linux-kernel - Re: Enabling peer to peer device transactions for PCIe devices

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Date:   Fri, 25 Nov 2016 15:51:15 -0500
From:   Felix Kuehling <felix.kuehling@....com>
To:     Christian König <deathsimple@...afone.de>,
        Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        Christian König <christian.koenig@....com>
CC:     Haggai Eran <haggaie@...lanox.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
        Serguei Sagalovitch <serguei.sagalovitch@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "Blinzer, Paul" <Paul.Blinzer@....com>,
        "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        Dan Williams <dan.j.williams@...el.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        "Sander, Ben" <ben.sander@....com>,
        "Linux-media@...r.kernel.org" <Linux-media@...r.kernel.org>
Subject: Re: Enabling peer to peer device transactions for PCIe devices


On 16-11-25 03:40 PM, Christian König wrote:
> Am 25.11.2016 um 20:32 schrieb Jason Gunthorpe:
>> This assumes the commands are fairly short lived of course, the
>> expectation of the mmu notifiers is that a flush is reasonably prompt
>
> Correct, this is another problem. GFX command submissions usually
> don't take longer than a few milliseconds, but compute command
> submission can easily take multiple hours.
>
> I can easily imagine what would happen when kswapd is blocked by a GPU
> command submission for an hour or so while the system is under memory
> pressure :)
>
> I'm thinking on this problem for about a year now and going in circles
> for quite a while. So if you have ideas on this even if they sound
> totally crazy, feel free to come up.

Our GPUs (at least starting with VI) support compute-wave-save-restore
and can swap out compute queues with fairly low latency. Yes, there is
some overhead (both memory usage and time), but it's a fairly regular
thing with our hardware scheduler (firmware, actually) when we need to
preempt running compute queues to update runlists or we overcommit the
hardware queue resources.

Regards,
  Felix