linux-kernel - Re: [PATCH v4 04/40] drm/sched: Add enqueue credit limit

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Message-ID: <aCY42rgJC4sQ4tp4@pollux>
Date: Thu, 15 May 2025 20:56:26 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Rob Clark <robdclark@...il.com>
Cc: Connor Abbott <cwabbott0@...il.com>, Rob Clark <robdclark@...omium.org>,
	phasta@...nel.org, dri-devel@...ts.freedesktop.org,
	freedreno@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
	Matthew Brost <matthew.brost@...el.com>,
	Christian König <ckoenig.leichtzumerken@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	open list <linux-kernel@...r.kernel.org>,
	Boris Brezillon <boris.brezillon@...labora.com>
Subject: Re: [PATCH v4 04/40] drm/sched: Add enqueue credit limit

On Thu, May 15, 2025 at 10:40:15AM -0700, Rob Clark wrote:
> On Thu, May 15, 2025 at 10:30 AM Danilo Krummrich <dakr@...nel.org> wrote:
> >
> > (Cc: Boris)
> >
> > On Thu, May 15, 2025 at 12:22:18PM -0400, Connor Abbott wrote:
> > > For some context, other drivers have the concept of a "synchronous"
> > > VM_BIND ioctl which completes immediately, and drivers implement it by
> > > waiting for the whole thing to finish before returning.
> >
> > Nouveau implements sync by issuing a normal async VM_BIND and subsequently
> > waits for the out-fence synchronously.
> 
> As Connor mentioned, we'd prefer it to be async rather than blocking,
> in normal cases, otherwise with drm native context for using native
> UMD in guest VM, you'd be blocking the single host/VMM virglrender
> thread.
> 
> The key is we want to keep it async in the normal cases, and not have
> weird edge case CTS tests blow up from being _too_ async ;-)

I really wonder why they don't blow up in Nouveau, which also support full
asynchronous VM_BIND. Mind sharing which tests blow up? :)

> > > But this
> > > doesn't work for native context, where everything has to be
> > > asynchronous, so we're trying a new approach where we instead submit
> > > an asynchronous bind for "normal" (non-sparse/driver internal)
> > > allocations and only attach its out-fence to the in-fence of
> > > subsequent submits to other queues.
> >
> > This is what nouveau does and I think other drivers like Xe and panthor do this
> > as well.
> 
> No one has added native context support for these drivers yet

Huh? What exactly do you mean with "native context" then?

> > > Once you do this then you need a
> > > limit like this to prevent memory usage from pending page table
> > > updates from getting out of control. Other drivers haven't needed this
> > > yet, but they will when they get native context support.
> >
> > What are the cases where you did run into this, i.e. which application in
> > userspace hit this? Was it the CTS, some game, something else?
> 
> CTS tests that do weird things with massive # of small bind/unbind.  I
> wouldn't expect to hit the blocking case in the real world.

As mentioned above, can you please share them? I'd like to play around a bit. :)

- Danilo