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From: Jason Gunthorpe <jgg@ziepe.ca>
To: Logan Gunthorpe <logang@deltatee.com>
Cc: "Alistair Popple" <apopple@nvidia.com>,
	"Felix Kuehling" <Felix.Kuehling@amd.com>,
	"Christoph Hellwig" <hch@lst.de>,
	"Dan Williams" <dan.j.williams@intel.com>,
	linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
	linux-block@vger.kernel.org, linux-pci@vger.kernel.org,
	linux-mm@kvack.org, iommu@lists.linux-foundation.org,
	"Stephen Bates" <sbates@raithlin.com>,
	"Christian König" <christian.koenig@amd.com>,
	"John Hubbard" <jhubbard@nvidia.com>,
	"Don Dutile" <ddutile@redhat.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Daniel Vetter" <daniel.vetter@ffwll.ch>,
	"Jakowski Andrzej" <andrzej.jakowski@intel.com>,
	"Minturn Dave B" <dave.b.minturn@intel.com>,
	"Jason Ekstrand" <jason@jlekstrand.net>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Xiong Jianxin" <jianxin.xiong@intel.com>,
	"Bjorn Helgaas" <helgaas@kernel.org>,
	"Ira Weiny" <ira.weiny@intel.com>,
	"Robin Murphy" <robin.murphy@arm.com>,
	"Martin Oliveira" <martin.oliveira@eideticom.com>,
	"Chaitanya Kulkarni" <ckulkarnilinux@gmail.com>
Subject: Re: [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap_p2pmem()
Date: Fri, 1 Oct 2021 19:46:05 -0300	[thread overview]
Message-ID: <20211001224605.GS3544071@ziepe.ca> (raw)
In-Reply-To: <8871549c-63b5-d062-87ea-9036605984d5@deltatee.com>

On Fri, Oct 01, 2021 at 04:22:28PM -0600, Logan Gunthorpe wrote:

> > It would close this issue, however synchronize_rcu() is very slow
> > (think > 1second) in some cases and thus cannot be inserted here.
> 
> It shouldn't be *that* slow, at least not the vast majority of the
> time... it seems a bit unreasonable that a CPU wouldn't schedule for
> more than a second. 

I've seen bug reports on exactly this, it is well known. Loaded
big multi-cpu systems have high delays here, for whatever reason.

> But these aren't fast paths and synchronize_rcu() already gets
> called in the unbind path for p2pdma a of couple times. I'm sure it
> would also be fine to slow down the vma_close() path as well.

vma_close is done in a loop destroying vma's and if each synchronize
costs > 1s it can take forever to close a process. We had to kill a
similar use of synchronize_rcu in RDMA because users were complaining
of > 40s process exit times.

The driver unload path is fine to be slow, and is probably done on an
unloaded system where synchronize_rcu is not so bad

Anyway, it is not really something for this series to fix, just
something we should all be aware of and probably ought to get fixed
before we do much more with ZONE_DEVICE pages

Jason

  reply	other threads:[~2021-10-01 22:47 UTC|newest]

