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Subject[PATCH v2] abort secondary CPU bring-up gracefully if do_boot_cpu timed out on cpu_callin_mask
Date
Hang is observed on virtual machines during CPU hotplug, especially
in big guests with many CPUs. (It happens more often if host is
over-committed).

Patch is present in RHEL6 since 2012 and it fixes issue there,
it also fixes issue in upstream kernel according to tests.

Below is detailed description of the problem:
-----
Master CPU may timeout before cpu_callin_mask is set and cancel
booting CPU, but being onlined CPU still continues to boot, sets
cpu_active_mask (CPU_STARTING notifiers) and spins in
check_tsc_sync_target() for master cpu to arrive. Following attempt
to online another cpu hangs in stop_machine, initiated from here:

smp_callin ->
smp_store_cpu_info ->
identify_secondary_cpu ->
mtrr_ap_init -> set_mtrr_from_inactive_cpu

stop_machine waits on completion of stop_work on all CPUs from
cpu_active_mask including a failed CPU that spins in check_tsc_sync_target().

Issue is fixed if being onlined CPU continues to boot and calls
notify_cpu_starting(cpuid) only when master CPU waits for it to
come online. If master CPU times out on cpu_callin_mask and goes via
cancel path, the being onlined CPU should gracefully shutdown itself.

Patch introduces cpu_may_complete_boot_mask to notify a being onlined
CPU that it may call notify_cpu_starting(cpuid) and continue to boot
when master CPU goes via normal boot path and going to wait till the
being onlined CPU completes its initialization.

- normal boot sequence will look like:
master CPU1 being onlined CPU2

* wait for CPU2 in cpu_callin_mask
---------------------------------------------------------------------
* set CPU2 in cpu_callin_mask
* wait till CPU1 set CPU2 bit
in cpu_may_complete_boot_mask
---------------------------------------------------------------------
* set CPU2 bit in
cpu_may_complete_boot_mask
* return from do_boot_cpu() and
wait in
- check_tsc_sync_source() or
- while (!cpu_online(CPU2))
---------------------------------------------------------------------
* call notify_cpu_starting()
and continue CPU2 initialization
* mark itself as ONLINE
---------------------------------------------------------------------
* return to _cpu_up and call
cpu_notify(CPU_ONLINE, ...)

- cancel/error path will look like:
master CPU1 being onlined CPU2

* time out on cpu_callin_mask
---------------------------------------------------------------------
* set CPU2 in cpu_callin_mask
* wait till CPU2 is set in
cpu_may_complete_boot_mask or
cleared in cpu_callout_mask
---------------------------------------------------------------------
* clear CPU2 in cpu_callout_mask
and return with error
---------------------------------------------------------------------
* do cleanups and play_dead()

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
v2:
* updated commit message with description of which systems are affected
but bug and under which conditions.
---
arch/x86/include/asm/cpumask.h | 1 +
arch/x86/kernel/cpu/common.c | 2 ++
arch/x86/kernel/smpboot.c | 37 +++++++++++++++++++++++++++++++++++--
3 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/cpumask.h b/arch/x86/include/asm/cpumask.h
index 61c852f..eacd269 100644
--- a/arch/x86/include/asm/cpumask.h
+++ b/arch/x86/include/asm/cpumask.h
@@ -7,6 +7,7 @@ extern cpumask_var_t cpu_callin_mask;
extern cpumask_var_t cpu_callout_mask;
extern cpumask_var_t cpu_initialized_mask;
extern cpumask_var_t cpu_sibling_setup_mask;
+extern cpumask_var_t cpu_may_complete_boot_mask;

extern void setup_cpu_local_masks(void);

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 8e28bf2..4797111 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -50,6 +50,7 @@
cpumask_var_t cpu_initialized_mask;
cpumask_var_t cpu_callout_mask;
cpumask_var_t cpu_callin_mask;
+cpumask_var_t cpu_may_complete_boot_mask;

/* representing cpus for which sibling maps can be computed */
cpumask_var_t cpu_sibling_setup_mask;
@@ -61,6 +62,7 @@ void __init setup_cpu_local_masks(void)
alloc_bootmem_cpumask_var(&cpu_callin_mask);
alloc_bootmem_cpumask_var(&cpu_callout_mask);
alloc_bootmem_cpumask_var(&cpu_sibling_setup_mask);
+ alloc_bootmem_cpumask_var(&cpu_may_complete_boot_mask);
}

static void default_init(struct cpuinfo_x86 *c)
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index a32da80..4e9a63b 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -104,6 +104,8 @@ EXPORT_PER_CPU_SYMBOL(cpu_info);

atomic_t init_deasserted;

+static void remove_siblinginfo(int cpu);
+
/*
* Report back to the Boot Processor during boot time or to the caller processor
* during CPU online.
@@ -202,12 +204,38 @@ static void smp_callin(void)
set_cpu_sibling_map(raw_smp_processor_id());
wmb();

- notify_cpu_starting(cpuid);
-
/*
* Allow the master to continue.
*/
cpumask_set_cpu(cpuid, cpu_callin_mask);
+
+ /*
+ * Wait for signal from master CPU to continue or abort.
+ */
+ while (!cpumask_test_cpu(cpuid, cpu_may_complete_boot_mask) &&
+ cpumask_test_cpu(cpuid, cpu_callout_mask)) {
+ cpu_relax();
+ }
+
+ /* die if master cancelled cpu_up */
+ if (!cpumask_test_cpu(cpuid, cpu_may_complete_boot_mask))
+ goto die;
+
+ notify_cpu_starting(cpuid);
+ return;
+
+die:
+#ifdef CONFIG_HOTPLUG_CPU
+ /* was set by smp_store_cpu_info->...->numa_add_cpu */
+ numa_remove_cpu(cpuid);
+ remove_siblinginfo(cpuid);
+ clear_local_APIC();
+ /* was set by cpu_init() */
+ cpumask_clear_cpu(cpuid, cpu_initialized_mask);
+ cpumask_clear_cpu(cpuid, cpu_callin_mask);
+ play_dead();
+#endif
+ panic("%s: Failed to online CPU%d!\n", __func__, cpuid);
}

static int cpu0_logical_apicid;
@@ -827,6 +855,8 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
}

if (cpumask_test_cpu(cpu, cpu_callin_mask)) {
+ /* Signal AP that it may continue to boot */
+ cpumask_set_cpu(cpu, cpu_may_complete_boot_mask);
print_cpu_msr(&cpu_data(cpu));
pr_debug("CPU%d: has booted.\n", cpu);
} else {
@@ -1085,6 +1115,7 @@ void __init native_smp_prepare_cpus(unsigned int max_cpus)
*/
smp_store_boot_cpu_info(); /* Final full version of the data */
cpumask_copy(cpu_callin_mask, cpumask_of(0));
+ cpumask_copy(cpu_may_complete_boot_mask, cpumask_of(0));
mb();

current_thread_info()->cpu = 0; /* needed? */
@@ -1294,6 +1325,8 @@ static void __ref remove_cpu_from_maps(int cpu)
cpumask_clear_cpu(cpu, cpu_callin_mask);
/* was set by cpu_init() */
cpumask_clear_cpu(cpu, cpu_initialized_mask);
+ /* set by do_boot_cpu() */
+ cpumask_clear_cpu(cpu, cpu_may_complete_boot_mask);
numa_remove_cpu(cpu);
}

--
1.7.1


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