[PATCH v4 0/4] util/thread-pool: Expose minimun and maximum size

[PATCH v4 0/4] util/thread-pool: Expose minimun and maximum size

[Nicolas Saenz Julienne]

As discussed on the previous RFC[1] the thread-pool's dynamic thread
management doesn't play well with real-time and latency sensitive
systems. This series introduces a set of controls that'll permit
achieving more deterministic behaviours, for example by fixing the
pool's size.

We first introduce a new common interface to event loop configuration by
moving iothread's already available properties into an abstract class
called 'EventLooopBackend' and have both 'IOThread' and the newly
created 'MainLoop' inherit the properties from that class.

With this new configuration interface in place it's relatively simple to
introduce new options to fix the even loop's thread pool sizes. The
resulting QAPI looks like this:

    -object main-loop,id=main-loop,thread-pool-min=1,thread-pool-max=1

Note that all patches are bisect friendly and pass all the tests.

[1] https://patchwork.ozlabs.org/project/qemu-devel/patch/20220202175234.656711-1-nsaenzju@redhat.com/

@Stefan I kept your Signed-off-by, since the changes trivial/not
thread-pool related

---
Changes since v3:
 - Avoid duplication in qom.json by creating EventLoopBaseProperties.
 - Fix failures on first compilation due to race between
   event-loop-base.o and qapi header generation.

Changes since v2:
 - Get rid of wrong locking/waiting
 - Fix qapi versioning
 - Better commit messages

Changes since v1:
 - Address all Stefan's comments
 - Introduce new fix

Nicolas Saenz Julienne (3):
  Introduce event-loop-base abstract class
  util/main-loop: Introduce the main loop into QOM
  util/event-loop-base: Introduce options to set the thread pool size

 event-loop-base.c                | 140 +++++++++++++++++++++++++++++++
 include/block/aio.h              |  10 +++
 include/block/thread-pool.h      |   3 +
 include/qemu/main-loop.h         |  10 +++
 include/sysemu/event-loop-base.h |  41 +++++++++
 include/sysemu/iothread.h        |   6 +-
 iothread.c                       |  68 +++++----------
 meson.build                      |  26 +++---
 qapi/qom.json                    |  40 +++++++--
 util/aio-posix.c                 |   1 +
 util/async.c                     |  20 +++++
 util/main-loop.c                 |  65 ++++++++++++++
 util/thread-pool.c               |  55 +++++++++++-
 13 files changed, 416 insertions(+), 69 deletions(-)
 create mode 100644 event-loop-base.c
 create mode 100644 include/sysemu/event-loop-base.h

-- 
2.35.1

[PATCH v4 1/3] Introduce event-loop-base abstract class

[Nicolas Saenz Julienne]

Introduce the 'event-loop-base' abstract class, it'll hold the
properties common to all event loops and provide the necessary hooks for
their creation and maintenance. Then have iothread inherit from it.

EventLoopBaseClass is defined as user creatable and provides a hook for
its children to attach themselves to the user creatable class 'complete'
function. It also provides an update_params() callback to propagate
property changes onto its children.

The new 'event-loop-base' class will live in the root directory. It is
built on its own using the 'link_whole' option (there are no direct
function dependencies between the class and its children, it all happens
trough 'constructor' magic). And also imposes new compilation
dependencies:

    qom <- event-loop-base <- blockdev (iothread.c)

And in subsequent patches:

    qom <- event-loop-base <- qemuutil (util/main-loop.c)

All this forced some amount of reordering in meson.build:

 - Moved qom build definition before qemuutil. Doing it the other way
   around (i.e. moving qemuutil after qom) isn't possible as a lot of
   core libraries that live in between the two depend on it.

 - Process the 'hw' subdir earlier, as it introduces files into the
   'qom' source set.

No functional changes intended.

Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

---
Changes since v3:
 - Fix event-loop-base compilation so it depends on qapi header
   generation (note the '+ genh' in event_loop_base).

Changes since v2:
 - reword commit message to better explain compilation dependencies.

