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-rw-r--r--cpus-common.c95
-rw-r--r--docs/tcg-exclusive.promela53
-rw-r--r--include/qom/cpu.h5
3 files changed, 133 insertions, 20 deletions
diff --git a/cpus-common.c b/cpus-common.c
index 38b1d55..3e11452 100644
--- a/cpus-common.c
+++ b/cpus-common.c
@@ -28,6 +28,9 @@ static QemuCond exclusive_cond;
static QemuCond exclusive_resume;
static QemuCond qemu_work_cond;
+/* >= 1 if a thread is inside start_exclusive/end_exclusive. Written
+ * under qemu_cpu_list_lock, read with atomic operations.
+ */
static int pending_cpus;
void qemu_init_cpu_list(void)
@@ -177,18 +180,26 @@ static inline void exclusive_idle(void)
void start_exclusive(void)
{
CPUState *other_cpu;
+ int running_cpus;
qemu_mutex_lock(&qemu_cpu_list_lock);
exclusive_idle();
/* Make all other cpus stop executing. */
- pending_cpus = 1;
+ atomic_set(&pending_cpus, 1);
+
+ /* Write pending_cpus before reading other_cpu->running. */
+ smp_mb();
+ running_cpus = 0;
CPU_FOREACH(other_cpu) {
- if (other_cpu->running) {
- pending_cpus++;
+ if (atomic_read(&other_cpu->running)) {
+ other_cpu->has_waiter = true;
+ running_cpus++;
qemu_cpu_kick(other_cpu);
}
}
+
+ atomic_set(&pending_cpus, running_cpus + 1);
while (pending_cpus > 1) {
qemu_cond_wait(&exclusive_cond, &qemu_cpu_list_lock);
}
@@ -203,7 +214,7 @@ void start_exclusive(void)
void end_exclusive(void)
{
qemu_mutex_lock(&qemu_cpu_list_lock);
- pending_cpus = 0;
+ atomic_set(&pending_cpus, 0);
qemu_cond_broadcast(&exclusive_resume);
qemu_mutex_unlock(&qemu_cpu_list_lock);
}
@@ -211,24 +222,78 @@ void end_exclusive(void)
/* Wait for exclusive ops to finish, and begin cpu execution. */
void cpu_exec_start(CPUState *cpu)
{
- qemu_mutex_lock(&qemu_cpu_list_lock);
- exclusive_idle();
- cpu->running = true;
- qemu_mutex_unlock(&qemu_cpu_list_lock);
+ atomic_set(&cpu->running, true);
+
+ /* Write cpu->running before reading pending_cpus. */
+ smp_mb();
+
+ /* 1. start_exclusive saw cpu->running == true and pending_cpus >= 1.
+ * After taking the lock we'll see cpu->has_waiter == true and run---not
+ * for long because start_exclusive kicked us. cpu_exec_end will
+ * decrement pending_cpus and signal the waiter.
+ *
+ * 2. start_exclusive saw cpu->running == false but pending_cpus >= 1.
+ * This includes the case when an exclusive item is running now.
+ * Then we'll see cpu->has_waiter == false and wait for the item to
+ * complete.
+ *
+ * 3. pending_cpus == 0. Then start_exclusive is definitely going to
+ * see cpu->running == true, and it will kick the CPU.
+ */
+ if (unlikely(atomic_read(&pending_cpus))) {
+ qemu_mutex_lock(&qemu_cpu_list_lock);
+ if (!cpu->has_waiter) {
+ /* Not counted in pending_cpus, let the exclusive item
+ * run. Since we have the lock, just set cpu->running to true
+ * while holding it; no need to check pending_cpus again.
+ */
+ atomic_set(&cpu->running, false);
+ exclusive_idle();
+ /* Now pending_cpus is zero. */
+ atomic_set(&cpu->running, true);
+ } else {
+ /* Counted in pending_cpus, go ahead and release the
+ * waiter at cpu_exec_end.
+ */
+ }
+ qemu_mutex_unlock(&qemu_cpu_list_lock);
+ }
}
/* Mark cpu as not executing, and release pending exclusive ops. */
void cpu_exec_end(CPUState *cpu)
{
- qemu_mutex_lock(&qemu_cpu_list_lock);
- cpu->running = false;
- if (pending_cpus > 1) {
- pending_cpus--;
- if (pending_cpus == 1) {
- qemu_cond_signal(&exclusive_cond);
+ atomic_set(&cpu->running, false);
+
+ /* Write cpu->running before reading pending_cpus. */
+ smp_mb();
+
+ /* 1. start_exclusive saw cpu->running == true. Then it will increment
+ * pending_cpus and wait for exclusive_cond. After taking the lock
+ * we'll see cpu->has_waiter == true.
+ *
+ * 2. start_exclusive saw cpu->running == false but here pending_cpus >= 1.
+ * This includes the case when an exclusive item started after setting
+ * cpu->running to false and before we read pending_cpus. Then we'll see
+ * cpu->has_waiter == false and not touch pending_cpus. The next call to
+ * cpu_exec_start will run exclusive_idle if still necessary, thus waiting
+ * for the item to complete.
