[Mesa-dev] [RFC] Linux Graphics Next: Explicit fences everywhere and no BO fences - initial proposal

[Christian König]

Am 20.04.21 um 17:07 schrieb Daniel Stone:
> On Tue, 20 Apr 2021 at 15:58, Christian König 
> <ckoenig.leichtzumerken at gmail.com 
> <mailto:ckoenig.leichtzumerken at gmail.com>> wrote:
>
>     Am 20.04.21 um 16:53 schrieb Daniel Stone:
>>     On Mon, 19 Apr 2021 at 11:48, Marek Olšák <maraeo at gmail.com
>>     <mailto:maraeo at gmail.com>> wrote:
>>
>>         Deadlock mitigation to recover from segfaults:
>>         - The kernel knows which process is obliged to signal which
>>         fence. This information is part of the Present request and
>>         supplied by userspace.
>>         - If the producer crashes, the kernel signals the submit
>>         fence, so that the consumer can make forward progress.
>>         - If the consumer crashes, the kernel signals the return
>>         fence, so that the producer can reclaim the buffer.
>>         - A GPU hang signals all fences. Other deadlocks will be
>>         handled like GPU hangs.
>>
>>
>>     Another thought: with completely arbitrary userspace fencing,
>>     none of this is helpful either. If the compositor can't guarantee
>>     that a hostile client has submitted a fence which will never be
>>     signaled, then it won't be waiting on it, so it already needs
>>     infrastructure to handle something like this.
>
>>     That already handles the crashed-client case, because if the
>>     client crashes, then its connection will be dropped, which will
>>     trigger the compositor to destroy all its resources anyway,
>>     including any pending waits.
>
>     Exactly that's the problem. A compositor isn't immediately
>     informed that the client crashed, instead it is still referencing
>     the buffer and trying to use it for compositing.
>
>
> If the compositor no longer has a guarantee that the buffer will be 
> ready for composition in a reasonable amount of time (which dma_fence 
> gives us, and this proposal does not appear to give us), then the 
> compositor isn't trying to use the buffer for compositing, it's 
> waiting asynchronously on a notification that the fence has signaled 
> before it attempts to use the buffer.
>
> Marek's initial suggestion is that the kernel signal the fence, which 
> would unblock composition (and presumably show garbage on screen, or 
> at best jump back to old content).
>
> My position is that the compositor will know the process has crashed 
> anyway - because its socket has been closed - at which point we 
> destroy all the client's resources including its windows and buffers 
> regardless. Signaling the fence doesn't give us any value here, 
> _unless_ the compositor is just blindly waiting for the fence to 
> signal ... which it can't do because there's no guarantee the fence 
> will ever signal.

Yeah, but that assumes that the compositor has change to not blindly 
wait for the client to finish rendering and as Daniel explained that is 
rather unrealistic.

What we need is a fallback mechanism which signals the fence after a 
timeout and gives a penalty to the one causing the timeout.

That gives us the same functionality we have today with the in software 
scheduler inside the kernel.

Regards,
Christian.

> Cheers,
> Daniel

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