On 2/9/24 19:52, Daniel Vetter wrote:
On Fri, Feb 09, 2024 at 06:41:32PM +0100, Danilo Krummrich wrote:
On 2/6/24 15:03, Daniel Vetter wrote:
On Mon, Feb 05, 2024 at 11:00:04PM +0100, Danilo Krummrich wrote:
On 2/5/24 22:08, Dave Airlie wrote:
On Tue, 6 Feb 2024 at 02:22, Danilo Krummrich <dakr@xxxxxxxxxx> wrote:
On 1/29/24 02:50, Dave Airlie wrote:
From: Dave Airlie <airlied@xxxxxxxxxx>
This should break the deadlock between the fctx lock and the irq lock.
This offloads the processing off the work from the irq into a workqueue.
Signed-off-by: Dave Airlie <airlied@xxxxxxxxxx>
Nouveau's scheduler uses a dedicated wq, hence from this perspective it's
safe deferring fence signalling to the kernel global wq. However, I wonder
if we could create deadlocks by building dependency chains into other
drivers / kernel code that, by chance, makes use of the kernel global wq as
well.
Admittedly, even if, it's gonna be extremely unlikely given that
WQ_MAX_ACTIVE == 512. But maybe it'd be safer to use a dedicated wq.
Also, do we need to CC stable?
I pushed this to Linus at the end of last week, since the hangs in 6.7
take out the complete system and I wanted it in stable.
It might be safer to use a dedicated wq, is the concern someone is
waiting on a fence in a workqueue somewhere else so we will never
signal it?
Yes, if some other work is waiting for this fence (or something else in the same
dependency chain) to signal it can prevent executing the work signaling this fence,
in case both are scheduled on the same wq. As mentioned, with the kernel global wq
this would be rather unlikely to lead to an actual stall with WQ_MAX_ACTIVE == 512,
but formally the race condition exists. I guess a malicious attacker could try to
intentionally push jobs directly or indirectly depending on this fence to a driver
which queues them up on a scheduler using the kernel global wq.
I think if you add dma_fence_signalling annotations (aside, there's some
patch from iirc Thomas Hellstrom to improve them and cut down on some
false positives, but I lost track) then I think you won't get any splats
because the wq subsystem assumes that WC_MAX_ACTIVE is close enough to
infinity to not matter.
As mentioned, for the kernel global wq it's 512. (Intentionally) feeding the kernel
with enough jobs to to provoke a deadlock doesn't seem impossible to me.
I think it'd be safer to just establish not to use the kernel global wq for executing
work in the fence signalling critical path.
We could also run into similar problems with a dedicated wq, e.g. when drivers share
a wq between drm_gpu_scheduler instances (see [1]), however, I'm not sure we can catch
that with lockdep.
I think if you want to fix it perfectly you'd need to set the max number
of wq to the number of engines (or for dynamic/fw scheduled engines to the
number of context) you have. Or whatever limit to the number of parallel
timelines there is.>
I guess this would need a new wq function to update? drm/sched code could
be able to set that for drivers, so drivers cannot get this wrong.
Not sure I can follow. The scheduler instance might be per context and bind
queue. In this case it gets the shared wq passed, but doesn't know how many
other scheduler instances share the same one.
Additionally, there might be drivers not using the DRM scheduler for for bind
queues at all (I think Xe does not).
If we don't do something like that then I'm not sure there's really much
benefit - instead of carefully timing 512 dma_fence on the global wq you
just need to create a pile of context (at least on intel's guc that's
absolutely no issue) and then careful time them.
Well, that's true. I'd still argue that there is a slight difference. From a
drivers isolated perspective using the global kernel wq might be entirely fine,
as in this patch. However, in combination with another driver doing the same
thing, things can blow up. For the case you illustrated it's at least possible
to spot it from a driver's perspective.
I still feel like we have bigger fish to fry ... But might be worth the
effort to at least make the parallel timeline limit a lot more explicit?
I agree, and it'd be great if we can find a solution such bugs can be detected
systematically (e.g. through lockdep), but maybe we can start to at least
document that we should never use the kernel global wq and where we need to be
careful in sharing driver wqs.
- Danilo
Cheers, Sima
[1] https://elixir.bootlin.com/linux/v6.8-rc3/source/drivers/gpu/drm/nouveau/nouveau_drm.c#L313
I'm not sure we should care differently, but I guess it'd be good to
annotate it all in case the wq subsystem's idea of how much such deadlocks
are real changes.
Also Teo is on a mission to get rid of all the global wq flushes, so there
should also be no source of deadlocks from that kind of cross-driver
dependency. Or at least shouldn't be in the future, I'm not sure it all
landed.
-Sima
