diff mbox

[RFC,v4,22/28] tcg: enable thread-per-vCPU

Message ID 1470929064-4092-23-git-send-email-alex.bennee@linaro.org
State New
Headers show

Commit Message

Alex Bennée Aug. 11, 2016, 3:24 p.m. UTC 
There are a couple of changes that occur at the same time here:

  - introduce a single vCPU qemu_tcg_cpu_thread_fn

  One of these is spawned per vCPU with its own Thread and Condition
  variables. qemu_tcg_rr_cpu_thread_fn is the new name for the old
  single threaded function.

  - the TLS current_cpu variable is now live for the lifetime of MTTCG
    vCPU threads. This is for future work where async jobs need to know
    the vCPU context they are operating in.

The user to switch on multi-thread behaviour and spawn a thread
per-vCPU. For a simple test kvm-unit-test like:

  ./arm/run ./arm/locking-test.flat -smp 4 -accel tcg,thread=multi

Will now use 4 vCPU threads and have an expected FAIL (instead of the
unexpected PASS) as the default mode of the test has no protection when
incrementing a shared variable.

However we still default to a single thread for all vCPUs as individual
front-end and back-ends need additional fixes to safely support:
  - atomic behaviour
  - tb invalidation
  - memory ordering

The function default_mttcg_enabled can be tweaked as support is added.

Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

[AJB: Some fixes, conditionally, commit rewording]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>


---
v1 (ajb):
  - fix merge conflicts
  - maintain single-thread approach
v2
  - re-base fixes (no longer has tb_find_fast lock tweak ahead)
  - remove bogus break condition on cpu->stop/stopped
  - only process exiting cpus exit_request
  - handle all cpus idle case (fixes shutdown issues)
  - sleep on EXCP_HALTED in mttcg mode (prevent crash on start-up)
  - move icount timer into helper
v3
  - update the commit message
  - rm kick_timer tweaks (move to earlier tcg_current_cpu tweaks)
  - ensure linux-user clears cpu->exit_request in loop
  - purging of global exit_request and tcg_current_cpu in earlier patches
  - fix checkpatch warnings
v4
  - don't break loop on stopped, we may never schedule next in RR mode
  - make sure we flush iorequests of current cpu if we exited on one
  - add tcg_cpu_exec_start/end wraps for async work functions
  - stop killing of current_cpu on loop exit
  - set current_cpu in the single thread function
  - remove sleep special case, add qemu_tcg_should_sleep() for mttcg
  - no need to atomic set cpu->exit_request going into the loop
  - removed extraneous setting of exit_request
  - split tb_lock() part of patch
  - rename single thread fn to qemu_tcg_rr_cpu_thread_fn
---
 cpu-exec.c |   5 ---
 cpus.c     | 133 +++++++++++++++++++++++++++++++++++++++++++++++--------------
 2 files changed, 103 insertions(+), 35 deletions(-)

-- 
2.7.4
diff mbox

Patch

diff --git a/cpu-exec.c b/cpu-exec.c
index d2e8b9e..9af7260 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -365,7 +365,6 @@  static inline bool cpu_handle_halt(CPUState *cpu)
         }
 #endif
         if (!cpu_has_work(cpu)) {
-            current_cpu = NULL;
             return true;
         }
 
@@ -506,7 +505,6 @@  static inline void cpu_handle_interrupt(CPUState *cpu,
 
     }
     if (unlikely(cpu->exit_request || replay_has_interrupt())) {
-        cpu->exit_request = 0;
         cpu->exception_index = EXCP_INTERRUPT;
         cpu_loop_exit(cpu);
     }
@@ -641,8 +639,5 @@  int cpu_exec(CPUState *cpu)
     cc->cpu_exec_exit(cpu);
     rcu_read_unlock();
 
-    /* fail safe : never use current_cpu outside cpu_exec() */
-    current_cpu = NULL;
-
     return ret;
 }
diff --git a/cpus.c b/cpus.c
index 4fc5e4c..d831d43 100644
--- a/cpus.c
+++ b/cpus.c
@@ -980,24 +980,31 @@  static inline void tcg_cpu_exec_end(CPUState *cpu)
 
 static void qemu_wait_io_event_common(CPUState *cpu)
 {
+    atomic_mb_set(&cpu->thread_kicked, false);
     if (cpu->stop) {
         cpu->stop = false;
         cpu->stopped = true;
         qemu_cond_broadcast(&qemu_pause_cond);
     }
     process_queued_cpu_work(cpu);
-    cpu->thread_kicked = false;
+}
+
+static bool qemu_tcg_should_sleep(CPUState *cpu)
+{
+    if (mttcg_enabled) {
+        return cpu_thread_is_idle(cpu);
+    } else {
+        return all_cpu_threads_idle();
+    }
 }
 
 static void qemu_tcg_wait_io_event(CPUState *cpu)
 {
-    while (all_cpu_threads_idle()) {
+    while (qemu_tcg_should_sleep(cpu)) {
         qemu_cond_wait(cpu->halt_cond, &qemu_global_mutex);
     }
 
