linux-kernel - Re: [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap

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Date:   Fri, 1 Oct 2021 19:46:05 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Alistair Popple <apopple@...dia.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Stephen Bates <sbates@...thlin.com>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Martin Oliveira <martin.oliveira@...eticom.com>,
        Chaitanya Kulkarni <ckulkarnilinux@...il.com>
Subject: Re: [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap_p2pmem()

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