[Xen-devel] [PATCH v7] x86: initialize secondary CPU only if master CPU will wait for it

[Igor Mammedov <imammedo@xxxxxxxxxx>]

Hang is observed on virtual machines during CPU hotplug,
especially in big guests with many CPUs. (It reproducible
more often if host is over-committed).

It happens because master CPU gives up waiting on
secondary CPU and allows it to run wild. As result
AP causes locking or crashing system. For example
as described here: https://lkml.org/lkml/2014/3/6/257

If master CPU have sent STARTUP IPI successfully,
and AP signalled to master CPU that it's ready
to start initialization, make master CPU wait
indefinitely till AP is onlined.
To ensure that AP won't ever run wild, make it
wait at early startup till master CPU confirms its
intention to wait for AP. If AP doesn't respond in 10
seconds, the master CPU will timeout and cancel
AP onlining.

Signed-off-by: Igor Mammedov <imammedo@xxxxxxxxxx>
---
v7:
 - fix stuck boot with non SMP config
 - fix stuck paravirtual Xen SMP boot with more than 1VCPU
   and CPU hotplug
v6:
 - no changes
v5:
 - add smp_mb() after clearing cpu_initialized_mask in do_boot_cpu()
 - add 10 sec timeout description into commit message.
v4:
 - move commont code in cpu_init() for x32/x64 in shared
   helper function wait_formaster_cpu()
 - add WARN_ON(cpumask_test_and_set_cpu(cpu, cpu_initialized_mask))
   to wait_formaster_cpu()
v3:
 - leave timeouts in do_boot_cpu(), so that master CPU
   won't hang if AP doesn't respond, use cpu_initialized_mask
   as a way for AP to signal to master CPU that it's ready
   to start initialzation.
v2:
 - ammend comment in cpu_init()
---
 arch/x86/kernel/cpu/common.c |   29 ++++++++-----
 arch/x86/kernel/smpboot.c    |   99 +++++++++++++-----------------------------
 arch/x86/xen/smp.c           |    2 +
 3 files changed, 51 insertions(+), 79 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index ef1b93f..0b94e57 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1258,6 +1258,19 @@ static void dbg_restore_debug_regs(void)
 #define dbg_restore_debug_regs()
 #endif /* ! CONFIG_KGDB */
 
+static void wait_for_master_cpu(int cpu)
+{
+#ifdef CONFIG_SMP
+       /*
+        * wait for ACK from master CPU before continuing
+        * with AP initialization
+        */
+       WARN_ON(cpumask_test_and_set_cpu(cpu, cpu_initialized_mask));
+       while (!cpumask_test_cpu(cpu, cpu_callout_mask))
+               cpu_relax();
+#endif
+}
+
 /*
  * cpu_init() initializes state that is per-CPU. Some data is already
  * initialized (naturally) in the bootstrap process, such as the GDT
@@ -1273,16 +1286,17 @@ void cpu_init(void)
        struct task_struct *me;
        struct tss_struct *t;
        unsigned long v;
-       int cpu;
+       int cpu = stack_smp_processor_id();
        int i;
 
+       wait_for_master_cpu(cpu);
+
        /*
         * Load microcode on this cpu if a valid microcode is available.
         * This is early microcode loading procedure.
         */
        load_ucode_ap();
 
-       cpu = stack_smp_processor_id();
        t = &per_cpu(init_tss, cpu);
        oist = &per_cpu(orig_ist, cpu);
 
@@ -1294,9 +1308,6 @@ void cpu_init(void)
 
        me = current;
 
-       if (cpumask_test_and_set_cpu(cpu, cpu_initialized_mask))
-               panic("CPU#%d already initialized!\n", cpu);
-
        pr_debug("Initializing CPU#%d\n", cpu);
 
        clear_in_cr4(X86_CR4_VME|X86_CR4_PVI|X86_CR4_TSD|X86_CR4_DE);
@@ -1373,13 +1384,9 @@ void cpu_init(void)
        struct tss_struct *t = &per_cpu(init_tss, cpu);
        struct thread_struct *thread = &curr->thread;
 
-       show_ucode_info_early();
+       wait_for_master_cpu(cpu);
 
-       if (cpumask_test_and_set_cpu(cpu, cpu_initialized_mask)) {
-               printk(KERN_WARNING "CPU#%d already initialized!\n", cpu);
-               for (;;)
-                       local_irq_enable();
-       }
+       show_ucode_info_early();
 
        printk(KERN_INFO "Initializing CPU#%d\n", cpu);
 
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 5492798..a1472b7 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -111,7 +111,6 @@ atomic_t init_deasserted;
 static void smp_callin(void)
 {
        int cpuid, phys_id;
-       unsigned long timeout;
 
