Re: Summary of the Arm ports BoF at DC17
- Date: Sun, 17 Sep 2017 22:13:37 +0100
- From: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
- Subject: Re: Summary of the Arm ports BoF at DC17
On Sun, 2017-09-17 at 12:59 -0700, Vagrant Cascadian wrote: > On 2017-09-15, Ben Hutchings wrote: > > On Thu, 2017-09-14 at 03:40 +0100, Steve McIntyre wrote: > > [...] > > > There is optional kernel support to trap the exceptions here > > > and emulate the instructions, but it's really not recommended for > > > serious use (e.g. on a build machine!). > > > > [...] > > > > Why is it not recommended? Terrible performance, or known bugs? > > On arm64 kernels building armhf packages for the reproducible builds > builders, I'm seeing hundreds or even thousands of kernel log warnings > per second when building anything that makes use of > CP15_BARRIER_EMULATION (e.g. anything using ghc): > > [85740.553537] cp15barrier_handler: 115316 callbacks suppressed > [85740.559358] "ghc-pkg" (29572) uses deprecated CP15 Barrier instruction at 0xf5beaa7c > [85740.567344] "ghc-pkg" (29572) uses deprecated CP15 Barrier instruction at 0xf5beaa9c > > That *might* be sufficient to actually impact performance; it certainly > makes it hard to read other kernel log messages on those machines... The log rate-limiting is done in the kernel, so the log daemon doesn't have to process all those messages. However, the rate-limiting state is protected with a mutex (actually a spin-lock) so even when the log message is suppressed it involves an additional two memory barriers. And I agree that this kind of rate-limiting isn't really sufficient. Warning once per process might be more reasonable, but there's currently no mechanism for that in the kernel. Ben. > There was a bug reported on ghc related to this: > > https://bugs.debian.org/864847 > > > live well, > vagrant -- Ben Hutchings friends: People who know you well, but like you anyway.
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