[RFC] QEMU/KVM: dedicated IO thread

Marcelo Tosatti
2008-03-25T13:53:31Z

Avi was concerned that this would cause problems with migration. I
haven't specifically tested it yet, but it seems there will be no
problems introduced by this change: the IO thread will stop all vcpu's
in the same way the vcpu0 thread did before.

QEMU/KVM: separate thread for IO handling

Move IO processing from vcpu0 to a dedicated thread.

This removes load on vcpu0 by allowing better cache locality and also
improves latency.

We can now block signal handling for IO events, so sigtimedwait won't
race with handlers:

- Currently the SIGALRM handler fails to set CPU_INTERRUPT_EXIT because
the "next_cpu" variable is not initialized in the KVM path, meaning that
processing of timer expiration might be delayed until the next vcpu0 exit.

- Processing of IO events will not be unnecessarily interrupted.

Index: kvm-userspace.io/libkvm/libkvm.c
===================================================================
--- kvm-userspace.io.orig/libkvm/libkvm.c
+++ kvm-userspace.io/libkvm/libkvm.c
@@ -388,9 +388,6 @@ int kvm_create(kvm_context_t kvm, unsign
if (r < 0)
return r;
kvm_create_irqchip(kvm);
- r = kvm_create_vcpu(kvm, 0);
- if (r < 0)
- return r;

return 0;
}
Index: kvm-userspace.io/qemu/qemu-kvm.c
===================================================================
--- kvm-userspace.io.orig/qemu/qemu-kvm.c
+++ kvm-userspace.io/qemu/qemu-kvm.c
@@ -38,6 +38,7 @@ struct qemu_kvm_signal_table {
};

static struct qemu_kvm_signal_table io_signal_table;
+static struct qemu_kvm_signal_table vcpu_signal_table;

#define SIG_IPI (SIGRTMIN+4)

@@ -51,6 +52,8 @@ struct vcpu_info {
int stopped;
} vcpu_info[256];

+pthread_t io_thread;
+
static inline unsigned long kvm_get_thread_id(void)
{
return syscall(SYS_gettid);
@@ -67,12 +70,19 @@ static void sig_ipi_handler(int n)

void kvm_update_interrupt_request(CPUState *env)
{
- if (env && vcpu && env != vcpu->env) {
- if (vcpu_info[env->cpu_index].signalled)
- return;
- vcpu_info[env->cpu_index].signalled = 1;
- if (vcpu_info[env->cpu_index].thread)
- pthread_kill(vcpu_info[env->cpu_index].thread, SIG_IPI);
+ int signal = 0;
+
+ if (env) {
+ if (!vcpu)
+ signal = 1;
+ if (vcpu && env != vcpu->env && !vcpu_info[env->cpu_index].signalled)
+ signal = 1;
+
+ if (signal) {
+ vcpu_info[env->cpu_index].signalled = 1;
+ if (vcpu_info[env->cpu_index].thread)
+ pthread_kill(vcpu_info[env->cpu_index].thread, SIG_IPI);
+ }
}
}

@@ -105,7 +115,7 @@ static void post_kvm_run(void *opaque, i

static int pre_kvm_run(void *opaque, int vcpu)
{
- CPUState *env = cpu_single_env;
+ CPUState *env = qemu_kvm_cpu_env(vcpu);

kvm_arch_pre_kvm_run(opaque, vcpu);

@@ -151,7 +161,8 @@ static int has_work(CPUState *env)
return kvm_arch_has_work(env);
}

-static int kvm_eat_signal(CPUState *env, int timeout)
+static int kvm_eat_signal(struct qemu_kvm_signal_table *waitset, CPUState *env,
+ int timeout)
{
struct timespec ts;
int r, e, ret = 0;
@@ -160,12 +171,12 @@ static int kvm_eat_signal(CPUState *env,

ts.tv_sec = timeout / 1000;
ts.tv_nsec = (timeout % 1000) * 1000000;
- r = sigtimedwait(&io_signal_table.sigset, &siginfo, &ts);
+ r = sigtimedwait(&waitset->sigset, &siginfo, &ts);
if (r == -1 && (errno == EAGAIN || errno == EINTR) && !timeout)
return 0;
e = errno;
pthread_mutex_lock(&qemu_mutex);
- if (vcpu)
+ if (env && vcpu)
cpu_single_env = vcpu->env;
if (r == -1 && !(errno == EAGAIN || errno == EINTR)) {
printf("sigtimedwait: %s\n", strerror(e));
@@ -181,7 +192,7 @@ static int kvm_eat_signal(CPUState *env,
if (env && vcpu_info[env->cpu_index].stop) {
vcpu_info[env->cpu_index].stop = 0;
vcpu_info[env->cpu_index].stopped = 1;
- pthread_kill(vcpu_info[0].thread, SIG_IPI);
+ pthread_kill(io_thread, SIGUSR1);
}
pthread_mutex_unlock(&qemu_mutex);

@@ -192,23 +203,18 @@ static int kvm_eat_signal(CPUState *env,
static void kvm_eat_signals(CPUState *env, int timeout)
{
int r = 0;
+ struct qemu_kvm_signal_table *waitset = &vcpu_signal_table;

- while (kvm_eat_signal(env, 0))
+ while (kvm_eat_signal(waitset, env, 0))
r = 1;
if (!r && timeout) {
- r = kvm_eat_signal(env, timeout);
+ r = kvm_eat_signal(waitset, env, timeout);
if (r)
- while (kvm_eat_signal(env, 0))
+ while (kvm_eat_signal(waitset, env, 0))
;
}
- /*
- * we call select() even if no signal was received, to account for
- * for which there is no signal handler installed.
- */
pthread_mutex_lock(&qemu_mutex);
cpu_single_env = vcpu->env;
- if (env->cpu_index == 0)
- main_loop_wait(0);
pthread_mutex_unlock(&qemu_mutex);
}

