Date: Sat, 4 Aug 2018 17:47:33 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Justin Hibbits <jhibbits@freebsd.org> Cc: John Baldwin <jhb@freebsd.org>, src-committers <src-committers@freebsd.org>, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r337273 - head/sys/dev/nvme Message-ID: <20180804144733.GO6049@kib.kiev.ua> In-Reply-To: <CAHSQbTCE%2Bbngg0NpDB38b3HBH6zugykt9U4mFXNYJTSYHD4asQ@mail.gmail.com> References: <201808032004.w73K46XJ053249@repo.freebsd.org> <20180804080840.GI6049@kib.kiev.ua> <c931140b-0334-de19-35f1-5b00cd10c2d9@FreeBSD.org> <20180804130315.GK6049@kib.kiev.ua> <CAHSQbTCE%2Bbngg0NpDB38b3HBH6zugykt9U4mFXNYJTSYHD4asQ@mail.gmail.com>
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On Sat, Aug 04, 2018 at 08:29:47AM -0500, Justin Hibbits wrote: > On Sat, Aug 4, 2018, 08:03 Konstantin Belousov <kostikbel@gmail.com> wrote: > > > On Sat, Aug 04, 2018 at 05:14:31AM -0700, John Baldwin wrote: > > > On 8/4/18 1:08 AM, Konstantin Belousov wrote: > > > > On Fri, Aug 03, 2018 at 08:04:06PM +0000, Justin Hibbits wrote: > > > >> Author: jhibbits > > > >> Date: Fri Aug 3 20:04:06 2018 > > > >> New Revision: 337273 > > > >> URL: https://svnweb.freebsd.org/changeset/base/337273 > > > >> > > > >> Log: > > > >> nvme(4): Add bus_dmamap_sync() at the end of the request path > > > >> > > > >> Summary: > > > >> Some architectures, in this case powerpc64, need explicit > > synchronization > > > >> barriers vs device accesses. > > > >> > > > >> Prior to this change, when running 'make buildworld -j72' on a > > 18-core > > > >> (72-thread) POWER9, I would see controller resets often. With this > > change, I > > > >> don't see these resets messages, though another tester still does, > > for yet to be > > > >> determined reasons, so this may not be a complete fix. > > Additionally, I see a > > > >> ~5-10% speed up in buildworld times, likely due to not needing to > > reset the > > > >> controller. > > > >> > > > >> Reviewed By: jimharris > > > >> Differential Revision: https://reviews.freebsd.org/D16570 > > > >> > > > >> Modified: > > > >> head/sys/dev/nvme/nvme_qpair.c > > > >> > > > >> Modified: head/sys/dev/nvme/nvme_qpair.c > > > >> > > ============================================================================== > > > >> --- head/sys/dev/nvme/nvme_qpair.c Fri Aug 3 19:24:04 2018 > > (r337272) > > > >> +++ head/sys/dev/nvme/nvme_qpair.c Fri Aug 3 20:04:06 2018 > > (r337273) > > > >> @@ -401,9 +401,13 @@ nvme_qpair_complete_tracker(struct nvme_qpair > > *qpair, > > > >> req->retries++; > > > >> nvme_qpair_submit_tracker(qpair, tr); > > > >> } else { > > > >> - if (req->type != NVME_REQUEST_NULL) > > > >> + if (req->type != NVME_REQUEST_NULL) { > > > >> + bus_dmamap_sync(qpair->dma_tag_payload, > > > >> + tr->payload_dma_map, > > > >> + BUS_DMASYNC_POSTREAD | BUS_DMASYNC_POSTWRITE); > > > >> bus_dmamap_unload(qpair->dma_tag_payload, > > > >> tr->payload_dma_map); > > > >> + } > > > >> > > > >> nvme_free_request(req); > > > >> tr->req = NULL; > > > >> @@ -487,6 +491,8 @@ nvme_qpair_process_completions(struct nvme_qpair > > *qpai > > > >> */ > > > >> return (false); > > > >> > > > >> + bus_dmamap_sync(qpair->dma_tag, qpair->queuemem_map, > > > >> + BUS_DMASYNC_POSTREAD | BUS_DMASYNC_POSTWRITE); > > > >> while (1) { > > > >> cpl = qpair->cpl[qpair->cq_head]; > > > >> > > > >> @@ -828,7 +834,16 @@ nvme_qpair_submit_tracker(struct nvme_qpair > > *qpair, st > > > >> if (++qpair->sq_tail == qpair->num_entries) > > > >> qpair->sq_tail = 0; > > > >> > > > >> + bus_dmamap_sync(qpair->dma_tag, qpair->queuemem_map, > > > >> + BUS_DMASYNC_PREREAD | BUS_DMASYNC_PREWRITE); > > > >> +#ifndef __powerpc__ > > > >> + /* > > > >> + * powerpc's bus_dmamap_sync() already includes a heavyweight > > sync, but > > > >> + * no other archs do. > > > >> + */ > > > >> wmb(); > > > >> +#endif > > > > What is the purpose of this call ? It is useless without paired read > > > > barrier. So where is the reciprocal rmb() ? > > > > > > For DMA, the rmb is in the device controller. However, architectures > > > that need this kind of ordering should do it in their bus_dmmap_sync op, > > > and this explicit one needs to be removed. (Alpha had a wmb in its > > > bus_dmamap_sync op for this reason.) > > Yes, if something special is needed, it should happen in platform-specific > > busdma code. > > > > Also, if wmb() is needed, then it is not a supposed semantic or > > wmb(), but a specific side-effects of one of the instruction in the > > implementation of wmb(). > > > > As I noted, on x86 it is not needed and detrimental to the performance. > > > > According to Jim Harris it is needed for x86. I tried to remove it thinking > it was unnecessary with the sync in place now. The wmb() was added way > back in r240616 by him, when only x86 was supported by the driver. > bus_dmamap_sync() for all archs except powerpc lack a barrier, from what I > can see. See the review for more discussion that took place. If it isn't > needed I will gladly remove it. I am very curious why x86 would need MFENCE there. I do not see an explanation in the review.
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