[Qemu-devel] [PATCH 12/12] cpus-common: lock-free fast path for cpu_exec_start/end

[Paolo Bonzini]

Set cpu->running without taking the cpu_list lock, only look at it if
there is a concurrent exclusive section.  This requires adding a new
field to CPUState, which records whether a running CPU is being counted
in pending_cpus.  When an exclusive section is started concurrently with
cpu_exec_start, cpu_exec_start can use the new field to wait for the end
of the exclusive section.

This a separate patch for easier bisection of issues.

Signed-off-by: Paolo Bonzini <address@hidden>
---
 cpus-common.c              |  72 +++++++++++++--
 docs/tcg-exclusive.promela | 224 +++++++++++++++++++++++++++++++++++++++++++++
 include/qom/cpu.h          |   5 +-
 3 files changed, 289 insertions(+), 12 deletions(-)
 create mode 100644 docs/tcg-exclusive.promela

diff --git a/cpus-common.c b/cpus-common.c
index 88cf5ec..443617a 100644
--- a/cpus-common.c
+++ b/cpus-common.c
@@ -170,8 +170,12 @@ void start_exclusive(void)
 
     /* Make all other cpus stop executing.  */
     pending_cpus = 1;
+
+    /* Write pending_cpus before reading other_cpu->running.  */
+    smp_mb();
     CPU_FOREACH(other_cpu) {
         if (other_cpu->running) {
+            other_cpu->has_waiter = true;
             pending_cpus++;
             qemu_cpu_kick(other_cpu);
         }
@@ -192,25 +196,73 @@ 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_mutex);
-    exclusive_idle();
     cpu->running = true;
-    qemu_mutex_unlock(&qemu_cpu_list_mutex);
+
+    /* 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_cpu == 0.  Then start_exclusive is definitely going to
+     * see cpu->running == true, and it will kick the CPU.
+     */
+    if (pending_cpus) {
+        qemu_mutex_lock(&qemu_cpu_list_mutex);
+        if (!cpu->has_waiter) {
+            /* Not counted in pending_cpus, let the exclusive item
+             * run.  Since we have the lock, set cpu->running to true
+             * while holding it instead of retrying.
+             */
+            cpu->running = false;
+            exclusive_idle();
+            cpu->running = true;
+        } else {
+            /* Counted in pending_cpus, go ahead.  */
+        }
+        qemu_mutex_unlock(&qemu_cpu_list_mutex);
+    }
 }
 
 /* Mark cpu as not executing, and release pending exclusive ops.  */
 void cpu_exec_end(CPUState *cpu)
 {
-    qemu_mutex_lock(&qemu_cpu_list_mutex);
     cpu->running = false;
-    if (pending_cpus > 1) {
-        pending_cpus--;
-        if (pending_cpus == 1) {
-            qemu_cond_signal(&exclusive_cond);
+
+    /* 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 is running now.
+     * Then we'll see cpu->has_waiter == false and not touch pending_cpus,
+     * but will run exclusive_idle to wait for the item to complete.
+     *
+     * 3. pending_cpu == 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 (pending_cpus) {
+        qemu_mutex_lock(&qemu_cpu_list_mutex);
+        if (cpu->has_waiter) {
+            cpu->has_waiter = false;
+            if (--pending_cpus == 1) {
+                qemu_cond_signal(&exclusive_cond);
+            }
+            exclusive_idle();
         }
+        qemu_mutex_unlock(&qemu_cpu_list_mutex);
     }
-    exclusive_idle();
-    qemu_mutex_unlock(&qemu_cpu_list_mutex);
 }
 