        /*
         * If waken up by an INIT in an 82489DX configuration
@@ -130,37 +129,6 @@ static void smp_callin(void)
         * (This works even if the APIC is not enabled.)
         */
        phys_id = read_apic_id();
-       if (cpumask_test_cpu(cpuid, cpu_callin_mask)) {
-               panic("%s: phys CPU#%d, CPU#%d already present??\n", __func__,
-                                       phys_id, cpuid);
-       }
-       pr_debug("CPU#%d (phys ID: %d) waiting for CALLOUT\n", cpuid, phys_id);
-
-       /*
-        * STARTUP IPIs are fragile beasts as they might sometimes
-        * trigger some glue motherboard logic. Complete APIC bus
-        * silence for 1 second, this overestimates the time the
-        * boot CPU is spending to send the up to 2 STARTUP IPIs
-        * by a factor of two. This should be enough.
-        */
-
-       /*
-        * Waiting 2s total for startup (udelay is not yet working)
-        */
-       timeout = jiffies + 2*HZ;
-       while (time_before(jiffies, timeout)) {
-               /*
-                * Has the boot CPU finished it's STARTUP sequence?
-                */
-               if (cpumask_test_cpu(cpuid, cpu_callout_mask))
-                       break;
-               cpu_relax();
-       }
-
-       if (!time_before(jiffies, timeout)) {
-               panic("%s: CPU%d started up but did not get a callout!\n",
-                     __func__, cpuid);
-       }
 
        /*
         * the boot CPU has finished the init stage and is spinning
@@ -757,8 +725,8 @@ static int do_boot_cpu(int apicid, int cpu, struct 
task_struct *idle)
        unsigned long start_ip = real_mode_header->trampoline_start;
 
        unsigned long boot_error = 0;
-       int timeout;
        int cpu0_nmi_registered = 0;
+       unsigned long timeout;
 
        /* Just in case we booted with a single CPU. */
        alternatives_enable_smp();
@@ -806,6 +774,15 @@ static int do_boot_cpu(int apicid, int cpu, struct 
task_struct *idle)
        }
 
        /*
+        * AP might wait on cpu_callout_mask in cpu_init() with
+        * cpu_initialized_mask set if previous attempt to online
+        * it timed-out. Clear cpu_initialized_mask so that after
+        * INIT/SIPI it could start with a clean state.
+        */
+       cpumask_clear_cpu(cpu, cpu_initialized_mask);
+       smp_mb();
+
+       /*
         * Wake up a CPU in difference cases:
         * - Use the method in the APIC driver if it's defined
         * Otherwise,
@@ -817,55 +794,41 @@ static int do_boot_cpu(int apicid, int cpu, struct 
task_struct *idle)
                boot_error = wakeup_cpu_via_init_nmi(cpu, start_ip, apicid,
                                                     &cpu0_nmi_registered);
 
+
        if (!boot_error) {
                /*
-                * allow APs to start initializing.
+                * Wait 10s total for a response from AP
                 */
-               pr_debug("Before Callout %d\n", cpu);
-               cpumask_set_cpu(cpu, cpu_callout_mask);
-               pr_debug("After Callout %d\n", cpu);
+               boot_error = -1;
+               timeout = jiffies + 10*HZ;
+               while (time_before(jiffies, timeout)) {
+                       if (cpumask_test_cpu(cpu, cpu_initialized_mask)) {
+                               /*
+                                * Tell AP to proceed with initialization
+                                */
+                               cpumask_set_cpu(cpu, cpu_callout_mask);
+                               boot_error = 0;
+                               break;
+                       }
+                       udelay(100);
+                       schedule();
+               }
+       }
 
+       if (!boot_error) {
                /*
-                * Wait 5s total for a response
+                * Wait till AP completes initial initialization
                 */
-               for (timeout = 0; timeout < 50000; timeout++) {
-                       if (cpumask_test_cpu(cpu, cpu_callin_mask))
-                               break;  /* It has booted */
-                       udelay(100);
+               while (!cpumask_test_cpu(cpu, cpu_callin_mask)) {
                        /*
                         * Allow other tasks to run while we wait for the
                         * AP to come online. This also gives a chance
                         * for the MTRR work(triggered by the AP coming online)
                         * to be completed in the stop machine context.
                         */
+                       udelay(100);
                        schedule();
                }
-
-               if (cpumask_test_cpu(cpu, cpu_callin_mask)) {
-                       print_cpu_msr(&cpu_data(cpu));
-                       pr_debug("CPU%d: has booted.\n", cpu);
-               } else {
-                       boot_error = 1;
-                       if (*trampoline_status == 0xA5A5A5A5)
-                               /* trampoline started but...? */
-                               pr_err("CPU%d: Stuck ??\n", cpu);
-                       else
-                               /* trampoline code not run */
-                               pr_err("CPU%d: Not responding\n", cpu);
-                       if (apic->inquire_remote_apic)
-                               apic->inquire_remote_apic(apicid);
-               }
-       }
-
-       if (boot_error) {
-               /* Try to put things back the way they were before ... */
-               numa_remove_cpu(cpu); /* was set by numa_add_cpu */
-
-               /* was set by do_boot_cpu() */
-               cpumask_clear_cpu(cpu, cpu_callout_mask);
-
-               /* was set by cpu_init() */
-               cpumask_clear_cpu(cpu, cpu_initialized_mask);
        }
 
        /* mark "stuck" area as not stuck */
diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
index 7005974..3631e71 100644
--- a/arch/x86/xen/smp.c
+++ b/arch/x86/xen/smp.c
@@ -360,6 +360,8 @@ cpu_initialize_context(unsigned int cpu, struct task_struct 
*idle)
        struct desc_struct *gdt;
        unsigned long gdt_mfn;
 
+       /* used to tell cpu_init() that it can proceed with initialization */
+       cpumask_set_cpu(cpu, cpu_callout_mask);
        if (cpumask_test_and_set_cpu(cpu, xen_cpu_initialized_map))
                return 0;
 
-- 
1.7.1


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