Am 02.07.20 um 15:29 schrieb Jason Gunthorpe:
On Thu, Jul 02, 2020 at 03:10:00PM +0200, Daniel Vetter wrote:
On Wed, Jul 01, 2020 at 02:15:24PM -0300, Jason Gunthorpe wrote:
On Wed, Jul 01, 2020 at 05:42:21PM +0200, Daniel Vetter wrote:
All you need is the ability to stop wait for ongoing accesses to end and
make sure that new ones grab a new mapping.
Swap and flush isn't a general HW ability either..
I'm unclear how this could be useful, it is guarenteed to corrupt
in-progress writes?
Did you mean pause, swap and resume? That's ODP.
Yes, something like this. And good to know, never heard of ODP.
Hm I thought ODP was full hw page faults at an individual page
level,
Yes
and this stop&resume is for the entire nic. Under the hood both apply
back-pressure on the network if a transmission can't be received,
but
NIC's don't do stop and resume, blocking the Rx pipe is very
problematic and performance destroying.
The strategy for something like ODP is more complex, and so far no NIC
has deployed it at any granularity larger than per-page.
So since Jason really doesn't like dma_fence much I think for rdma
synchronous it is. And it shouldn't really matter, since waiting for a
small transaction to complete at rdma wire speed isn't really that
long an operation.
Even if DMA fence were to somehow be involved, how would it look?
Well above you're saying it would be performance destroying, but let's
pretend that's not a problem :-) Also, I have no clue about rdma, so this
is really just the flow we have on the gpu side.
I see, no, this is not workable, the command flow in RDMA is not at
all like GPU - what you are a proposing is a global 'stop the whole
chip' Tx and Rx flows for an undetermined time. Not feasible
What we can do is use ODP techniques and pause only the MR attached to
the DMA buf with the process you outline below. This is not so hard to
implement.
Well it boils down to only two requirements:
1. You can stop accessing the memory or addresses exported by the DMA-buf.
2. Before the next access you need to acquire a new mapping.
How you do this is perfectly up to you. E.g. you can stop everything,
just prevent access to this DMA-buf, or just pause the users of this
DMA-buf....
3. rdma driver worker gets busy to restart rx:
1. lock all dma-buf that are currently in use (dma_resv_lock).
thanks to ww_mutex deadlock avoidance this is possible
Why all? Why not just lock the one that was invalidated to restore the
mappings? That is some artifact of the GPU approach?
No, but you must make sure that mapping one doesn't invalidate others
you need.
Otherwise you can end up in a nice live lock :)
And why is this done with work queues and locking instead of a
callback saying the buffer is valid again?
You can do this as well, but a work queue is usually easier to handle
than a notification in an interrupt context of a foreign driver.
Regards,
Christian.
Jason
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