Changes since v1:
 - Rename to event-loop-base
 - Move event-loop-base into root directory
 - Build event-loop-base on its own, use link_whole to avoid the problem
   of the object file not being linked due to lacking direct calls from
   dependencies.
 - Move poll parameters into iothread, as main loop can't poll
 - Update Authorship (I took what iothread.c had and added myself, I
   hope that's fine)
 - Introduce update_params() callback

 event-loop-base.c                | 104 +++++++++++++++++++++++++++++++
 include/sysemu/event-loop-base.h |  36 +++++++++++
 include/sysemu/iothread.h        |   6 +-
 iothread.c                       |  65 ++++++-------------
 meson.build                      |  23 ++++---
 5 files changed, 175 insertions(+), 59 deletions(-)
 create mode 100644 event-loop-base.c
 create mode 100644 include/sysemu/event-loop-base.h

diff --git a/event-loop-base.c b/event-loop-base.c
new file mode 100644
index 0000000000..a924c73a7c
--- /dev/null
+++ b/event-loop-base.c
@@ -0,0 +1,104 @@
+/*
+ * QEMU event-loop base
+ *
+ * Copyright (C) 2022 Red Hat Inc
+ *
+ * Authors:
+ *  Stefan Hajnoczi <stefanha@redhat.com>
+ *  Nicolas Saenz Julienne <nsaenzju@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#include "qemu/osdep.h"
+#include "qom/object_interfaces.h"
+#include "qapi/error.h"
+#include "sysemu/event-loop-base.h"
+
+typedef struct {
+    const char *name;
+    ptrdiff_t offset; /* field's byte offset in EventLoopBase struct */
+} EventLoopBaseParamInfo;
+
+static EventLoopBaseParamInfo aio_max_batch_info = {
+    "aio-max-batch", offsetof(EventLoopBase, aio_max_batch),
+};
+
+static void event_loop_base_get_param(Object *obj, Visitor *v,
+        const char *name, void *opaque, Error **errp)
+{
+    EventLoopBase *event_loop_base = EVENT_LOOP_BASE(obj);
+    EventLoopBaseParamInfo *info = opaque;
+    int64_t *field = (void *)event_loop_base + info->offset;
+
+    visit_type_int64(v, name, field, errp);
+}
+
+static void event_loop_base_set_param(Object *obj, Visitor *v,
+        const char *name, void *opaque, Error **errp)
+{
+    EventLoopBaseClass *bc = EVENT_LOOP_BASE_GET_CLASS(obj);
+    EventLoopBase *base = EVENT_LOOP_BASE(obj);
+    EventLoopBaseParamInfo *info = opaque;
+    int64_t *field = (void *)base + info->offset;
+    int64_t value;
+
+    if (!visit_type_int64(v, name, &value, errp)) {
+        return;
+    }
+
+    if (value < 0) {
+        error_setg(errp, "%s value must be in range [0, %" PRId64 "]",
+                   info->name, INT64_MAX);
+        return;
+    }
+
+    *field = value;
+
+    if (bc->update_params) {
+        bc->update_params(base, errp);
+    }
+
+    return;
+}
+
+static void event_loop_base_complete(UserCreatable *uc, Error **errp)
+{
+    EventLoopBaseClass *bc = EVENT_LOOP_BASE_GET_CLASS(uc);
+    EventLoopBase *base = EVENT_LOOP_BASE(uc);
+
+    if (bc->init) {
+        bc->init(base, errp);
+    }
+}
+
+static void event_loop_base_class_init(ObjectClass *klass, void *class_data)
+{
+    UserCreatableClass *ucc = USER_CREATABLE_CLASS(klass);
+    ucc->complete = event_loop_base_complete;
+
+    object_class_property_add(klass, "aio-max-batch", "int",
+                              event_loop_base_get_param,
+                              event_loop_base_set_param,
+                              NULL, &aio_max_batch_info);
+}
+
+static const TypeInfo event_loop_base_info = {
+    .name = TYPE_EVENT_LOOP_BASE,
+    .parent = TYPE_OBJECT,
+    .instance_size = sizeof(EventLoopBase),
+    .class_size = sizeof(EventLoopBaseClass),
+    .class_init = event_loop_base_class_init,
+    .abstract = true,
+    .interfaces = (InterfaceInfo[]) {
+        { TYPE_USER_CREATABLE },
+        { }
+    }
+};
+
+static void register_types(void)
+{
+    type_register_static(&event_loop_base_info);
+}
+type_init(register_types);
diff --git a/include/sysemu/event-loop-base.h b/include/sysemu/event-loop-base.h
new file mode 100644
index 0000000000..8e77d8b69f
--- /dev/null
+++ b/include/sysemu/event-loop-base.h
@@ -0,0 +1,36 @@
+/*
+ * QEMU event-loop backend
+ *
+ * Copyright (C) 2022 Red Hat Inc
+ *
+ * Authors:
+ *  Nicolas Saenz Julienne <nsaenzju@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+#ifndef QEMU_EVENT_LOOP_BASE_H
+#define QEMU_EVENT_LOOP_BASE_H
+
+#include "qom/object.h"
+#include "block/aio.h"
+#include "qemu/typedefs.h"
+
+#define TYPE_EVENT_LOOP_BASE         "event-loop-base"
+OBJECT_DECLARE_TYPE(EventLoopBase, EventLoopBaseClass,
+                    EVENT_LOOP_BASE)
+
+struct EventLoopBaseClass {
+    ObjectClass parent_class;
+
+    void (*init)(EventLoopBase *base, Error **errp);
+    void (*update_params)(EventLoopBase *base, Error **errp);
+};
+
+struct EventLoopBase {
+    Object parent;
+
+    /* AioContext AIO engine parameters */
+    int64_t aio_max_batch;
+};
+#endif
diff --git a/include/sysemu/iothread.h b/include/sysemu/iothread.h
index 7f714bd136..8f8601d6ab 100644
--- a/include/sysemu/iothread.h
+++ b/include/sysemu/iothread.h
@@ -17,11 +17,12 @@
 #include "block/aio.h"
 #include "qemu/thread.h"
 #include "qom/object.h"
+#include "sysemu/event-loop-base.h"
 
