On Fri, 2023-03-24 at 00:48 +0100, Thomas Gleixner wrote: > On Thu, Mar 23 2023 at 22:49, David Woodhouse wrote: > > On Thu, 2023-03-23 at 23:36 +0100, Thomas Gleixner wrote: > > > There is no point in special casing this. All architectures can invoke > > > the CPUHP_BP_* states before CPUHP_BRINGUP_CPU for each to be brought up > > > CPU first. So this can be made unconditional and common exercised code. > > > > > > > There were three paragraphs in the commit message explaining why I > > didn't want to do that. It didn't work for x86 before I started, and I > > haven't reviewed *every* other architecture to ensure that it will work > > there. It was opt-in for a reason. :) > > I noticed myself before reading your reply :) > > > > Aside of that this dynamic state range is pretty pointless. There is > > > really nothing dynamic here and there is no real good reason to have > > > four dynamic parallel states just because. > > > > The original patch set did use more than one state; the plan to do > > microcode updates in parallel does involve doing at least one more, I > > believe. > > I don't think so. The micro code muck can completely serialize itself > and does not need control CPU assistance if done right. If we go there > we have to fix that mess and not just proliferatng it with moar duct tape. > > > > + /* > > > + * Fully per AP serialized bringup from here on. If the > > > + * architecture does no register the CPUHP_BP_PARALLEL_STARTUP > > > + * state, this step sends the startup IPI first. > > > + */ > > > > Not sure I'd conceded that yet; the APs do their *own* bringup from > > here, and that really ought to be able to run in parallel. > > Somewhere in the distance future once we've > > 1) sorted the mandatory synchronization points, e.g. TSC sync in the > early bootup phase. That's why we have four of them... :) > 2) audited _all_ AP state callbacks that they are able to cope with > parallel bringup. > > That's a long road as there are tons of assumptions about the > implicit CPU hotplug serialization in those callbacks. > > Just because it did not explode in your face yet does not mean this > is safe. > > I just looked at 10 randomly picked AP online callbacks and found 5 > of them being not ready :) Oh, it's totally hosed, absolutely. I don't think even my most ambitious hacks had even tried it yet. But I want to, so I wasn't about to add a comment saying the opposite. But it's fine; removing the comment is the *least* of the work to be done in making that bit actually work in parallel :)
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