obviously. But I would be wrong... because
> pgdat_balanced is doing this:
> /*
> * A special case here:
> *
> * balance_pgdat() skips over all_unreclaimable after
> * DEF_PRIORITY. Effectively, it considers them balanced so
> * they must be considered balanced here as well!
> */
> if (!zone_reclaimable(zone)) {
> balanced_pages += zone->managed_pages;
> continue;
> }
>
> and zone_reclaimable is false for you as you didn't have any
> zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it
> would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those
> zones right?) and so the kswapd would be woken up easily. So it looks
> like a mess.
Yeah, looks like a much cleaner/complete solution would be to remove
such zones from zonelists. But that means covering all situations when
these hugepages are allocated/removed and the approach then looks
similar to memory hotplug.
Also I'm not sure if the ability to actually allocate the reserved
hugepage would be impossible due to not being reachable by a zonelist...
> There are possibly other places which rely on populated_zone or
> for_each_populated_zone without checking reclaimability. Are those
> working as expected?
Yeah. At least the wakeup_kswapd case should be fixed IMHO. No point in
waking it up just to let it immediately go to sleep again.
> That being said. I am not objecting to this patch. I am just trying to
> wrap my head around possible issues from such a weird configuration and
> all the consequences.
>
>> Signed-off-by: Nishanth Aravamudan <[email protected]>
>
> The patch as is doesn't seem to be harmful.
>
> Reviewed-by: Michal Hocko <[email protected]>
>
>> ---
>> v1 -> v2:
>> Check against zone_reclaimable_pages, rather zone_reclaimable, based
>> upon feedback from Dave Hansen.
>
> Dunno, but shouldn't we use the same thing here and in pgdat_balanced?
> zone_reclaimable_pages seems to be used only from zone_reclaimable().
pgdat_balanced() has a different goal than pfmemalloc_watermark_ok() and
needs to match what balance_pgdat() does, which includes considering
NR_PAGES_SCANNED through zone_reclaimable(). For the situation
considered in this patch, result of zone_reclaimable() will match the
test zone_reclaimable_pages(zone) == 0, so it is fine I think.
What I find somewhat worrying though is that we could potentially break
the pfmemalloc_watermark_ok() test in situations where
zone_reclaimable_pages(zone) == 0 is a transient situation (and not a
permanently allocated hugepage). In that case, the throttling is
supposed to help system recover, and we might be breaking that ability
with this patch, no?
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 5e8eadd71bac..c627fa4c991f 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -2646,7 +2646,8 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
>>
>> for (i = 0; i <= ZONE_NORMAL; i++) {
>> zone = &pgdat->node_zones[i];
>> - if (!populated_zone(zone))
>> + if (!populated_zone(zone) ||
>> + zone_reclaimable_pages(zone) == 0)
>> continue;
>>
>> pfmemalloc_reserve += min_wmark_pages(zone);
>>
>
On 03.04.2015 [09:57:35 +0200], Vlastimil Babka wrote:
> On 03/31/2015 11:48 AM, Michal Hocko wrote:
> >On Fri 27-03-15 15:23:50, Nishanth Aravamudan wrote:
> >>On 27.03.2015 [13:17:59 -0700], Dave Hansen wrote:
> >>>On 03/27/2015 12:28 PM, Nishanth Aravamudan wrote:
> >>>>@@ -2585,7 +2585,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
> >>>>
> >>>> for (i = 0; i <= ZONE_NORMAL; i++) {
> >>>> zone = &pgdat->node_zones[i];
> >>>>- if (!populated_zone(zone))
> >>>>+ if (!populated_zone(zone) || !zone_reclaimable(zone))
> >>>> continue;
> >>>>
> >>>> pfmemalloc_reserve += min_wmark_pages(zone);
> >>>
> >>>Do you really want zone_reclaimable()? Or do you want something more
> >>>direct like "zone_reclaimable_pages(zone) == 0"?
