Date: Fri, 02 Nov 2007 11:34:38 +1000 From: Glen <glen.leeder@nokia.com> To: ext Nate Lawson <nate@root.org> Cc: ACPI mailing list <freebsd-acpi@freebsd.org> Subject: Re: SMP system shutdown hang (acpi_cpu_shutdown - smp_rendezvous) Message-ID: <472A7EAE.6050608@nokia.com> In-Reply-To: <472A72AB.4000809@root.org> References: <472A53B2.6030901@nokia.com> <472A72AB.4000809@root.org>
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ext Nate Lawson wrote: > Glen wrote: > >> Hi, >> >> I have been seeing intermittent hangs in the acpi shutdown code on a >> Intel 2.4GHz 8 CPU system. I am running a with a Freebsd6.1 code base >> but cannot see a reason why this can't happen in other Freebsd versions. >> The hang is very irregular, I am recreating it using an expect script >> that repeatedly reboots the system. Sometimes, I can do up to 200 >> reboots before observing the hang, sometimes, it happens after 5-20 >> reboots. >> >> It has been difficult to pin down the hang as the system is not >> responding to NMI events but using breakpoints I believe the hang is in >> acpi_cpu.c:acpi_cpu_shutdown with the call to smp_rendezvous. >> > > First, thank you for your careful debugging help. This is wonderful. > > >> My theory is that one of the CPUs does not respond to ipi_all_but_self >> and that all the other CPUs are waiting for it in smp_rendezvous_action. >> The smp_rv_waiters[0] < mp_ncpus condition never gets met and the system >> hangs. This maybe happen due to other activity (or a deadlock?) on that >> CPU. >> >> I noticed a few threads relating to this and have already tried stuff >> like changing kern.sched.ipiwakeup.enabled & machdep.cpu_idle_hlt. >> Neither had any effect. >> > > Very interesting. I didn't think anything could cause an IPI not to get > delivered eventually but during shutdown interrupts may be disabled at > some point. > > It was only a theory; I couldn't think of any other reasons why one of the CPUs doesn't rendezvous, interrupts being disabled is a good reason though! >> 1) I tried removing the call to smp_rendezvous in acpi_cpu_shutdown and >> this stops the hang from happening. Does anyone know the purpose of this >> call in the shutdown code or if I might suffer some consequence by >> removing it? >> > > Yes, I put it in to break all APs out of their potential C1-3 sleep. > This way they are not halted when shutdown needs to synchronize and stop > them. But that code sends its own IPI so there is no reason to do it > again here. I will remove smp_rendezvous() now. > It sounds like removing smp_rendezvous is a safe thing to do, thanks for your insight. > >> 2) Has anyone got any suggestions for debugging this further given that >> I can't break into the debugger? I thought I could maybe instrument some >> counters in i386/i386/local_apic.c & kern_smp.c with the aim of >> identifying a root cause. >> > > Sounds reasonable. Thanks again for a detailed problem report. > > I will notify the list if I find anything further regarding this problem. Thanks for your response.
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