linux_dsm_epyc7002/arch
Thomas Gleixner 506a66f374 Revert "x86/apic: Ignore secondary threads if nosmt=force"
Dave Hansen reported, that it's outright dangerous to keep SMT siblings
disabled completely so they are stuck in the BIOS and wait for SIPI.

The reason is that Machine Check Exceptions are broadcasted to siblings and
the soft disabled sibling has CR4.MCE = 0. If a MCE is delivered to a
logical core with CR4.MCE = 0, it asserts IERR#, which shuts down or
reboots the machine. The MCE chapter in the SDM contains the following
blurb:

    Because the logical processors within a physical package are tightly
    coupled with respect to shared hardware resources, both logical
    processors are notified of machine check errors that occur within a
    given physical processor. If machine-check exceptions are enabled when
    a fatal error is reported, all the logical processors within a physical
    package are dispatched to the machine-check exception handler. If
    machine-check exceptions are disabled, the logical processors enter the
    shutdown state and assert the IERR# signal. When enabling machine-check
    exceptions, the MCE flag in control register CR4 should be set for each
    logical processor.

Reverting the commit which ignores siblings at enumeration time solves only
half of the problem. The core cpuhotplug logic needs to be adjusted as
well.

This thoughtful engineered mechanism also turns the boot process on all
Intel HT enabled systems into a MCE lottery. MCE is enabled on the boot CPU
before the secondary CPUs are brought up. Depending on the number of
physical cores the window in which this situation can happen is smaller or
larger. On a HSW-EX it's about 750ms:

MCE is enabled on the boot CPU:

[    0.244017] mce: CPU supports 22 MCE banks

The corresponding sibling  boots:

[    1.008005] .... node  , CPUs:    

That means if an MCE hits on physical core 0 (logical CPUs 0 and 72)
between these two points the machine is going to shutdown. At least it's a
known safe state.

It's obvious that the early boot can be hit by an MCE as well and then runs
into the same situation because MCEs are not yet enabled on the boot CPU.
But after enabling them on the boot CPU, it does not make any sense to
prevent the kernel from recovering.

Adjust the nosmt kernel parameter documentation as well.

Reverts: 2207def700 ("x86/apic: Ignore secondary threads if nosmt=force")
Reported-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Tony Luck <tony.luck@intel.com>
2018-07-02 11:25:28 +02:00
..
alpha Kbuild updates for v4.18 2018-06-06 11:00:15 -07:00
arc mm: introduce ARCH_HAS_PTE_SPECIAL 2018-06-07 17:34:35 -07:00
arm docs: Fix some broken references 2018-06-15 18:10:01 -03:00
arm64 docs: Fix some broken references 2018-06-15 18:10:01 -03:00
c6x Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2018-06-04 20:27:54 -07:00
h8300 Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2018-06-04 20:27:54 -07:00
hexagon hexagon: drop the unused variable zero_page_mask 2018-06-15 07:55:25 +09:00
ia64 - Error path bug fix for overflow tests (Dan) 2018-06-12 18:28:00 -07:00
m68k Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/gerg/m68knommu 2018-06-05 10:51:30 -07:00
microblaze docs: Fix some broken references 2018-06-15 18:10:01 -03:00
mips Merge branch 'akpm' (patches from Andrew) 2018-06-15 08:51:42 +09:00
nds32 Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2018-06-04 20:27:54 -07:00
nios2 Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2018-06-04 20:27:54 -07:00
openrisc Kbuild updates for v4.18 2018-06-06 11:00:15 -07:00
parisc arch/*: Kconfig: fix documentation for NMI watchdog 2018-06-15 12:48:59 -03:00
powerpc Mostly the PPC part of the release, but also switching to Arnd's fix 2018-06-16 06:37:04 +09:00
riscv RISC-V Updates for the 4.18 Merge Window 2018-06-16 06:42:43 +09:00
s390 - Error path bug fix for overflow tests (Dan) 2018-06-12 18:28:00 -07:00
sh arch/*: Kconfig: fix documentation for NMI watchdog 2018-06-15 12:48:59 -03:00
sparc arch/*: Kconfig: fix documentation for NMI watchdog 2018-06-15 12:48:59 -03:00
um Solve a series of broken links for files under Documentation: 2018-06-17 05:25:18 +09:00
unicore32 docs: Fix some broken references 2018-06-15 18:10:01 -03:00
x86 Revert "x86/apic: Ignore secondary threads if nosmt=force" 2018-07-02 11:25:28 +02:00
xtensa docs: Fix some broken references 2018-06-15 18:10:01 -03:00
.gitignore
Kconfig cpu/hotplug: Provide knobs to control SMT 2018-06-21 14:20:58 +02:00