From: Joonsoo Kim <[email protected]>
Currently, we use classzone_idx to calculate lowmem reserve proetection
for an allocation request. This classzone_idx causes a problem
on NUMA systems when the lowmem reserve protection exists for some zones
on a node that do not exist on other nodes.
Before further explanation, I should first clarify how to compute
the classzone_idx and the high_zoneidx.
- ac->high_zoneidx is computed via the arcane gfp_zone(gfp_mask) and
represents the index of the highest zone the allocation can use
- classzone_idx was supposed to be the index of the highest zone on
the local node that the allocation can use, that is actually available
in the system
Think about following example. Node 0 has 4 populated zone,
DMA/DMA32/NORMAL/MOVABLE. Node 1 has 1 populated zone, NORMAL. Some zones,
such as MOVABLE, doesn't exist on node 1 and this makes following
difference.
Assume that there is an allocation request whose gfp_zone(gfp_mask) is
the zone, MOVABLE. Then, it's high_zoneidx is 3. If this allocation is
initiated on node 0, it's classzone_idx is 3 since actually
available/usable zone on local (node 0) is MOVABLE. If this allocation
is initiated on node 1, it's classzone_idx is 2 since actually
available/usable zone on local (node 1) is NORMAL.
You can see that classzone_idx of the allocation request are different
according to their starting node, even if their high_zoneidx is the same.
Think more about these two allocation requests. If they are processed
on local, there is no problem. However, if allocation is initiated
on node 1 are processed on remote, in this example, at the NORMAL zone
on node 0, due to memory shortage, problem occurs. Their different
classzone_idx leads to different lowmem reserve and then different
min watermark. See the following example.
root@ubuntu:/sys/devices/system/memory# cat /proc/zoneinfo
Node 0, zone DMA
per-node stats
...
pages free 3965
min 5
low 8
high 11
spanned 4095
present 3998
managed 3977
protection: (0, 2961, 4928, 5440)
...
Node 0, zone DMA32
pages free 757955
min 1129
low 1887
high 2645
spanned 1044480
present 782303
managed 758116
protection: (0, 0, 1967, 2479)
...
Node 0, zone Normal
pages free 459806
min 750
low 1253
high 1756
spanned 524288
present 524288
managed 503620
protection: (0, 0, 0, 4096)
...
Node 0, zone Movable
pages free 130759
min 195
low 326
high 457
spanned 1966079
present 131072
managed 131072
protection: (0, 0, 0, 0)
...
Node 1, zone DMA
pages free 0
min 0
low 0
high 0
spanned 0
present 0
managed 0
protection: (0, 0, 1006, 1006)
Node 1, zone DMA32
pages free 0
min 0
low 0
high 0
spanned 0
present 0
managed 0
protection: (0, 0, 1006, 1006)
Node 1, zone Normal
per-node stats
...
pages free 233277
min 383
low 640
high 897
spanned 262144
present 262144
managed 257744
protection: (0, 0, 0, 0)
...
Node 1, zone Movable
pages free 0
min 0
low 0
high 0
spanned 262144
present 0
managed 0
protection: (0, 0, 0, 0)
- static min watermark for the NORMAL zone on node 0 is 750.
- lowmem reserve for the request with classzone idx 3 at the NORMAL
on node 0 is 4096.
- lowmem reserve for the request with classzone idx 2 at the NORMAL
on node 0 is 0.
So, overall min watermark is:
allocation initiated on node 0 (classzone_idx 3): 750 + 4096 = 4846
allocation initiated on node 1 (classzone_idx 2): 750 + 0 = 750
allocation initiated on node 1 will have some precedence than allocation
initiated on node 0 because min watermark of the former allocation is
lower than the other. So, allocation initiated on node 1 could succeed
on node 0 when allocation initiated on node 0 could not, and, this could
cause too many numa_miss allocation. Then, performance could be
downgraded.
Recently, there was a regression report about this problem on CMA patches
since CMA memory are placed in ZONE_MOVABLE by those patches. I checked
that problem is disappeared with this fix that uses high_zoneidx
for classzone_idx.
http://lkml.kernel.org/r/20180102063528.GG30397@yexl-desktop
Using high_zoneidx for classzone_idx is more consistent way than previous
approach because system's memory layout doesn't affect anything to it.
With this patch, both classzone_idx on above example will be 3 so will
have the same min watermark.
allocation initiated on node 0: 750 + 4096 = 4846
allocation initiated on node 1: 750 + 4096 = 4846
One could wonder if there is a side effect that allocation initiated on
node 1 will use higher bar when allocation is handled on local since
classzone_idx could be higher than before. It will not happen because
the zone without managed page doesn't contributes lowmem_reserve at all.
Reported-by: Ye Xiaolong <[email protected]>
Tested-by: Ye Xiaolong <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Signed-off-by: Joonsoo Kim <[email protected]>
---
mm/internal.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/internal.h b/mm/internal.h
index c39c895..aebaa33 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -119,7 +119,7 @@ struct alloc_context {
bool spread_dirty_pages;
};
-#define ac_classzone_idx(ac) zonelist_zone_idx(ac->preferred_zoneref)
+#define ac_classzone_idx(ac) (ac->high_zoneidx)
/*
* Locate the struct page for both the matching buddy in our
--
2.7.4
On 03/23/20 at 01:49pm, [email protected] wrote:
> From: Joonsoo Kim <[email protected]>
>
> Currently, we use classzone_idx to calculate lowmem reserve proetection
> for an allocation request. This classzone_idx causes a problem
> on NUMA systems when the lowmem reserve protection exists for some zones
> on a node that do not exist on other nodes.