> >>
> >>Yeah, I guess in my testing this worked out to be the same, since
> >>zone_reclaimable_pages(zone) is 0 and so zone_reclaimable(zone) will
> >>always be false. Thanks!
> >>
> >>Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc
> >>reserves if node has no ZONE_NORMAL") from Mel.
> >>
> >>We have a system with the following topology:
> >>
> >># numactl -H
> >>available: 3 nodes (0,2-3)
> >>node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
> >>23 24 25 26 27 28 29 30 31
> >>node 0 size: 28273 MB
> >>node 0 free: 27323 MB
> >>node 2 cpus:
> >>node 2 size: 16384 MB
> >>node 2 free: 0 MB
> >>node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
> >>node 3 size: 30533 MB
> >>node 3 free: 13273 MB
> >>node distances:
> >>node 0 2 3
> >> 0: 10 20 20
> >> 2: 20 10 20
> >> 3: 20 20 10
> >>
> >>Node 2 has no free memory, because:
> >># cat /sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages
> >>1
> >>
> >>This leads to the following zoneinfo:
> >>
> >>Node 2, zone DMA
> >> pages free 0
> >> min 1840
> >> low 2300
> >> high 2760
> >> scanned 0
> >> spanned 262144
> >> present 262144
> >> managed 262144
> >>...
> >> all_unreclaimable: 1
> >
> >Blee, this is a weird configuration.
> >
> >>If one then attempts to allocate some normal 16M hugepages via
> >>
> >>echo 37 > /proc/sys/vm/nr_hugepages
> >>
> >>The echo never returns and kswapd2 consumes CPU cycles.
> >>
> >>This is because throttle_direct_reclaim ends up calling
> >>wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...).
> >>pfmemalloc_watermark_ok() in turn checks all zones on the node if there
> >>are any reserves, and if so, then indicates the watermarks are ok, by
> >>seeing if there are sufficient free pages.
> >>
> >>675becce15 added a condition already for memoryless nodes. In this case,
> >>though, the node has memory, it is just all consumed (and not
> >>reclaimable). Effectively, though, the result is the same on this call
> >>to pfmemalloc_watermark_ok() and thus seems like a reasonable additional
> >>condition.
> >>
> >>With this change, the afore-mentioned 16M hugepage allocation attempt
> >>succeeds and correctly round-robins between Nodes 1 and 3.
> >
> >I am just wondering whether this is the right/complete fix. Don't we
> >need a similar treatment at more places?
> >I would expect kswapd would be looping endlessly because the zone
> >wouldn't be balanced obviously. But I would be wrong... because
> >pgdat_balanced is doing this:
> > /*
> > * A special case here:
> > *
> > * balance_pgdat() skips over all_unreclaimable after
> > * DEF_PRIORITY. Effectively, it considers them balanced so
> > * they must be considered balanced here as well!
> > */
> > if (!zone_reclaimable(zone)) {
> > balanced_pages += zone->managed_pages;
> > continue;
> > }
> >
> >and zone_reclaimable is false for you as you didn't have any
> >zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it
> >would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those
> >zones right?) and so the kswapd would be woken up easily. So it looks
> >like a mess.
>
> Yeah, looks like a much cleaner/complete solution would be to remove
> such zones from zonelists. But that means covering all situations
> when these hugepages are allocated/removed and the approach then
> looks similar to memory hotplug.
> Also I'm not sure if the ability to actually allocate the reserved
> hugepage would be impossible due to not being reachable by a
> zonelist...
>
> >There are possibly other places which rely on populated_zone or
> >for_each_populated_zone without checking reclaimability. Are those
> >working as expected?
>
> Yeah. At least the wakeup_kswapd case should be fixed IMHO. No point
> in waking it up just to let it immediately go to sleep again.
>
> >That being said. I am not objecting to this patch. I am just trying to
> >wrap my head around possible issues from such a weird configuration and
> >all the consequences.
> >
> >>Signed-off-by: Nishanth Aravamudan <[email protected]>
> >
> >The patch as is doesn't seem to be harmful.