 #define TYPE_IOTHREAD "iothread"
 
 struct IOThread {
-    Object parent_obj;
+    EventLoopBase parent_obj;
 
     QemuThread thread;
     AioContext *ctx;
@@ -37,9 +38,6 @@ struct IOThread {
     int64_t poll_max_ns;
     int64_t poll_grow;
     int64_t poll_shrink;
-
-    /* AioContext AIO engine parameters */
-    int64_t aio_max_batch;
 };
 typedef struct IOThread IOThread;
 
diff --git a/iothread.c b/iothread.c
index 0f98af0f2a..8fa2f3bfb8 100644
--- a/iothread.c
+++ b/iothread.c
@@ -17,6 +17,7 @@
 #include "qemu/module.h"
 #include "block/aio.h"
 #include "block/block.h"
+#include "sysemu/event-loop-base.h"
 #include "sysemu/iothread.h"
 #include "qapi/error.h"
 #include "qapi/qapi-commands-misc.h"
@@ -152,10 +153,15 @@ static void iothread_init_gcontext(IOThread *iothread)
     iothread->main_loop = g_main_loop_new(iothread->worker_context, TRUE);
 }
 
-static void iothread_set_aio_context_params(IOThread *iothread, Error **errp)
+static void iothread_set_aio_context_params(EventLoopBase *base, Error **errp)
 {
+    IOThread *iothread = IOTHREAD(base);
     ERRP_GUARD();
 
+    if (!iothread->ctx) {
+        return;
+    }
+
     aio_context_set_poll_params(iothread->ctx,
                                 iothread->poll_max_ns,
                                 iothread->poll_grow,
@@ -166,14 +172,15 @@ static void iothread_set_aio_context_params(IOThread *iothread, Error **errp)
     }
 
     aio_context_set_aio_params(iothread->ctx,
-                               iothread->aio_max_batch,
+                               iothread->parent_obj.aio_max_batch,
                                errp);
 }
 