-    CPU_FOREACH(cpu) {
-        qemu_wait_io_event_common(cpu);
-    }
+    qemu_wait_io_event_common(cpu);
 
     wait_safe_cpu_work();
 }
@@ -1070,6 +1077,7 @@  static void *qemu_dummy_cpu_thread_fn(void *arg)
     qemu_thread_get_self(cpu->thread);
     cpu->thread_id = qemu_get_thread_id();
     cpu->can_do_io = 1;
+    current_cpu = cpu;
 
     sigemptyset(&waitset);
     sigaddset(&waitset, SIG_IPI);
@@ -1078,9 +1086,7 @@  static void *qemu_dummy_cpu_thread_fn(void *arg)
     cpu->created = true;
     qemu_cond_signal(&qemu_cpu_cond);
 
-    current_cpu = cpu;
     while (1) {
-        current_cpu = NULL;
         qemu_mutex_unlock_iothread();
         do {
             int sig;
@@ -1091,7 +1097,6 @@  static void *qemu_dummy_cpu_thread_fn(void *arg)
             exit(1);
         }
         qemu_mutex_lock_iothread();
-        current_cpu = cpu;
         qemu_wait_io_event_common(cpu);
     }
 
@@ -1230,7 +1235,7 @@  static void kick_tcg_thread(void *opaque)
     qemu_cpu_kick_rr_cpu();
 }
 
-static void *qemu_tcg_cpu_thread_fn(void *arg)
+static void *qemu_tcg_rr_cpu_thread_fn(void *arg)
 {
     CPUState *cpu = arg;
     QEMUTimer *kick_timer;
@@ -1253,6 +1258,7 @@  static void *qemu_tcg_cpu_thread_fn(void *arg)
 
         /* process any pending work */
         CPU_FOREACH(cpu) {
+            current_cpu = cpu;
             qemu_wait_io_event_common(cpu);
         }
     }
@@ -1279,6 +1285,7 @@  static void *qemu_tcg_cpu_thread_fn(void *arg)
 
         while (cpu && !cpu->exit_request) {
             atomic_mb_set(&tcg_current_rr_cpu, cpu);
+            current_cpu = cpu;
 
             qemu_clock_enable(QEMU_CLOCK_VIRTUAL,
                               (cpu->singlestep_enabled & SSTEP_NOTIMER) == 0);
@@ -1292,7 +1299,7 @@  static void *qemu_tcg_cpu_thread_fn(void *arg)
                     cpu_handle_guest_debug(cpu);
                     break;
                 }
-            } else if (cpu->stop || cpu->stopped) {
+            } else if (cpu->stop) {
                 if (cpu->unplug) {
                     cpu = CPU_NEXT(cpu);
                 }
@@ -1311,13 +1318,73 @@  static void *qemu_tcg_cpu_thread_fn(void *arg)
 
         handle_icount_deadline();
 
-        qemu_tcg_wait_io_event(QTAILQ_FIRST(&cpus));
+        qemu_tcg_wait_io_event(cpu ? cpu : QTAILQ_FIRST(&cpus));
         deal_with_unplugged_cpus();
     }
 
