On Tue, May 29, 2018 at 11:12 PM Greg Thelen <gthelen@xxxxxxxxxx> 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 <gthelen@xxxxxxxxxx>
> Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
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
>
