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Fri, 5 Dec 2025 13:15:01 +0000 Message-ID: Date: Fri, 5 Dec 2025 14:14:56 +0100 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/4] dma-buf/fence: give some reasonable maximum signaling timeout To: Lucas Stach , phasta@kernel.org, alexdeucher@gmail.com, simona.vetter@ffwll.ch, faith@gfxstrand.net, sumit.semwal@linaro.org Cc: linaro-mm-sig@lists.linaro.org, dri-devel@lists.freedesktop.org References: <20251120150018.27385-1-christian.koenig@amd.com> <20251120150018.27385-2-christian.koenig@amd.com> <380012b9d6f0e9ee3c2f125cfe2f37f65c1979e0.camel@mailbox.org> <1c1a14d42d0a4a25ebce26a2af0a61dc1b7813fc.camel@mailbox.org> <508ff709-0f05-4982-8e15-5fea3bbd12e7@amd.com> <52d484c5-6dfb-4e2f-9caa-a61cf1d94801@amd.com> <207d50fe-eef3-4baa-97a7-567598e56b55@amd.com> <54d6e863fc606d22f245e30012bb5120d5bee7ee.camel@mailbox.org> <6151e7c5-1de2-4857-ae64-1e2fd6cb7513@amd.com> <5267e8fbb93a8fe722f845331d17c61e29f1a705.camel@pengutronix.de> Content-Language: en-US From: =?UTF-8?Q?Christian_K=C3=B6nig?= In-Reply-To: <5267e8fbb93a8fe722f845331d17c61e29f1a705.camel@pengutronix.de> Content-Type: text/plain; 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And eliminating many >> other problems we know well from drm/sched. > > It doesn't really matter if you have firmware scheduling or not for > preemption to be a hard problem on GPUs. CPUs have limited software > visible state that needs to be saved/restored on a context switch and > even there people start complaining now that they need to context > switch the AVX512 register set. Yeah, that has been discussed for the last 20 years or so when the first MMX extension came out. > GPUs have megabytes of software visible state. Which needs to be > saved/restored on the context switch if you want fine grained > preemption with low preemption latency. There might be points in the > command execution where you can ignore most of that state, but reaching > those points can have basically unbounded latency. So either you can > reliably save/restore lots of state or you are limited to very coarse > grained preemption with all the usual issues of timeouts and DoS > vectors. > I'm not totally up to speed with the current state across all relevant > GPUs, but until recently NVidia was the only vendor to have real > reliable fine-grained preemption. Completely agree. You won't believe how often that is a topic in discussions. AMD has Compute Wave Save Restore now on newer HW, but both the reliability and performance are unfortunately questionable at best. Regards, Christian. > > Regards, > Lucas > >