 static void async_safe_run_on_cpu_fn(CPUState *cpu, void *data)
diff --git a/docs/tcg-exclusive.promela b/docs/tcg-exclusive.promela
new file mode 100644
index 0000000..293b530
--- /dev/null
+++ b/docs/tcg-exclusive.promela
@@ -0,0 +1,224 @@
+/*
+ * This model describes the implementation of exclusive sections in
+ * cpus-common.c (start_exclusive, end_exclusive, cpu_exec_start,
+ * cpu_exec_end).
+ *
+ * Author: Paolo Bonzini <address@hidden>
+ *
+ * This file is in the public domain.  If you really want a license,
+ * the WTFPL will do.
+ *
+ * To verify it:
+ *     spin -a docs/event.promela
+ *     ./a.out -a
+ *
+ * Tunable processor macros: N_CPUS, N_EXCLUSIVE, N_CYCLES, USE_MUTEX,
+ *                           TEST_EXPENSIVE.
+ */
+
+// Define the missing parameters for the model
+#ifndef N_CPUS
+#define N_CPUS 2
+#warning defaulting to 2 CPU processes
+#endif
+
+// 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
+
+#ifndef N_EXCLUSIVE
+# if !defined N_CYCLES || N_CYCLES <= 1 || defined TEST_EXPENSIVE
+#  define N_EXCLUSIVE     2
+#  warning defaulting to 2 concurrent exclusive sections
+# else
+#  define N_EXCLUSIVE     1
+#  warning defaulting to 1 concurrent exclusive sections
+# endif
+#endif
+#ifndef N_CYCLES
+# if N_EXCLUSIVE <= 1 || defined TEST_EXPENSIVE
+#  define N_CYCLES        2
+#  warning defaulting to 2 CPU cycles
+# else
+#  define N_CYCLES        1
+#  warning defaulting to 1 CPU cycles
+# endif
+#endif
+
+
+// synchronization primitives.  condition variables require a
+// process-local "cond_t saved;" variable.
+
+#define mutex_t              byte
+#define MUTEX_LOCK(m)        atomic { m == 0 -> m = 1 }
+#define MUTEX_UNLOCK(m)      m = 0
+
+#define cond_t               int
+#define COND_WAIT(c, m)      {                                  \
+                               saved = c;                       \
+                               MUTEX_UNLOCK(m);                 \
+                               c != saved -> MUTEX_LOCK(m);     \
+                             }
+#define COND_BROADCAST(c)    c++
+
+// this is the logic from cpus-common.c
+
+mutex_t mutex;
+cond_t exclusive_cond;
+cond_t exclusive_resume;
+byte pending_cpus;
+
+byte running[N_CPUS];
+byte has_waiter[N_CPUS];
+
+#define exclusive_idle()                                          \
+  do                                                              \
+      :: pending_cpus -> COND_WAIT(exclusive_resume, mutex);      \
+      :: else         -> break;                                   \
+  od
+
+#define start_exclusive()                                         \
+    MUTEX_LOCK(mutex);                                            \
+    exclusive_idle();                                             \
+    pending_cpus = 1;                                             \
+                                                                  \
+    i = 0;                                                        \
+    do                                                            \
+       :: i < N_CPUS -> {                                         \
+           if                                                     \
+              :: running[i] -> has_waiter[i] = 1; pending_cpus++; \
+              :: else       -> skip;                              \
+           fi;                                                    \
+           i++;                                                   \
+       }                                                          \
+       :: else -> break;                                          \
+    od;                                                           \
+                                                                  \
+    do                                                            \
+      :: pending_cpus > 1 -> COND_WAIT(exclusive_cond, mutex);    \
+      :: else             -> break;                               \
+    od;
+
+#define end_exclusive()                                           \
+    pending_cpus = 0;                                             \
+    COND_BROADCAST(exclusive_resume);                             \
+    MUTEX_UNLOCK(mutex);
+
+#ifdef USE_MUTEX
+// Simple version using mutexes
+#define cpu_exec_start(id)                                                   \
+    MUTEX_LOCK(mutex);                                                       \
+    exclusive_idle();                                                        \
+    running[id] = 1;                                                         \
+    MUTEX_UNLOCK(mutex);
+
+#define cpu_exec_end(id)                                                     \
+    MUTEX_LOCK(mutex);                                                       \
+    running[id] = 0;                                                         \
+    if                                                                       \
+        :: pending_cpus -> {                                                 \
+            pending_cpus--;                                                  \
+            if                                                               \
+                :: pending_cpus == 1 -> COND_BROADCAST(exclusive_cond);      \
+                :: else -> skip;                                             \
+            fi;                                                              \
+            exclusive_idle();                                                \
+        }                                                                    \
+        :: 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;                                                      \
+                    exclusive_idle();                                        \
+                }                                                            \
+                :: else -> skip;                                             \
+            fi;                                                              \
+            MUTEX_UNLOCK(mutex);                                             \
+        }                                                                    \
+        :: else -> skip;                                                     \
+    fi
+#endif
+
+// Promela processes
+
+byte done_cpu;
+byte in_cpu;
+active[N_CPUS] proctype cpu()
+{
+    byte id = _pid % N_CPUS;
+    byte cycles = 0;
+    cond_t saved;
+
+    do
+       :: cycles == N_CYCLES -> break;
+       :: else -> {
+           cycles++;
+           cpu_exec_start(id)
+           in_cpu++;
+           done_cpu++;
+           in_cpu--;
+           cpu_exec_end(id)
+       }
+    od;
+}
+
+byte done_exclusive;
+byte in_exclusive;
+active[N_EXCLUSIVE] proctype exclusive()
+{
+    cond_t saved;
+    byte i;
+
+    start_exclusive();
+    in_exclusive = 1;
+    done_exclusive++;
+    in_exclusive = 0;
+    end_exclusive();
+}
+
+#define LIVENESS   (done_cpu == N_CPUS * N_CYCLES && done_exclusive == 
N_EXCLUSIVE)
+#define SAFETY     !(in_exclusive && in_cpu)
+
+never {    /* ! ([] SAFETY && <> [] LIVENESS) */
+    do
+    // once the liveness property is satisfied, this is not executable
+    // and the never clause is not accepted
+    :: ! LIVENESS -> accept_liveness: skip
+    :: 1          -> assert(SAFETY)
+    od;
+}
diff --git a/include/qom/cpu.h b/include/qom/cpu.h
index 3817a98..14cf97c 100644
--- a/include/qom/cpu.h
+++ b/include/qom/cpu.h
@@ -249,7 +249,8 @@ 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.
@@ -303,7 +304,7 @@ 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;
-- 
2.7.4