+ *
+ * 3. pending_cpus == 0. Then start_exclusive is definitely going to
+ * see cpu->running == false, and it can ignore this CPU until the
+ * next cpu_exec_start.
+ */
+ if (unlikely(atomic_read(&pending_cpus))) {
+ qemu_mutex_lock(&qemu_cpu_list_lock);
+ if (cpu->has_waiter) {
+ cpu->has_waiter = false;
+ atomic_set(&pending_cpus, pending_cpus - 1);
+ if (pending_cpus == 1) {
+ qemu_cond_signal(&exclusive_cond);
+ }
}
+ qemu_mutex_unlock(&qemu_cpu_list_lock);
}
- qemu_mutex_unlock(&qemu_cpu_list_lock);
}
void async_safe_run_on_cpu(CPUState *cpu, run_on_cpu_func func, void *data)
diff --git a/docs/tcg-exclusive.promela b/docs/tcg-exclusive.promela
index feac679..c91cfca 100644
--- a/docs/tcg-exclusive.promela
+++ b/docs/tcg-exclusive.promela
@@ -13,7 +13,8 @@
* gcc pan.c -O2
* ./a.out -a
*
- * Tunable processor macros: N_CPUS, N_EXCLUSIVE, N_CYCLES, TEST_EXPENSIVE.
+ * Tunable processor macros: N_CPUS, N_EXCLUSIVE, N_CYCLES, USE_MUTEX,
+ * TEST_EXPENSIVE.
*/
// Define the missing parameters for the model
@@ -22,8 +23,10 @@
#warning defaulting to 2 CPU processes
#endif
-// the expensive test is not so expensive for <= 3 CPUs
-#if N_CPUS <= 3
+// the expensive test is not so expensive for <= 2 CPUs
+// If the mutex is used, it's also cheap (300 MB / 4 seconds) for 3 CPUs
+// For 3 CPUs and the lock-free option it needs 1.5 GB of RAM
+#if N_CPUS <= 2 || (N_CPUS <= 3 && defined USE_MUTEX)
#define TEST_EXPENSIVE
#endif
@@ -107,6 +110,8 @@ byte has_waiter[N_CPUS];
COND_BROADCAST(exclusive_resume); \
MUTEX_UNLOCK(mutex);
+#ifdef USE_MUTEX
+// Simple version using mutexes
#define cpu_exec_start(id) \
MUTEX_LOCK(mutex); \
exclusive_idle(); \
@@ -127,6 +132,48 @@ byte has_waiter[N_CPUS];
:: else -> skip; \
fi; \
MUTEX_UNLOCK(mutex);
+#else
+// Wait-free fast path, only needs mutex when concurrent with
+// an exclusive section
+#define cpu_exec_start(id) \
+ running[id] = 1; \
+ if \
+ :: pending_cpus -> { \
+ MUTEX_LOCK(mutex); \
+ if \
+ :: !has_waiter[id] -> { \
+ running[id] = 0; \
+ exclusive_idle(); \
+ running[id] = 1; \
+ } \
+ :: else -> skip; \
+ fi; \
+ MUTEX_UNLOCK(mutex); \
+ } \
+ :: else -> skip; \
+ fi;
+
+#define cpu_exec_end(id) \
+ running[id] = 0; \
+ if \
+ :: pending_cpus -> { \
+ MUTEX_LOCK(mutex); \
+ if \
+ :: has_waiter[id] -> { \
+ has_waiter[id] = 0; \
+ pending_cpus--; \
+ if \
+ :: pending_cpus == 1 -> COND_BROADCAST(exclusive_cond); \
+ :: else -> skip; \
+ fi; \
+ } \
+ :: else -> skip; \
+ fi; \
+ MUTEX_UNLOCK(mutex); \
+ } \
+ :: else -> skip; \
+ fi
+#endif
// Promela processes
diff --git a/include/qom/cpu.h b/include/qom/cpu.h
index 5dfe74a..22b54d6 100644
--- a/include/qom/cpu.h
+++ b/include/qom/cpu.h
@@ -242,7 +242,8 @@ struct qemu_work_item;
* @nr_threads: Number of threads within this CPU.
* @numa_node: NUMA node this CPU is belonging to.
* @host_tid: Host thread ID.
- * @running: #true if CPU is currently running;
+ * @running: #true if CPU is currently running (lockless).
+ * @has_waiter: #true if a CPU is currently waiting for the cpu_exec_end;
* valid under cpu_list_lock.
* @created: Indicates whether the CPU thread has been successfully created.
* @interrupt_request: Indicates a pending interrupt request.
@@ -296,7 +297,7 @@ struct CPUState {
#endif
int thread_id;
uint32_t host_tid;
- bool running;
+ bool running, has_waiter;
struct QemuCond *halt_cond;
bool thread_kicked;
bool created;
generated by cgit v1.1 at 2024-12-24 16:17:52 +0000