[Qemu-devel] [RFC PATCH 2/3] cpus-common: Cache allocated work items

[Pranith Kumar]

Using heaptrack, I found that quite a few of our temporary allocations
are coming from allocating work items. Instead of doing this
continously, we can cache the allocated items and reuse them instead
of freeing them.

This reduces the number of allocations by 25% (200000 -> 150000 for
ARM64 boot+shutdown test).

Signed-off-by: Pranith Kumar <address@hidden>
---
 cpus-common.c | 85 ++++++++++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 70 insertions(+), 15 deletions(-)

diff --git a/cpus-common.c b/cpus-common.c
index 59f751ecf9..a1c4c7d1a3 100644
--- a/cpus-common.c
+++ b/cpus-common.c
@@ -24,6 +24,7 @@
 #include "sysemu/cpus.h"
 
 static QemuMutex qemu_cpu_list_lock;
+static QemuMutex qemu_wi_pool_lock;
 static QemuCond exclusive_cond;
 static QemuCond exclusive_resume;
 static QemuCond qemu_work_cond;
@@ -33,6 +34,58 @@ static QemuCond qemu_work_cond;
  */
 static int pending_cpus;
 
+typedef struct qemu_work_item {
+    struct qemu_work_item *next;
+    run_on_cpu_func func;
+    run_on_cpu_data data;
+    bool free, exclusive, done;
+} qemu_work_item;
+
+typedef struct qemu_wi_pool {
+    qemu_work_item *first, *last;
+} qemu_wi_pool;
+
+qemu_wi_pool *wi_free_pool;
+
+static void qemu_init_workitem_pool(void)
+{
+    wi_free_pool = g_malloc0(sizeof(qemu_wi_pool));
+    wi_free_pool->first = NULL;
+    wi_free_pool->last  = NULL;
+}
+
+static void qemu_wi_pool_insert(qemu_work_item *item)
+{
+    qemu_mutex_lock(&qemu_wi_pool_lock);
+    if (wi_free_pool->last == NULL) {
+        wi_free_pool->first = item;
+        wi_free_pool->last = item;
+    } else {
+        wi_free_pool->last->next = item;
+        wi_free_pool->last = item;
+    }
+    qemu_mutex_unlock(&qemu_wi_pool_lock);
+}
+
+static qemu_work_item* qemu_wi_pool_remove(void)
+{
+    qemu_mutex_lock(&qemu_wi_pool_lock);
+    qemu_work_item *ret = wi_free_pool->first;
+
+    if (ret == NULL)
+        goto out;
+
+    wi_free_pool->first = ret->next;
+    if (wi_free_pool->last == ret) {
+        wi_free_pool->last = NULL;
+    }
+    ret->next = NULL;
+
+ out:
+    qemu_mutex_unlock(&qemu_wi_pool_lock);
+    return ret;
+}
+
 void qemu_init_cpu_list(void)
 {
     /* This is needed because qemu_init_cpu_list is also called by the
@@ -43,6 +96,9 @@ void qemu_init_cpu_list(void)
     qemu_cond_init(&exclusive_cond);
     qemu_cond_init(&exclusive_resume);
     qemu_cond_init(&qemu_work_cond);
+
+    qemu_init_workitem_pool();
+    qemu_mutex_init(&qemu_wi_pool_lock);
 }
 
 void cpu_list_lock(void)
@@ -106,14 +162,7 @@ void cpu_list_remove(CPUState *cpu)
     qemu_mutex_unlock(&qemu_cpu_list_lock);
 }
 
-struct qemu_work_item {
-    struct qemu_work_item *next;
-    run_on_cpu_func func;
-    run_on_cpu_data data;
-    bool free, exclusive, done;
-};
-
-static void queue_work_on_cpu(CPUState *cpu, struct qemu_work_item *wi)
+static void queue_work_on_cpu(CPUState *cpu, qemu_work_item *wi)
 {
     qemu_mutex_lock(&cpu->work_mutex);
     if (cpu->queued_work_first == NULL) {
@@ -132,7 +181,7 @@ static void queue_work_on_cpu(CPUState *cpu, struct 
qemu_work_item *wi)
 void do_run_on_cpu(CPUState *cpu, run_on_cpu_func func, run_on_cpu_data data,
                    QemuMutex *mutex)
 {
-    struct qemu_work_item wi;
+    qemu_work_item wi;
 
     if (qemu_cpu_is_self(cpu)) {
         func(cpu, data);
@@ -145,6 +194,7 @@ void do_run_on_cpu(CPUState *cpu, run_on_cpu_func func, 
run_on_cpu_data data,
     wi.free = false;
     wi.exclusive = false;
 
+
     queue_work_on_cpu(cpu, &wi);
     while (!atomic_mb_read(&wi.done)) {
         CPUState *self_cpu = current_cpu;
@@ -156,9 +206,11 @@ void do_run_on_cpu(CPUState *cpu, run_on_cpu_func func, 
run_on_cpu_data data,
 
 void async_run_on_cpu(CPUState *cpu, run_on_cpu_func func, run_on_cpu_data 
data)
 {
-    struct qemu_work_item *wi;
+    qemu_work_item *wi = qemu_wi_pool_remove();
 
-    wi = g_malloc0(sizeof(struct qemu_work_item));
+    if (!wi) {
+        wi = g_malloc0(sizeof(qemu_work_item));
+    }
     wi->func = func;
     wi->data = data;
     wi->free = true;
@@ -299,9 +351,11 @@ void cpu_exec_end(CPUState *cpu)
 void async_safe_run_on_cpu(CPUState *cpu, run_on_cpu_func func,
                            run_on_cpu_data data)
 {
-    struct qemu_work_item *wi;
+    qemu_work_item *wi = qemu_wi_pool_remove();
 
-    wi = g_malloc0(sizeof(struct qemu_work_item));
+    if (!wi) {
+        wi = g_malloc0(sizeof(qemu_work_item));
+    }
     wi->func = func;
     wi->data = data;
     wi->free = true;
@@ -312,7 +366,7 @@ void async_safe_run_on_cpu(CPUState *cpu, run_on_cpu_func 
func,
 
 void process_queued_cpu_work(CPUState *cpu)
 {
-    struct qemu_work_item *wi;
+    qemu_work_item *wi;
 
     if (cpu->queued_work_first == NULL) {
         return;
@@ -343,7 +397,8 @@ void process_queued_cpu_work(CPUState *cpu)
         }
         qemu_mutex_lock(&cpu->work_mutex);
         if (wi->free) {
-            g_free(wi);
+            memset(wi, 0, sizeof(qemu_work_item));
+            qemu_wi_pool_insert(wi);
         } else {
             atomic_mb_set(&wi->done, true);
         }
-- 
2.13.0