-static void iothread_complete(UserCreatable *obj, Error **errp)
+
+static void iothread_init(EventLoopBase *base, Error **errp)
 {
     Error *local_error = NULL;
-    IOThread *iothread = IOTHREAD(obj);
+    IOThread *iothread = IOTHREAD(base);
     char *thread_name;
 
     iothread->stopping = false;
@@ -189,7 +196,7 @@ static void iothread_complete(UserCreatable *obj, Error **errp)
      */
     iothread_init_gcontext(iothread);
 
-    iothread_set_aio_context_params(iothread, &local_error);
+    iothread_set_aio_context_params(base, &local_error);
     if (local_error) {
         error_propagate(errp, local_error);
         aio_context_unref(iothread->ctx);
@@ -201,7 +208,7 @@ static void iothread_complete(UserCreatable *obj, Error **errp)
      * to inherit.
      */
     thread_name = g_strdup_printf("IO %s",
-                        object_get_canonical_path_component(OBJECT(obj)));
+                        object_get_canonical_path_component(OBJECT(base)));
     qemu_thread_create(&iothread->thread, thread_name, iothread_run,
                        iothread, QEMU_THREAD_JOINABLE);
     g_free(thread_name);
@@ -226,9 +233,6 @@ static IOThreadParamInfo poll_grow_info = {
 static IOThreadParamInfo poll_shrink_info = {
     "poll-shrink", offsetof(IOThread, poll_shrink),
 };
-static IOThreadParamInfo aio_max_batch_info = {
-    "aio-max-batch", offsetof(IOThread, aio_max_batch),
-};
 
 static void iothread_get_param(Object *obj, Visitor *v,
         const char *name, IOThreadParamInfo *info, Error **errp)
@@ -288,35 +292,12 @@ static void iothread_set_poll_param(Object *obj, Visitor *v,
     }
 }
 
-static void iothread_get_aio_param(Object *obj, Visitor *v,
-        const char *name, void *opaque, Error **errp)
-{
-    IOThreadParamInfo *info = opaque;
-
-    iothread_get_param(obj, v, name, info, errp);
-}
-
-static void iothread_set_aio_param(Object *obj, Visitor *v,
-        const char *name, void *opaque, Error **errp)
-{
-    IOThread *iothread = IOTHREAD(obj);
-    IOThreadParamInfo *info = opaque;
-
-    if (!iothread_set_param(obj, v, name, info, errp)) {
-        return;
-    }
-
-    if (iothread->ctx) {
-        aio_context_set_aio_params(iothread->ctx,
-                                   iothread->aio_max_batch,
-                                   errp);
-    }
-}
-
 static void iothread_class_init(ObjectClass *klass, void *class_data)
 {
-    UserCreatableClass *ucc = USER_CREATABLE_CLASS(klass);
-    ucc->complete = iothread_complete;
+    EventLoopBaseClass *bc = EVENT_LOOP_BASE_CLASS(klass);
+
+    bc->init = iothread_init;
+    bc->update_params = iothread_set_aio_context_params;
 
     object_class_property_add(klass, "poll-max-ns", "int",
                               iothread_get_poll_param,
@@ -330,23 +311,15 @@ static void iothread_class_init(ObjectClass *klass, void *class_data)
                               iothread_get_poll_param,
                               iothread_set_poll_param,
                               NULL, &poll_shrink_info);
-    object_class_property_add(klass, "aio-max-batch", "int",
-                              iothread_get_aio_param,
-                              iothread_set_aio_param,
-                              NULL, &aio_max_batch_info);
 }
 
 static const TypeInfo iothread_info = {
     .name = TYPE_IOTHREAD,
-    .parent = TYPE_OBJECT,
+    .parent = TYPE_EVENT_LOOP_BASE,
     .class_init = iothread_class_init,
     .instance_size = sizeof(IOThread),
     .instance_init = iothread_instance_init,
     .instance_finalize = iothread_instance_finalize,
-    .interfaces = (InterfaceInfo[]) {
-        {TYPE_USER_CREATABLE},
-        {}
-    },
 };
 
 static void iothread_register_types(void)
@@ -383,7 +356,7 @@ 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->aio_max_batch = iothread->aio_max_batch;
+    info->aio_max_batch = iothread->parent_obj.aio_max_batch;
 
