On Sat, May 04, 2019 at 04:59:12PM +1000, Nicholas Piggin wrote: > Frederic Weisbecker's on May 4, 2019 10:27 am: > > On Fri, May 03, 2019 at 10:47:37AM -0700, tip-bot for Nicholas Piggin wrote: > >> Commit-ID: 9219565aa89033a9cfdae788c1940473a1253d6c > >> Gitweb: > >> https://git.kernel.org/tip/9219565aa89033a9cfdae788c1940473a1253d6c > >> Author: Nicholas Piggin <npig...@gmail.com> > >> AuthorDate: Thu, 11 Apr 2019 13:34:47 +1000 > >> Committer: Ingo Molnar <mi...@kernel.org> > >> CommitDate: Fri, 3 May 2019 19:42:58 +0200 > >> > >> sched/isolation: Require a present CPU in housekeeping mask > >> > >> During housekeeping mask setup, currently a possible CPU is required. > >> That does not guarantee the CPU would be available at boot time, so > >> check to ensure that at least one present CPU is in the mask. > > > > I have a doubt about the requirements and semantics of cpu_present_mask. > > IIUC a present CPU means that it is physically plugged in (from ACPI > > perspective) but might not be logically plugged in (set on cpu_online_mask). > > Right, a superset of cpu_possible_mask, subset of cpu_online_mask. It > means that CPU can be brought online at any time. > > > But do we have the guarantee that a present CPU _will_ be online at least > > once > > right after the boot? After all, kernel parameters such as "maxcpus=" can > > prevent > > from turning some CPUs on. I guess there are even more creative ways to > > achieve > > that. > > > > In any case we really require the housekeeper to be forced online. Perhaps > > I missed that enforcement somewhere in the patchset? > > No I think you're right, that may be able to boot without anything in > the housekeeping mask. Maybe we can just cpu_up() a CPU in the > housekeeping mask with a warning that it has overidden their SMP > command line option. I'll take a look at it.
But then what if cpu_up() fails? In this case I can think of only two answers: * Force the boot CPU as the housekeeper. * Rollback the whole thing: nohz and all isolation. The second solution looks sane to me. After all if the user doesn't include CPU 0 in the housekeeping set, forcing it isn't going to help much. But that means we must enhance the isolation code (nohz included) to be able to dynamically add/del CPUs to the houseeeping/isolation set. That's not going to be easy but it's a necessary evolution of that subsystem since we want to drive it through cpusets. I should start working on that.
