On 5/6/22 4:59 AM, Bjoern A. Zeeb wrote:
On Thu, 21 Apr 2022, John Baldwin wrote:
The branch main has been updated by jhb:
URL:
https://cgit.FreeBSD.org/src/commit/?id=92e40a9b9241313b3d16543819ccd8d642bb11a5
commit 92e40a9b9241313b3d16543819ccd8d642bb11a5
Author: John Baldwin <[email protected]>
AuthorDate: 2022-04-21 19:01:55 +0000
Commit: John Baldwin <[email protected]>
CommitDate: 2022-04-21 19:01:55 +0000
busdma_bounce: Batch bounce page free operations when possible.
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D34968
---
sys/kern/subr_busdma_bounce.c | 69 +++++++++++++++++++++----------------------
1 file changed, 34 insertions(+), 35 deletions(-)
diff --git a/sys/kern/subr_busdma_bounce.c b/sys/kern/subr_busdma_bounce.c
index 243da8e9487f..f3699cf2ad27 100644
--- a/sys/kern/subr_busdma_bounce.c
+++ b/sys/kern/subr_busdma_bounce.c
@@ -382,55 +382,54 @@ add_bounce_page(bus_dma_tag_t dmat, bus_dmamap_t map,
vm_offset_t vaddr,
}
static void
-free_bounce_page(bus_dma_tag_t dmat, struct bounce_page *bpage)
+free_bounce_pages(bus_dma_tag_t dmat, bus_dmamap_t map)
{
...
mtx_lock(&bounce_lock);
- STAILQ_INSERT_HEAD(&bz->bounce_page_list, bpage, links);
...
+ STAILQ_CONCAT(&bz->bounce_page_list, &map->bpages);
This changes an aspect we had discussed in a different context of a
change I had proposed to keep the pages ordered in order to fullfill
multi-page nseg=1 requests better.
You could fix that by doing two CONCAT operations perhaps:
STAILQ_CONCAT(&map->bpages, &bz->bounce_page_list);
STAILQ_CONCAT(&bz->bounce_page_list, &map->bpages);
You'd definitely need a comment explaining why and ideally some use
case that shows a benefit. That said, STAILQ_CONCAT are O(1) operations
that I think are 4 assignments each, so relatively cheap.
In the new case your "possibly still cache hot" pages are no longer
at the beginning of the bounce_page_list but at the end?
If this is no longer considered to be an issue, contigous page space
still is, so I wondere if a change to keep the pages ordered would be
accepted more likely now?
FWIW, this change actually keeps multi-segment page batches more
in order as they would retain their relative order in bz->bounce_page_list
on free rather than being reversed.
--
John Baldwin
