Reclaim priorities range from 0..12(DEF_PRIORITY).
scan_control.priority is a 4 byte int, which is overkill.
Since commit 6538b8ea886e ("x86_64: expand kernel stack to 16K") x86_64
stack overflows are not an issue. But it's inefficient to use 4 bytes
for priority.
Use s8 (signed byte) rather than u8 to allow for loops like:
do {
...
} while (--sc.priority >= 0);
This reduces sizeof(struct scan_control) from 96 => 88 bytes (x86_64),
which saves some stack.
scan_control.priority field order is changed to occupy otherwise unused
padding.
Signed-off-by: Greg Thelen <[email protected]>
---
mm/vmscan.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9b697323a88c..541c334bd176 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -83,9 +83,6 @@ struct scan_control {
*/
struct mem_cgroup *target_mem_cgroup;
- /* Scan (total_size >> priority) pages at once */
- int priority;
-
/* The highest zone to isolate pages for reclaim from */
enum zone_type reclaim_idx;
@@ -111,6 +108,9 @@ struct scan_control {
/* One of the zones is ready for compaction */
unsigned int compaction_ready:1;
+ /* Scan (total_size >> priority) pages at once */
+ s8 priority;
+
/* Incremented by the number of inactive pages that were scanned */
unsigned long nr_scanned;
--
2.17.0.921.gf22659ad46-goog
On Mon, May 28, 2018 at 07:40:25PM -0700, Greg Thelen wrote:
> Reclaim priorities range from 0..12(DEF_PRIORITY).
> scan_control.priority is a 4 byte int, which is overkill.
>
> Since commit 6538b8ea886e ("x86_64: expand kernel stack to 16K") x86_64
> stack overflows are not an issue. But it's inefficient to use 4 bytes
> for priority.
If you're looking to shave a few more bytes, allocation order can fit
in a u8 too (can't be more than 6 bits, and realistically won't be more
than 4 bits). reclaim_idx likewise will fit in a u8, and actually won't
be more than 3 bits.
I am sceptical that nr_to_reclaim should really be an unsigned long; I
don't think we should be trying to free 4 billion pages in a single call.
nr_scanned might be over 4 billion (!) but nr_reclaimed can probably
shrink to unsigned int along with nr_to_reclaim.
Matthew Wilcox <[email protected]> wrote:
> On Mon, May 28, 2018 at 07:40:25PM -0700, Greg Thelen wrote:
>> Reclaim priorities range from 0..12(DEF_PRIORITY).
>> scan_control.priority is a 4 byte int, which is overkill.
>>
>> Since commit 6538b8ea886e ("x86_64: expand kernel stack to 16K") x86_64
>> stack overflows are not an issue. But it's inefficient to use 4 bytes
>> for priority.
>
> If you're looking to shave a few more bytes, allocation order can fit
> in a u8 too (can't be more than 6 bits, and realistically won't be more
> than 4 bits). reclaim_idx likewise will fit in a u8, and actually won't
> be more than 3 bits.
Nod. Good tip. Included in ("[PATCH v2] mm: condense scan_control").
> I am sceptical that nr_to_reclaim should really be an unsigned long; I
> don't think we should be trying to free 4 billion pages in a single call.
> nr_scanned might be over 4 billion (!) but nr_reclaimed can probably
> shrink to unsigned int along with nr_to_reclaim.
Agreed. For patch simplicity, I'll pass on this for now.
Use smaller scan_control fields for order, priority, and reclaim_idx.
Convert fields from int => s8. All easily fit within a byte:
* allocation order range: 0..MAX_ORDER(64?)
* priority range: 0..12(DEF_PRIORITY)
* reclaim_idx range: 0..6(__MAX_NR_ZONES)
Since commit 6538b8ea886e ("x86_64: expand kernel stack to 16K") x86_64
stack overflows are not an issue. But it's inefficient to use ints.
Use s8 (signed byte) rather than u8 to allow for loops like:
do {
...
} while (--sc.priority >= 0);
Add BUILD_BUG_ON to verify that s8 is capable of storing max values.
This reduces sizeof(struct scan_control):
* 96 => 80 bytes (x86_64)
* 68 => 56 bytes (i386)
scan_control structure field order is changed to utilize padding.
After this patch there is 1 bit of scan_control padding.
Signed-off-by: Greg Thelen <[email protected]>
Suggested-by: Matthew Wilcox <[email protected]>
---
mm/vmscan.c | 32 ++++++++++++++++++++------------
1 file changed, 20 insertions(+), 12 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9b697323a88c..42731faea306 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -65,12 +65,6 @@ struct scan_control {
/* How many pages shrink_list() should reclaim */
unsigned long nr_to_reclaim;
- /* This context's GFP mask */
- gfp_t gfp_mask;
-
- /* Allocation order */
- int order;
-
/*
* Nodemask of nodes allowed by the caller. If NULL, all nodes
* are scanned.
