Date: Fri, 6 May 2022 08:59:41 -0700 From: John Baldwin <jhb@FreeBSD.org> To: "Bjoern A. Zeeb" <bz@FreeBSD.org> Cc: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: Re: git: 92e40a9b9241 - main - busdma_bounce: Batch bounce page free operations when possible. Message-ID: <f15da69b-bb1f-78ac-5ec0-012b42ee1efc@FreeBSD.org> In-Reply-To: <alpine.BSF.2.00.2205061153080.68830@ai.fobar.qr> References: <202204211902.23LJ2bug035892@gitrepo.freebsd.org> <alpine.BSF.2.00.2205061153080.68830@ai.fobar.qr>
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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 <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. 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
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