     return NULL;
 }
 
+/* Multi-threaded TCG
+ *
+ * In the multi-threaded case each vCPU has its own thread. The TLS
+ * variable current_cpu can be used deep in the code to find the
+ * current CPUState for a given thread.
+ */
+
+static void *qemu_tcg_cpu_thread_fn(void *arg)
+{
+    CPUState *cpu = arg;
+
+    rcu_register_thread();
+
+    qemu_mutex_lock_iothread();
+    qemu_thread_get_self(cpu->thread);
+
+    cpu->thread_id = qemu_get_thread_id();
+    cpu->created = true;
+    cpu->can_do_io = 1;
+    current_cpu = cpu;
+    qemu_cond_signal(&qemu_cpu_cond);
+
+    /* process any pending work */
+    cpu->exit_request = 1;
+
+    while (1) {
+        if (cpu_can_run(cpu)) {
+            int r;
+            tcg_cpu_exec_start(cpu);
+            r = tcg_cpu_exec(cpu);
+            tcg_cpu_exec_end(cpu);
+            switch (r) {
+            case EXCP_DEBUG:
+                cpu_handle_guest_debug(cpu);
+                break;
+            case EXCP_HALTED:
+                /* during start-up the vCPU is reset and the thread is
+                 * kicked several times. If we don't ensure we go back
+                 * to sleep in the halted state we won't cleanly
+                 * start-up when the vCPU is enabled.
+                 *
+                 * cpu->halted should ensure we sleep in wait_io_event
+                 */
+                g_assert(cpu->halted);
+                break;
+            default:
+                /* Ignore everything else? */
+                break;
+            }
+        }
+
+        handle_icount_deadline();
+
+        atomic_mb_set(&cpu->exit_request, 0);
+        qemu_tcg_wait_io_event(cpu);
+    }
+
+    return NULL;
+}
+
 static void qemu_cpu_kick_thread(CPUState *cpu)
 {
 #ifndef _WIN32
@@ -1342,7 +1409,7 @@  void qemu_cpu_kick(CPUState *cpu)
     qemu_cond_broadcast(cpu->halt_cond);
     if (tcg_enabled()) {
         cpu_exit(cpu);
-        /* Also ensure current RR cpu is kicked */
+        /* NOP unless doing single-thread RR */
         qemu_cpu_kick_rr_cpu();
     } else {
         qemu_cpu_kick_thread(cpu);
@@ -1409,13 +1476,6 @@  void pause_all_vcpus(void)
 
     if (qemu_in_vcpu_thread()) {
         cpu_stop_current();
-        if (!kvm_enabled()) {
-            CPU_FOREACH(cpu) {
-                cpu->stop = false;
-                cpu->stopped = true;
-            }
-            return;
-        }
     }
 
     while (!all_vcpus_paused()) {
@@ -1464,29 +1524,42 @@  void cpu_remove_sync(CPUState *cpu)
 static void qemu_tcg_init_vcpu(CPUState *cpu)
 {
     char thread_name[VCPU_THREAD_NAME_SIZE];
-    static QemuCond *tcg_halt_cond;
-    static QemuThread *tcg_cpu_thread;
+    static QemuCond *single_tcg_halt_cond;
+    static QemuThread *single_tcg_cpu_thread;
 
-    /* share a single thread for all cpus with TCG */
-    if (!tcg_cpu_thread) {
+    if (qemu_tcg_mttcg_enabled() || !single_tcg_cpu_thread) {
         cpu->thread = g_malloc0(sizeof(QemuThread));
         cpu->halt_cond = g_malloc0(sizeof(QemuCond));
         qemu_cond_init(cpu->halt_cond);
-        tcg_halt_cond = cpu->halt_cond;
-        snprintf(thread_name, VCPU_THREAD_NAME_SIZE, "CPU %d/TCG",
+
+        if (qemu_tcg_mttcg_enabled()) {
+            /* create a thread per vCPU with TCG (MTTCG) */
+            snprintf(thread_name, VCPU_THREAD_NAME_SIZE, "CPU %d/TCG",
                  cpu->cpu_index);
-        qemu_thread_create(cpu->thread, thread_name, qemu_tcg_cpu_thread_fn,
-                           cpu, QEMU_THREAD_JOINABLE);
+
+            qemu_thread_create(cpu->thread, thread_name, qemu_tcg_cpu_thread_fn,
+                               cpu, QEMU_THREAD_JOINABLE);
+
+        } else {
+            /* share a single thread for all cpus with TCG */
+            snprintf(thread_name, VCPU_THREAD_NAME_SIZE, "ALL CPUs/TCG");
+            qemu_thread_create(cpu->thread, thread_name,
+                               qemu_tcg_rr_cpu_thread_fn,
+                               cpu, QEMU_THREAD_JOINABLE);
+
+            single_tcg_halt_cond = cpu->halt_cond;
+            single_tcg_cpu_thread = cpu->thread;
+        }
 #ifdef _WIN32
         cpu->hThread = qemu_thread_get_handle(cpu->thread);
 #endif
         while (!cpu->created) {
             qemu_cond_wait(&qemu_cpu_cond, &qemu_global_mutex);
         }
-        tcg_cpu_thread = cpu->thread;
     } else {
-        cpu->thread = tcg_cpu_thread;
-        cpu->halt_cond = tcg_halt_cond;
+        /* For non-MTTCG cases we share the thread */
+        cpu->thread = single_tcg_cpu_thread;
+        cpu->halt_cond = single_tcg_halt_cond;
     }
 }