Re: git: 92e40a9b9241 - main - busdma_bounce: Batch bounce page free operations when possible.

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 <jhb@FreeBSD.org>
> AuthorDate: 2022-04-21 19:01:55 +0000
> Commit:     John Baldwin <jhb@FreeBSD.org>
> 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.

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?


...
> -static void
> -free_bounce_pages(bus_dma_tag_t dmat, bus_dmamap_t map)
> -{
> -	struct bounce_page *bpage;
> -
> -	while ((bpage = STAILQ_FIRST(&map->bpages)) != NULL) {
> -		STAILQ_REMOVE_HEAD(&map->bpages, links);
> -		free_bounce_page(dmat, bpage);
> -	}
> -}

-- 
Bjoern A. Zeeb                                                     r15:7