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 <npiggin@xxxxxxxxx> >> AuthorDate: Thu, 11 Apr 2019 13:34:47 +1000 >> Committer: Ingo Molnar <mingo@xxxxxxxxxx> >> 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. Thanks, Nick
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