>
> Before further explanation, I should first clarify how to compute
> the classzone_idx and the high_zoneidx.
>
> - ac->high_zoneidx is computed via the arcane gfp_zone(gfp_mask) and
> represents the index of the highest zone the allocation can use
> - classzone_idx was supposed to be the index of the highest zone on
> the local node that the allocation can use, that is actually available
> in the system
>
> Think about following example. Node 0 has 4 populated zone,
> DMA/DMA32/NORMAL/MOVABLE. Node 1 has 1 populated zone, NORMAL. Some zones,
> such as MOVABLE, doesn't exist on node 1 and this makes following
> difference.
>
> Assume that there is an allocation request whose gfp_zone(gfp_mask) is
> the zone, MOVABLE. Then, it's high_zoneidx is 3. If this allocation is
> initiated on node 0, it's classzone_idx is 3 since actually
> available/usable zone on local (node 0) is MOVABLE. If this allocation
> is initiated on node 1, it's classzone_idx is 2 since actually
> available/usable zone on local (node 1) is NORMAL.
>
> You can see that classzone_idx of the allocation request are different
> according to their starting node, even if their high_zoneidx is the same.
>
> Think more about these two allocation requests. If they are processed
> on local, there is no problem. However, if allocation is initiated
> on node 1 are processed on remote, in this example, at the NORMAL zone
> on node 0, due to memory shortage, problem occurs. Their different
> classzone_idx leads to different lowmem reserve and then different
> min watermark. See the following example.
>
> root@ubuntu:/sys/devices/system/memory# cat /proc/zoneinfo
> Node 0, zone DMA
> per-node stats
> ...
> pages free 3965
> min 5
> low 8
> high 11
> spanned 4095
> present 3998
> managed 3977
> protection: (0, 2961, 4928, 5440)
> ...
> Node 0, zone DMA32
> pages free 757955
> min 1129
> low 1887
> high 2645
> spanned 1044480
> present 782303
> managed 758116
> protection: (0, 0, 1967, 2479)
> ...
> Node 0, zone Normal
> pages free 459806
> min 750
> low 1253
> high 1756
> spanned 524288
> present 524288
> managed 503620
> protection: (0, 0, 0, 4096)
> ...
> Node 0, zone Movable
> pages free 130759
> min 195
> low 326
> high 457
> spanned 1966079
> present 131072
> managed 131072
> protection: (0, 0, 0, 0)
> ...
> Node 1, zone DMA
> pages free 0
> min 0
> low 0
> high 0
> spanned 0
> present 0
> managed 0
> protection: (0, 0, 1006, 1006)
> Node 1, zone DMA32
> pages free 0
> min 0
> low 0
> high 0
> spanned 0
> present 0
> managed 0
> protection: (0, 0, 1006, 1006)
> Node 1, zone Normal
> per-node stats
> ...
> pages free 233277
> min 383
> low 640
> high 897
> spanned 262144
> present 262144
> managed 257744
> protection: (0, 0, 0, 0)
> ...
> Node 1, zone Movable
> pages free 0
> min 0
> low 0
> high 0
> spanned 262144
> present 0
> managed 0
> protection: (0, 0, 0, 0)
>
> - static min watermark for the NORMAL zone on node 0 is 750.
> - lowmem reserve for the request with classzone idx 3 at the NORMAL
> on node 0 is 4096.
> - lowmem reserve for the request with classzone idx 2 at the NORMAL
> on node 0 is 0.
>
> So, overall min watermark is:
> allocation initiated on node 0 (classzone_idx 3): 750 + 4096 = 4846
> allocation initiated on node 1 (classzone_idx 2): 750 + 0 = 750
>
> allocation initiated on node 1 will have some precedence than allocation
> initiated on node 0 because min watermark of the former allocation is
> lower than the other. So, allocation initiated on node 1 could succeed
> on node 0 when allocation initiated on node 0 could not, and, this could
> cause too many numa_miss allocation. Then, performance could be
> downgraded.
>
> Recently, there was a regression report about this problem on CMA patches
> since CMA memory are placed in ZONE_MOVABLE by those patches. I checked
> that problem is disappeared with this fix that uses high_zoneidx
> for classzone_idx.
>
> http://lkml.kernel.org/r/20180102063528.GG30397@yexl-desktop
>
> Using high_zoneidx for classzone_idx is more consistent way than previous
> approach because system's memory layout doesn't affect anything to it.
> With this patch, both classzone_idx on above example will be 3 so will
> have the same min watermark.
>
> allocation initiated on node 0: 750 + 4096 = 4846
> allocation initiated on node 1: 750 + 4096 = 4846
>
> One could wonder if there is a side effect that allocation initiated on
> node 1 will use higher bar when allocation is handled on local since
> classzone_idx could be higher than before. It will not happen because
> the zone without managed page doesn't contributes lowmem_reserve at all.
>
> Reported-by: Ye Xiaolong <[email protected]>
> Tested-by: Ye Xiaolong <[email protected]>
> Acked-by: Vlastimil Babka <[email protected]>
> Signed-off-by: Joonsoo Kim <[email protected]>
> ---
> mm/internal.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index c39c895..aebaa33 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -119,7 +119,7 @@ struct alloc_context {
> bool spread_dirty_pages;
> };
>
> -#define ac_classzone_idx(ac) zonelist_zone_idx(ac->preferred_zoneref)
> +#define ac_classzone_idx(ac) (ac->high_zoneidx)
Reviewed-by: Baoquan He <[email protected]>