Re: CONFIG_ISOLATION=y (was: [PATCH 0/6] support "dataplane" mode for nohz_full)
From: Frederic Weisbecker <hidden>
Date: 2015-05-12 15:36:39
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On Tue, May 12, 2015 at 02:34:40PM +0200, Ingo Molnar wrote:
* Peter Zijlstra [off-list ref] wrote:quoted
On Tue, May 12, 2015 at 11:10:32AM +0200, Ingo Molnar wrote:quoted
So I'd vote for Frederic's CONFIG_ISOLATION=y, mostly because this is a high level kernel feature, so it won't conflict with isolation concepts in lower level subsystems such as IOMMU isolation - and other higher level features like scheduler isolation are basically another partial implementation we want to merge with all this...But why do we need a CONFIG flag for something that has no content? That is, I do not see anything much; except the 'I want to stay in userspace and kill me otherwise' flag, and I'm not sure that warrants a CONFIG flag like this. Other than that, its all a combination of NOHZ_FULL and cpusets/isolcpus and whatnot.Yes, that's what I meant: CONFIG_ISOLATION would trigger what is NO_HZ_FULL today - we could possibly even remove CONFIG_NO_HZ_FULL as an individual Kconfig option?
Right, we could return to what we had previously: CONFIG_NO_HZ. A config that enables dynticks-idle by default and allows full dynticks if nohz_full= boot option is passed (or something driven by higher level isolation interface). Because eventually, distros enable NO_HZ_FULL so that their 0.0001% users can use it. Well at least Red Hat does.
CONFIG_ISOLATION=y would express the guarantee from the kernel that it's possible for user-space to configure itself to run undisturbed - instead of the current inconsistent set of options and facilities. A bit like CONFIG_PREEMPT_RT is more than just preemptable spinlocks, it also tries to offer various facilities and tune the defaults to turn the kernel hard-rt. Does that make sense to you?
Right although distros tend to want features to be enabled dynamically so that they have a single kernel to maintain. Things like PREEMPT_RT really need to be a different kernel because fundamental primitives like spinlocks must be implemented statically. But isolation can be a boot-enabled, or even runtime-enabled, as it's only about timer,irq,task affinity. Full Nohz is more complicated but it can be runtime toggled in the future. So we can bring CONFIG_CPU_ISOLATION, at least for distros that are really not interested in that so they can disable it. CONFIG_CPU_ISOLATION=y would bring an ability which is default-disabled and driven dynamically through whatever interface.