@@ -83,12 +77,6 @@ struct scan_control {
*/
struct mem_cgroup *target_mem_cgroup;
- /* Scan (total_size >> priority) pages at once */
- int priority;
-
- /* The highest zone to isolate pages for reclaim from */
- enum zone_type reclaim_idx;
-
/* Writepage batching in laptop mode; RECLAIM_WRITE */
unsigned int may_writepage:1;
@@ -111,6 +99,18 @@ struct scan_control {
/* One of the zones is ready for compaction */
unsigned int compaction_ready:1;
+ /* Allocation order */
+ s8 order;
+
+ /* Scan (total_size >> priority) pages at once */
+ s8 priority;
+
+ /* The highest zone to isolate pages for reclaim from */
+ s8 reclaim_idx;
+
+ /* This context's GFP mask */
+ gfp_t gfp_mask;
+
/* Incremented by the number of inactive pages that were scanned */
unsigned long nr_scanned;
@@ -3047,6 +3047,14 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
.may_swap = 1,
};
+ /*
+ * scan_control uses s8 fields for order, priority, and reclaim_idx.
+ * Confirm they are large enough for max values.
+ */
+ BUILD_BUG_ON(MAX_ORDER > S8_MAX);
+ BUILD_BUG_ON(DEF_PRIORITY > S8_MAX);
+ BUILD_BUG_ON(MAX_NR_ZONES > S8_MAX);
+
/*
* Do not enter reclaim if fatal signal was delivered while throttled.
* 1 is returned so that the page allocator does not OOM kill at this
--
2.17.0.921.gf22659ad46-goog
On Tue, May 29, 2018 at 11:12 PM Greg Thelen <[email protected]> wrote:
>
> Use smaller scan_control fields for order, priority, and reclaim_idx.
> Convert fields from int => s8. All easily fit within a byte:
> * allocation order range: 0..MAX_ORDER(64?)
> * priority range: 0..12(DEF_PRIORITY)
> * reclaim_idx range: 0..6(__MAX_NR_ZONES)
>
> Since commit 6538b8ea886e ("x86_64: expand kernel stack to 16K") x86_64
> stack overflows are not an issue. But it's inefficient to use ints.
>
> Use s8 (signed byte) rather than u8 to allow for loops like:
> do {
> ...
> } while (--sc.priority >= 0);
>
> Add BUILD_BUG_ON to verify that s8 is capable of storing max values.
>
> This reduces sizeof(struct scan_control):
> * 96 => 80 bytes (x86_64)
> * 68 => 56 bytes (i386)
>
> scan_control structure field order is changed to utilize padding.
> After this patch there is 1 bit of scan_control padding.
>
> Signed-off-by: Greg Thelen <[email protected]>
> Suggested-by: Matthew Wilcox <[email protected]>
Is there any interest in this? Less stack usage could mean less
dcache and dtlb pressure. But I understand if the complexity is
distasteful.
> ---
> mm/vmscan.c | 32 ++++++++++++++++++++------------
> 1 file changed, 20 insertions(+), 12 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 9b697323a88c..42731faea306 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -65,12 +65,6 @@ struct scan_control {
> /* How many pages shrink_list() should reclaim */
> unsigned long nr_to_reclaim;
>
> - /* This context's GFP mask */
> - gfp_t gfp_mask;
> -
> - /* Allocation order */
> - int order;
> -
> /*
> * Nodemask of nodes allowed by the caller. If NULL, all nodes
> * are scanned.
> @@ -83,12 +77,6 @@ struct scan_control {
> */
> struct mem_cgroup *target_mem_cgroup;
>
> - /* Scan (total_size >> priority) pages at once */
> - int priority;
> -
> - /* The highest zone to isolate pages for reclaim from */
> - enum zone_type reclaim_idx;
> -
> /* Writepage batching in laptop mode; RECLAIM_WRITE */
> unsigned int may_writepage:1;
>
> @@ -111,6 +99,18 @@ struct scan_control {
> /* One of the zones is ready for compaction */
> unsigned int compaction_ready:1;
>
> + /* Allocation order */
> + s8 order;
> +
> + /* Scan (total_size >> priority) pages at once */
> + s8 priority;
> +
> + /* The highest zone to isolate pages for reclaim from */
> + s8 reclaim_idx;
> +
> + /* This context's GFP mask */
> + gfp_t gfp_mask;
> +
> /* Incremented by the number of inactive pages that were scanned */
> unsigned long nr_scanned;
>
> @@ -3047,6 +3047,14 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
> .may_swap = 1,
> };
>
> + /*
> + * scan_control uses s8 fields for order, priority, and reclaim_idx.
> + * Confirm they are large enough for max values.
> + */
> + BUILD_BUG_ON(MAX_ORDER > S8_MAX);
> + BUILD_BUG_ON(DEF_PRIORITY > S8_MAX);
> + BUILD_BUG_ON(MAX_NR_ZONES > S8_MAX);
> +
> /*
> * Do not enter reclaim if fatal signal was delivered while throttled.
> * 1 is returned so that the page allocator does not OOM kill at this
> --
> 2.17.0.921.gf22659ad46-goog
>