qapi/iothread: introduce poll-weight parameter for aio-poll

Introduce a configurable poll-weight parameter for adaptive polling
in IOThread. This parameter replaces the hardcoded POLL_WEIGHT_SHIFT
constant, allowing runtime control over how much the most recent
event interval affects the next polling duration calculation.

The poll-weight parameter uses a shift value where larger values
decrease the weight of the current interval, enabling more gradual
adjustments. When set to 0, a default value of 3 is used (meaning
the current interval contributes approximately 1/8 to the weighted
average).

This patch also removes the hardcoded default value checks from
adjust_polling_time(). Instead, poll-grow, poll-shrink, and
poll-weight now use default values initialized in iothread.c
during IOThread creation.

Signed-off-by: Jaehoon Kim <jhkim@linux.ibm.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20260423195918.661299-4-jhkim@linux.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 34 insertions, 13 deletions

diff --git a/iothread.c b/iothread.c
index caf68e0764..3558535b40 100644
--- a/iothread.c
+++ b/iothread.c
@@ -25,17 +25,6 @@
 #include "qemu/rcu.h"
 #include "qemu/main-loop.h"
 
-
-#ifdef CONFIG_POSIX
-/* Benchmark results from 2016 on NVMe SSD drives show max polling times around
- * 16-32 microseconds yield IOPS improvements for both iodepth=1 and iodepth=32
- * workloads.
- */
-#define IOTHREAD_POLL_MAX_NS_DEFAULT 32768ULL
-#else
-#define IOTHREAD_POLL_MAX_NS_DEFAULT 0ULL
-#endif
-
 static void *iothread_run(void *opaque)
 {
     IOThread *iothread = opaque;
@@ -103,6 +92,10 @@ static void iothread_instance_init(Object *obj)
     IOThread *iothread = IOTHREAD(obj);
 
     iothread->poll_max_ns = IOTHREAD_POLL_MAX_NS_DEFAULT;
+    iothread->poll_grow = IOTHREAD_POLL_GROW_DEFAULT;
+    iothread->poll_shrink = IOTHREAD_POLL_SHRINK_DEFAULT;
+    iothread->poll_weight = IOTHREAD_POLL_WEIGHT_DEFAULT;
+
     iothread->thread_id = -1;
     qemu_sem_init(&iothread->init_done_sem, 0);
     /* By default, we don't run gcontext */
@@ -164,6 +157,7 @@ static void iothread_set_aio_context_params(EventLoopBase *base, Error **errp)
                                 iothread->poll_max_ns,
                                 iothread->poll_grow,
                                 iothread->poll_shrink,
+                                iothread->poll_weight,
                                 errp);
     if (*errp) {
         return;
@@ -233,6 +227,9 @@ static IOThreadParamInfo poll_grow_info = {
 static IOThreadParamInfo poll_shrink_info = {
     "poll-shrink", offsetof(IOThread, poll_shrink),
 };
+static IOThreadParamInfo poll_weight_info = {
+    "poll-weight", offsetof(IOThread, poll_weight),
+};
 
 static void iothread_get_param(Object *obj, Visitor *v,
         const char *name, IOThreadParamInfo *info, Error **errp)
@@ -254,13 +251,31 @@ static bool iothread_set_param(Object *obj, Visitor *v,
         return false;
     }
 
-    if (value < 0) {
+    if (info->offset == offsetof(IOThread, poll_weight)) {
+        if (value < 0 || value > 63) {
+            error_setg(errp, "%s value must be in range [0, 63]",
+                       info->name);
+            return false;
+        }
+    } else if (value < 0) {
         error_setg(errp, "%s value must be in range [0, %" PRId64 "]",
                    info->name, INT64_MAX);
         return false;
     }
 
-    *field = value;
+    if (value == 0) {
+        if (info->offset == offsetof(IOThread, poll_grow)) {
+            *field = IOTHREAD_POLL_GROW_DEFAULT;
+        } else if (info->offset == offsetof(IOThread, poll_shrink)) {
+            *field = IOTHREAD_POLL_SHRINK_DEFAULT;
+        } else if (info->offset == offsetof(IOThread, poll_weight)) {
+            *field = IOTHREAD_POLL_WEIGHT_DEFAULT;
+        } else {
+            *field = value;
+        }
+    } else {
+        *field = value;
+    }
 
     return true;
 }
@@ -288,6 +303,7 @@ static void iothread_set_poll_param(Object *obj, Visitor *v,
                                     iothread->poll_max_ns,
                                     iothread->poll_grow,
                                     iothread->poll_shrink,
+                                    iothread->poll_weight,
                                     errp);
     }
 }
@@ -311,6 +327,10 @@ static void iothread_class_init(ObjectClass *klass, const void *class_data)
                               iothread_get_poll_param,
                               iothread_set_poll_param,
                               NULL, &poll_shrink_info);
+    object_class_property_add(klass, "poll-weight", "int",
+                              iothread_get_poll_param,
+                              iothread_set_poll_param,
+                              NULL, &poll_weight_info);
 }
 
 static const TypeInfo iothread_info = {
@@ -356,6 +376,7 @@ static int query_one_iothread(Object *object, void *opaque)
     info->poll_max_ns = iothread->poll_max_ns;
     info->poll_grow = iothread->poll_grow;
     info->poll_shrink = iothread->poll_shrink;
+    info->poll_weight = iothread->poll_weight;
     info->aio_max_batch = iothread->parent_obj.aio_max_batch;
 
     QAPI_LIST_APPEND(*tail, info);