@@ -225,29 +231,29 @@ static int all_threads_paused(void)
{
int i;

- for (i = 1; i < smp_cpus; ++i)
+ for (i = 0; i < smp_cpus; ++i)
if (vcpu_info[i].stopped)
return 0;
return 1;
}

-static void pause_other_threads(void)
+static void pause_all_threads(void)
{
int i;

- for (i = 1; i < smp_cpus; ++i) {
+ for (i = 0; i < smp_cpus; ++i) {
vcpu_info[i].stop = 1;
pthread_kill(vcpu_info[i].thread, SIG_IPI);
}
while (!all_threads_paused())
- kvm_eat_signals(vcpu->env, 0);
+ kvm_eat_signal(&io_signal_table, NULL, 1000);
}

-static void resume_other_threads(void)
+static void resume_all_threads(void)
{
int i;

- for (i = 1; i < smp_cpus; ++i) {
+ for (i = 0; i < smp_cpus; ++i) {
vcpu_info[i].stop = 0;
vcpu_info[i].stopped = 0;
pthread_kill(vcpu_info[i].thread, SIG_IPI);
@@ -257,9 +263,9 @@ static void resume_other_threads(void)
static void kvm_vm_state_change_handler(void *context, int running)
{
if (running)
- resume_other_threads();
+ resume_all_threads();
else
- pause_other_threads();
+ pause_all_threads();
}

static void update_regs_for_sipi(CPUState *env)
@@ -281,8 +287,6 @@ static void setup_kernel_sigmask(CPUStat

sigprocmask(SIG_BLOCK, NULL, &set);
sigdelset(&set, SIG_IPI);
- if (env->cpu_index == 0)
- sigandset(&set, &set, &io_signal_table.negsigset);

kvm_set_signal_mask(kvm_context, env->cpu_index, &set);
}
@@ -364,38 +368,59 @@ void kvm_init_new_ap(int cpu, CPUState *
pthread_create(&vcpu_info[cpu].thread, NULL, ap_main_loop, env);
}

+static void qemu_kvm_init_signal_tables(void)
+{
+ qemu_kvm_init_signal_table(&io_signal_table);
+ qemu_kvm_init_signal_table(&vcpu_signal_table);
+
+ kvm_add_signal(&io_signal_table, SIGIO);
+ kvm_add_signal(&io_signal_table, SIGALRM);
+ kvm_add_signal(&io_signal_table, SIGUSR1);
+ kvm_add_signal(&io_signal_table, SIGUSR2);
+
+ kvm_add_signal(&vcpu_signal_table, SIG_IPI);
+
+ sigprocmask(SIG_BLOCK, &io_signal_table.sigset, NULL);
+}
+
int kvm_init_ap(void)
{
- CPUState *env = first_cpu->next_cpu;
+ CPUState *env = first_cpu;
int i;

#ifdef TARGET_I386
kvm_tpr_opt_setup();
#endif
qemu_add_vm_change_state_handler(kvm_vm_state_change_handler, NULL);
- qemu_kvm_init_signal_table(&io_signal_table);
- kvm_add_signal(&io_signal_table, SIGIO);
- kvm_add_signal(&io_signal_table, SIGALRM);
- kvm_add_signal(&io_signal_table, SIGUSR2);
- kvm_add_signal(&io_signal_table, SIG_IPI);
- sigprocmask(SIG_BLOCK, &io_signal_table.sigset, NULL);
+ qemu_kvm_init_signal_tables();

- vcpu = &vcpu_info[0];
- vcpu->env = first_cpu;
- vcpu->env->thread_id = kvm_get_thread_id();
signal(SIG_IPI, sig_ipi_handler);
- for (i = 1; i < smp_cpus; ++i) {
+ for (i = 0; i < smp_cpus; ++i) {
kvm_init_new_ap(i, env);
env = env->next_cpu;
}
return 0;
}

+/*
+ * The IO thread has all signals that inform machine events
+ * blocked (io_signal_table), so it won't get interrupted
+ * while processing in main_loop_wait().
+ */
+
int kvm_main_loop(void)
{
- vcpu_info[0].thread = pthread_self();
+ io_thread = pthread_self();
pthread_mutex_unlock(&qemu_mutex);
- return kvm_main_loop_cpu(first_cpu);
+ cpu_single_env = NULL;
+ while (1) {
+ kvm_eat_signal(&io_signal_table, NULL, 1000);
+ pthread_mutex_lock(&qemu_mutex);
+ cpu_single_env = NULL;
+ main_loop_wait(0);
+ pthread_mutex_unlock(&qemu_mutex);
+ }
+ return 0;
}

static int kvm_debug(void *opaque, int vcpu)
@@ -749,12 +774,16 @@ void qemu_kvm_aio_wait_start(void)

void qemu_kvm_aio_wait(void)
{
- if (!cpu_single_env || cpu_single_env->cpu_index == 0) {
- pthread_mutex_unlock(&qemu_mutex);
- kvm_eat_signal(cpu_single_env, 1000);
- pthread_mutex_lock(&qemu_mutex);
+ CPUState *cpu_single = cpu_single_env;
+
+ if (!cpu_single_env) {
+ pthread_mutex_unlock(&qemu_mutex);
+ kvm_eat_signal(&io_signal_table, cpu_single_env, 1000);
+ pthread_mutex_lock(&qemu_mutex);
+ cpu_single_env = NULL;
} else {
- pthread_cond_wait(&qemu_aio_cond, &qemu_mutex);
+ pthread_cond_wait(&qemu_aio_cond, &qemu_mutex);
+ cpu_single_env = cpu_single;
}
}

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