> >
> >Reviewed-by: Michal Hocko <[email protected]>
> >
> >>---
> >>v1 -> v2:
> >> Check against zone_reclaimable_pages, rather zone_reclaimable, based
> >> upon feedback from Dave Hansen.
> >
> >Dunno, but shouldn't we use the same thing here and in pgdat_balanced?
> >zone_reclaimable_pages seems to be used only from zone_reclaimable().
>
> pgdat_balanced() has a different goal than pfmemalloc_watermark_ok()
> and needs to match what balance_pgdat() does, which includes
> considering NR_PAGES_SCANNED through zone_reclaimable(). For the
> situation considered in this patch, result of zone_reclaimable()
> will match the test zone_reclaimable_pages(zone) == 0, so it is fine
> I think.
Right.
> What I find somewhat worrying though is that we could potentially
> break the pfmemalloc_watermark_ok() test in situations where
> zone_reclaimable_pages(zone) == 0 is a transient situation (and not
> a permanently allocated hugepage). In that case, the throttling is
> supposed to help system recover, and we might be breaking that
> ability with this patch, no?
Well, if it's transient, we'll skip it this time through, and once there
are reclaimable pages, we should notice it again.
I'm not familiar enough with this logic, so I'll read through the code
again soon to see if your concern is valid, as best I can.
Thanks,
Nish
On 03.04.2015 [10:45:56 -0700], Nishanth Aravamudan wrote:
> On 03.04.2015 [09:57:35 +0200], Vlastimil Babka wrote:
> > On 03/31/2015 11:48 AM, Michal Hocko wrote:
> > >On Fri 27-03-15 15:23:50, Nishanth Aravamudan wrote:
> > >>On 27.03.2015 [13:17:59 -0700], Dave Hansen wrote:
> > >>>On 03/27/2015 12:28 PM, Nishanth Aravamudan wrote:
> > >>>>@@ -2585,7 +2585,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
> > >>>>
> > >>>> for (i = 0; i <= ZONE_NORMAL; i++) {
> > >>>> zone = &pgdat->node_zones[i];
> > >>>>- if (!populated_zone(zone))
> > >>>>+ if (!populated_zone(zone) || !zone_reclaimable(zone))
> > >>>> continue;
> > >>>>
> > >>>> pfmemalloc_reserve += min_wmark_pages(zone);
> > >>>
> > >>>Do you really want zone_reclaimable()? Or do you want something more
> > >>>direct like "zone_reclaimable_pages(zone) == 0"?
> > >>
> > >>Yeah, I guess in my testing this worked out to be the same, since
> > >>zone_reclaimable_pages(zone) is 0 and so zone_reclaimable(zone) will
> > >>always be false. Thanks!
> > >>
> > >>Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc
> > >>reserves if node has no ZONE_NORMAL") from Mel.
> > >>
> > >>We have a system with the following topology:
> > >>
> > >># numactl -H
> > >>available: 3 nodes (0,2-3)
> > >>node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
> > >>23 24 25 26 27 28 29 30 31
> > >>node 0 size: 28273 MB
> > >>node 0 free: 27323 MB
> > >>node 2 cpus:
> > >>node 2 size: 16384 MB
> > >>node 2 free: 0 MB
> > >>node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
> > >>node 3 size: 30533 MB
> > >>node 3 free: 13273 MB
> > >>node distances:
> > >>node 0 2 3
> > >> 0: 10 20 20
> > >> 2: 20 10 20
> > >> 3: 20 20 10
> > >>
> > >>Node 2 has no free memory, because:
> > >># cat /sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages
> > >>1
> > >>
> > >>This leads to the following zoneinfo:
> > >>
> > >>Node 2, zone DMA
> > >> pages free 0
> > >> min 1840
> > >> low 2300
> > >> high 2760
> > >> scanned 0
> > >> spanned 262144
> > >> present 262144
> > >> managed 262144
> > >>...
> > >> all_unreclaimable: 1
> > >
> > >Blee, this is a weird configuration.
