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

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Date:   Fri, 1 Oct 2021 16:27:43 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Jason Gunthorpe <jgg@...pe.ca>,
        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>,
        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 10/1/21 15:46, Jason Gunthorpe wrote:
> 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.
> 

So have I. One reason is that synchronize_rcu() doesn't merely wait
for a context switch on each CPU--it also waits for callbacks (such as
those set up by call_rcu(), if I understand correctly) to run.

These can really add up to something quite substantial. In fact, I don't
think there is an upper limit on the running times, anywhere.

thanks,
-- 
John Hubbard
NVIDIA