Thread overview: 87+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-16 23:40 [PATCH v3 00/20] Userspace P2PDMA with O_DIRECT NVMe devices Logan Gunthorpe
2021-09-16 23:40 ` [PATCH v3 01/20] lib/scatterlist: add flag for indicating P2PDMA segments in an SGL Logan Gunthorpe
2021-09-28 18:32   ` Jason Gunthorpe
2021-09-29 21:15     ` Logan Gunthorpe
2021-09-30  4:47   ` Chaitanya Kulkarni
2021-09-30 16:49     ` Logan Gunthorpe
2021-09-30  4:57   ` Chaitanya Kulkarni
2021-09-16 23:40 ` [PATCH v3 02/20] PCI/P2PDMA: attempt to set map_type if it has not been set Logan Gunthorpe
2021-09-27 18:50   ` Bjorn Helgaas
2021-09-16 23:40 ` [PATCH v3 03/20] PCI/P2PDMA: make pci_p2pdma_map_type() non-static Logan Gunthorpe
2021-09-27 18:46   ` Bjorn Helgaas
2021-09-28 18:48   ` Jason Gunthorpe
2021-09-16 23:40 ` [PATCH v3 04/20] PCI/P2PDMA: introduce helpers for dma_map_sg implementations Logan Gunthorpe
2021-09-27 18:53   ` Bjorn Helgaas
2021-09-27 19:59     ` Logan Gunthorpe
2021-09-28 18:55   ` Jason Gunthorpe
2021-09-29 21:26     ` Logan Gunthorpe
2021-09-28 22:05   ` [PATCH v3 4/20] " Jason Gunthorpe
2021-09-29 21:30     ` Logan Gunthorpe
2021-09-29 22:46       ` Jason Gunthorpe
2021-09-29 23:00         ` Logan Gunthorpe
2021-09-29 23:40           ` Jason Gunthorpe
2021-09-16 23:40 ` [PATCH v3 05/20] dma-mapping: allow EREMOTEIO return code for P2PDMA transfers Logan Gunthorpe
2021-09-28 18:57   ` Jason Gunthorpe
2021-09-16 23:40 ` [PATCH v3 06/20] dma-direct: support PCI P2PDMA pages in dma-direct map_sg Logan Gunthorpe
2021-09-28 19:08   ` Jason Gunthorpe
2021-09-16 23:40 ` [PATCH v3 07/20] dma-mapping: add flags to dma_map_ops to indicate PCI P2PDMA support Logan Gunthorpe
2021-09-28 19:11   ` Jason Gunthorpe
2021-09-16 23:40 ` [PATCH v3 08/20] iommu/dma: support PCI P2PDMA pages in dma-iommu map_sg Logan Gunthorpe
2021-09-28 19:15   ` Jason Gunthorpe
2021-09-16 23:40 ` [PATCH v3 09/20] nvme-pci: check DMA ops when indicating support for PCI P2PDMA Logan Gunthorpe
2021-09-30  5:06   ` Chaitanya Kulkarni
2021-09-30 16:51     ` Logan Gunthorpe
2021-09-30 17:19       ` Chaitanya Kulkarni
2021-09-16 23:40 ` [PATCH v3 10/20] nvme-pci: convert to using dma_map_sgtable() Logan Gunthorpe
2021-10-05 22:29   ` Max Gurtovoy
2021-09-16 23:40 ` [PATCH v3 11/20] RDMA/core: introduce ib_dma_pci_p2p_dma_supported() Logan Gunthorpe
2021-09-28 19:17   ` Jason Gunthorpe
2021-10-05 22:31   ` Max Gurtovoy
2021-09-16 23:40 ` [PATCH v3 12/20] RDMA/rw: use dma_map_sgtable() Logan Gunthorpe
2021-09-28 19:43   ` Jason Gunthorpe
2021-09-29 22:56     ` Logan Gunthorpe
2021-10-05 22:40     ` Max Gurtovoy
2021-09-16 23:40 ` [PATCH v3 13/20] PCI/P2PDMA: remove pci_p2pdma_[un]map_sg() Logan Gunthorpe
2021-09-27 18:50   ` Bjorn Helgaas
2021-09-28 19:43   ` Jason Gunthorpe
2021-10-05 22:42   ` Max Gurtovoy
2021-09-16 23:40 ` [PATCH v3 14/20] mm: introduce FOLL_PCI_P2PDMA to gate getting PCI P2PDMA pages Logan Gunthorpe
2021-09-28 19:47   ` Jason Gunthorpe
2021-09-29 21:34     ` Logan Gunthorpe
2021-09-29 22:48       ` Jason Gunthorpe
2021-09-16 23:40 ` [PATCH v3 15/20] iov_iter: introduce iov_iter_get_pages_[alloc_]flags() Logan Gunthorpe
2021-09-16 23:40 ` [PATCH v3 16/20] block: set FOLL_PCI_P2PDMA in __bio_iov_iter_get_pages() Logan Gunthorpe
2021-09-16 23:40 ` [PATCH v3 17/20] block: set FOLL_PCI_P2PDMA in bio_map_user_iov() Logan Gunthorpe
2021-09-16 23:40 ` [PATCH v3 18/20] mm: use custom page_free for P2PDMA pages Logan Gunthorpe
2021-09-16 23:40 ` [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap_p2pmem() Logan Gunthorpe
2021-09-27 18:49   ` Bjorn Helgaas
2021-09-28 19:55   ` Jason Gunthorpe
2021-09-29 21:42     ` Logan Gunthorpe
2021-09-29 23:05       ` Jason Gunthorpe
2021-09-29 23:27         ` Logan Gunthorpe
2021-09-29 23:35           ` Jason Gunthorpe
2021-09-29 23:49             ` Logan Gunthorpe
2021-09-30  0:36               ` Jason Gunthorpe
2021-10-01 13:48                 ` Jason Gunthorpe
2021-10-01 17:01                   ` Logan Gunthorpe
2021-10-01 17:45                     ` Jason Gunthorpe
2021-10-01 20:13                       ` Logan Gunthorpe
2021-10-01 22:14                         ` Jason Gunthorpe
2021-10-01 22:22                           ` Logan Gunthorpe
2021-10-01 22:46                             ` Jason Gunthorpe [this message]
2021-10-01 23:27                               ` John Hubbard
2021-10-01 23:34                               ` Logan Gunthorpe
2021-10-04  6:58                       ` Christian König
2021-10-04 13:11                         ` Jason Gunthorpe
2021-10-04 13:22                           ` Christian König
2021-10-04 13:27                             ` Jason Gunthorpe
2021-10-04 14:54                               ` Christian König
2021-09-28 20:05   ` Jason Gunthorpe
2021-09-29 21:46     ` Logan Gunthorpe
2021-09-16 23:41 ` [PATCH v3 20/20] nvme-pci: allow mmaping the CMB in userspace Logan Gunthorpe
2021-09-28 20:02 ` [PATCH v3 00/20] Userspace P2PDMA with O_DIRECT NVMe devices Jason Gunthorpe
2021-09-29 21:50   ` Logan Gunthorpe
2021-09-29 23:21     ` Jason Gunthorpe
2021-09-29 23:28       ` Logan Gunthorpe
2021-09-29 23:36         ` Jason Gunthorpe
2021-09-29 23:52           ` Logan Gunthorpe

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