> > >
> > >>If one then attempts to allocate some normal 16M hugepages via
> > >>
> > >>echo 37 > /proc/sys/vm/nr_hugepages
> > >>
> > >>The echo never returns and kswapd2 consumes CPU cycles.
> > >>
> > >>This is because throttle_direct_reclaim ends up calling
> > >>wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...).
> > >>pfmemalloc_watermark_ok() in turn checks all zones on the node if there
> > >>are any reserves, and if so, then indicates the watermarks are ok, by
> > >>seeing if there are sufficient free pages.
> > >>
> > >>675becce15 added a condition already for memoryless nodes. In this case,
> > >>though, the node has memory, it is just all consumed (and not
> > >>reclaimable). Effectively, though, the result is the same on this call
> > >>to pfmemalloc_watermark_ok() and thus seems like a reasonable additional
> > >>condition.
> > >>
> > >>With this change, the afore-mentioned 16M hugepage allocation attempt
> > >>succeeds and correctly round-robins between Nodes 1 and 3.
> > >
> > >I am just wondering whether this is the right/complete fix. Don't we
> > >need a similar treatment at more places?
> > >I would expect kswapd would be looping endlessly because the zone
> > >wouldn't be balanced obviously. But I would be wrong... because
> > >pgdat_balanced is doing this:
> > > /*
> > > * A special case here:
> > > *
> > > * balance_pgdat() skips over all_unreclaimable after
> > > * DEF_PRIORITY. Effectively, it considers them balanced so
> > > * they must be considered balanced here as well!
> > > */
> > > if (!zone_reclaimable(zone)) {
> > > balanced_pages += zone->managed_pages;
> > > continue;
> > > }
> > >
> > >and zone_reclaimable is false for you as you didn't have any
> > >zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it
> > >would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those
> > >zones right?) and so the kswapd would be woken up easily. So it looks
> > >like a mess.
> >
> > Yeah, looks like a much cleaner/complete solution would be to remove
> > such zones from zonelists. But that means covering all situations
> > when these hugepages are allocated/removed and the approach then
> > looks similar to memory hotplug.
> > Also I'm not sure if the ability to actually allocate the reserved
> > hugepage would be impossible due to not being reachable by a
> > zonelist...
> >
> > >There are possibly other places which rely on populated_zone or
> > >for_each_populated_zone without checking reclaimability. Are those
> > >working as expected?
> >
> > Yeah. At least the wakeup_kswapd case should be fixed IMHO. No point
> > in waking it up just to let it immediately go to sleep again.
> >
> > >That being said. I am not objecting to this patch. I am just trying to
> > >wrap my head around possible issues from such a weird configuration and
> > >all the consequences.
> > >
> > >>Signed-off-by: Nishanth Aravamudan <[email protected]>
> > >
> > >The patch as is doesn't seem to be harmful.
> > >
> > >Reviewed-by: Michal Hocko <[email protected]>
> > >
> > >>---
> > >>v1 -> v2:
> > >> Check against zone_reclaimable_pages, rather zone_reclaimable, based
> > >> upon feedback from Dave Hansen.
> > >
> > >Dunno, but shouldn't we use the same thing here and in pgdat_balanced?
> > >zone_reclaimable_pages seems to be used only from zone_reclaimable().
> >
> > pgdat_balanced() has a different goal than pfmemalloc_watermark_ok()
> > and needs to match what balance_pgdat() does, which includes
> > considering NR_PAGES_SCANNED through zone_reclaimable(). For the
> > situation considered in this patch, result of zone_reclaimable()
> > will match the test zone_reclaimable_pages(zone) == 0, so it is fine
> > I think.
>
> Right.
>
> > What I find somewhat worrying though is that we could potentially
> > break the pfmemalloc_watermark_ok() test in situations where
> > zone_reclaimable_pages(zone) == 0 is a transient situation (and not
> > a permanently allocated hugepage). In that case, the throttling is
> > supposed to help system recover, and we might be breaking that
> > ability with this patch, no?
