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To:     Jason Gunthorpe <jgg@ziepe.ca>
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From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <ab49e8a7-8e81-29bb-250f-b395942b4da1@deltatee.com>
Date:   Fri, 1 Oct 2021 17:34:30 -0600
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Subject: Re: [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap_p2pmem()
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On 2021-10-01 4:46 p.m., 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.
> 
>> 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.

Ah, fair. This adds a bit of complexity, but we could do a call_rcu() in
 vma_close to do the page frees.

Logan