>
> Well, if it's transient, we'll skip it this time through, and once there
> are reclaimable pages, we should notice it again.
>
> I'm not familiar enough with this logic, so I'll read through the code
> again soon to see if your concern is valid, as best I can.
In reviewing the code, I think that transiently unreclaimable zones will
lead to some higher direct reclaim rates and possible contention, but
shouldn't cause any major harm. The likelihood of that situation, as
well, in a non-reserved memory setup like the one I described, seems
exceedingly low.
Thanks,
Nish
On 05/06/2015 12:09 AM, Nishanth Aravamudan wrote:
> On 03.04.2015 [10:45:56 -0700], Nishanth Aravamudan wrote:
>>> What I find somewhat worrying though is that we could potentially
>>> break the pfmemalloc_watermark_ok() test in situations where
>>> zone_reclaimable_pages(zone) == 0 is a transient situation (and not
>>> a permanently allocated hugepage). In that case, the throttling is
>>> supposed to help system recover, and we might be breaking that
>>> ability with this patch, no?
>>
>> Well, if it's transient, we'll skip it this time through, and once there
>> are reclaimable pages, we should notice it again.
>>
>> I'm not familiar enough with this logic, so I'll read through the code
>> again soon to see if your concern is valid, as best I can.
>
> In reviewing the code, I think that transiently unreclaimable zones will
> lead to some higher direct reclaim rates and possible contention, but
> shouldn't cause any major harm. The likelihood of that situation, as
> well, in a non-reserved memory setup like the one I described, seems
> exceedingly low.
OK, I guess when a reasonably configured system has nothing to reclaim,
it's already busted and throttling won't change much.
Consider the patch Acked-by: Vlastimil Babka <[email protected]>
> Thanks,
> Nish
>
On Wed, 06 May 2015 11:28:12 +0200 Vlastimil Babka <[email protected]> wrote:
> On 05/06/2015 12:09 AM, Nishanth Aravamudan wrote:
> > On 03.04.2015 [10:45:56 -0700], Nishanth Aravamudan wrote:
> >>> What I find somewhat worrying though is that we could potentially
> >>> break the pfmemalloc_watermark_ok() test in situations where
> >>> zone_reclaimable_pages(zone) == 0 is a transient situation (and not
> >>> a permanently allocated hugepage). In that case, the throttling is
> >>> supposed to help system recover, and we might be breaking that
> >>> ability with this patch, no?
> >>
> >> Well, if it's transient, we'll skip it this time through, and once there
> >> are reclaimable pages, we should notice it again.
> >>
> >> I'm not familiar enough with this logic, so I'll read through the code
> >> again soon to see if your concern is valid, as best I can.
> >
> > In reviewing the code, I think that transiently unreclaimable zones will
> > lead to some higher direct reclaim rates and possible contention, but
> > shouldn't cause any major harm. The likelihood of that situation, as
> > well, in a non-reserved memory setup like the one I described, seems
> > exceedingly low.
>
> OK, I guess when a reasonably configured system has nothing to reclaim,
> it's already busted and throttling won't change much.
>
> Consider the patch Acked-by: Vlastimil Babka <[email protected]>
OK, thanks, I'll move this patch into the queue for 4.2-rc1.
Or is it important enough to merge into 4.1?
From: Nishanth Aravamudan <[email protected]>
Subject: mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable pages
Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc
reserves if node has no ZONE_NORMAL") from Mel.
We have a system with the following topology:
# numactl -H
available: 3 nodes (0,2-3)
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
23 24 25 26 27 28 29 30 31
node 0 size: 28273 MB
node 0 free: 27323 MB
node 2 cpus:
node 2 size: 16384 MB
node 2 free: 0 MB
node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
node 3 size: 30533 MB
node 3 free: 13273 MB
node distances:
node 0 2 3
0: 10 20 20
2: 20 10 20
3: 20 20 10
Node 2 has no free memory, because:
# cat /sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages
1
This leads to the following zoneinfo:
Node 2, zone DMA
pages free 0
min 1840
low 2300
high 2760
scanned 0
spanned 262144
present 262144
managed 262144
...
all_unreclaimable: 1
If one then attempts to allocate some normal 16M hugepages via
echo 37 > /proc/sys/vm/nr_hugepages
The echo never returns and kswapd2 consumes CPU cycles.
This is because throttle_direct_reclaim ends up calling
wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...).
pfmemalloc_watermark_ok() in turn checks all zones on the node if there
are any reserves, and if so, then indicates the watermarks are ok, by
seeing if there are sufficient free pages.
675becce15 added a condition already for memoryless nodes. In this case,
though, the node has memory, it is just all consumed (and not
reclaimable). Effectively, though, the result is the same on this call to
pfmemalloc_watermark_ok() and thus seems like a reasonable additional
condition.
With this change, the afore-mentioned 16M hugepage allocation attempt
succeeds and correctly round-robins between Nodes 1 and 3.
Signed-off-by: Nishanth Aravamudan <[email protected]>
Reviewed-by: Michal Hocko <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Anton Blanchard <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Dan Streetman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
---
mm/vmscan.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff -puN mm/vmscan.c~mm-vmscan-do-not-throttle-based-on-pfmemalloc-reserves-if-node-has-no-reclaimable-pages mm/vmscan.c
--- a/mm/vmscan.c~mm-vmscan-do-not-throttle-based-on-pfmemalloc-reserves-if-node-has-no-reclaimable-pages
+++ a/mm/vmscan.c
@@ -2646,7 +2646,8 @@ static bool pfmemalloc_watermark_ok(pg_d
for (i = 0; i <= ZONE_NORMAL; i++) {
zone = &pgdat->node_zones[i];
- if (!populated_zone(zone))
+ if (!populated_zone(zone) ||
+ zone_reclaimable_pages(zone) == 0)
continue;
pfmemalloc_reserve += min_wmark_pages(zone);
_
On 08.05.2015 [15:47:26 -0700], Andrew Morton wrote:
> On Wed, 06 May 2015 11:28:12 +0200 Vlastimil Babka <[email protected]> wrote:
>
> > On 05/06/2015 12:09 AM, Nishanth Aravamudan wrote:
> > > On 03.04.2015 [10:45:56 -0700], Nishanth Aravamudan wrote:
> > >>> What I find somewhat worrying though is that we could potentially
> > >>> break the pfmemalloc_watermark_ok() test in situations where
> > >>> zone_reclaimable_pages(zone) == 0 is a transient situation (and not
> > >>> a permanently allocated hugepage). In that case, the throttling is
> > >>> supposed to help system recover, and we might be breaking that
> > >>> ability with this patch, no?
> > >>
> > >> Well, if it's transient, we'll skip it this time through, and once there
> > >> are reclaimable pages, we should notice it again.
> > >>
> > >> I'm not familiar enough with this logic, so I'll read through the code
> > >> again soon to see if your concern is valid, as best I can.
> > >
> > > In reviewing the code, I think that transiently unreclaimable zones will
> > > lead to some higher direct reclaim rates and possible contention, but
> > > shouldn't cause any major harm. The likelihood of that situation, as
> > > well, in a non-reserved memory setup like the one I described, seems
> > > exceedingly low.
> >
> > OK, I guess when a reasonably configured system has nothing to reclaim,
> > it's already busted and throttling won't change much.
> >
> > Consider the patch Acked-by: Vlastimil Babka <[email protected]>
>
> OK, thanks, I'll move this patch into the queue for 4.2-rc1.
Thank you!
> Or is it important enough to merge into 4.1?
I think 4.2 is sufficient, but I wonder now if I should have included a
stable tag? The issue has been around for a while and there's a
relatively easily workaround (use the per-node sysfs files to manually
round-robin around the exhausted node) in older kernels, so I had
decided against it before.
Thanks,
Nish