     QAPI_LIST_APPEND(*tail, info);
     return 0;
diff --git a/meson.build b/meson.build
index aef724ad3c..89aa3a2518 100644
--- a/meson.build
+++ b/meson.build
@@ -2802,6 +2802,7 @@ subdir('qom')
 subdir('authz')
 subdir('crypto')
 subdir('ui')
+subdir('hw')
 
 
 if enable_modules
@@ -2809,6 +2810,18 @@ if enable_modules
   modulecommon = declare_dependency(link_whole: libmodulecommon, compile_args: '-DBUILD_DSO')
 endif
 
+qom_ss = qom_ss.apply(config_host, strict: false)
+libqom = static_library('qom', qom_ss.sources() + genh,
+                        dependencies: [qom_ss.dependencies()],
+                        name_suffix: 'fa')
+qom = declare_dependency(link_whole: libqom)
+
+event_loop_base = files('event-loop-base.c')
+event_loop_base = static_library('event-loop-base', sources: event_loop_base + genh,
+                                 build_by_default: true)
+event_loop_base = declare_dependency(link_whole: event_loop_base,
+                                     dependencies: [qom])
+
 stub_ss = stub_ss.apply(config_all, strict: false)
 
 util_ss.add_all(trace_ss)
@@ -2895,7 +2908,6 @@ subdir('monitor')
 subdir('net')
 subdir('replay')
 subdir('semihosting')
-subdir('hw')
 subdir('tcg')
 subdir('fpu')
 subdir('accel')
@@ -3020,13 +3032,6 @@ qemu_syms = custom_target('qemu.syms', output: 'qemu.syms',
                              capture: true,
                              command: [undefsym, nm, '@INPUT@'])
 
-qom_ss = qom_ss.apply(config_host, strict: false)
-libqom = static_library('qom', qom_ss.sources() + genh,
-                        dependencies: [qom_ss.dependencies()],
-                        name_suffix: 'fa')
-
-qom = declare_dependency(link_whole: libqom)
-
 authz_ss = authz_ss.apply(config_host, strict: false)
 libauthz = static_library('authz', authz_ss.sources() + genh,
                           dependencies: [authz_ss.dependencies()],
@@ -3079,7 +3084,7 @@ libblockdev = static_library('blockdev', blockdev_ss.sources() + genh,
                              build_by_default: false)
 
 blockdev = declare_dependency(link_whole: [libblockdev],
-                              dependencies: [block])
+                              dependencies: [block, event_loop_base])
 
 qmp_ss = qmp_ss.apply(config_host, strict: false)
 libqmp = static_library('qmp', qmp_ss.sources() + genh,
-- 
2.35.1

[PATCH v4 2/3] util/main-loop: Introduce the main loop into QOM

[Nicolas Saenz Julienne]

'event-loop-base' provides basic property handling for all 'AioContext'
based event loops. So let's define a new 'MainLoopClass' that inherits
from it. This will permit tweaking the main loop's properties through
qapi as well as through the command line using the '-object' keyword[1].
Only one instance of 'MainLoopClass' might be created at any time.

'EventLoopBaseClass' learns a new callback, 'can_be_deleted()' so as to
mark 'MainLoop' as non-deletable.

[1] For example:
      -object main-loop,id=main-loop,aio-max-batch=<value>

Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
---
Changes since v3:
 - Rework qom.json

Changes since v2:
 - Fix mainloop's qapi versioning

Changes since v1:
 - Fix json files to differentiate between iothread and main-loop
 - Use OBJECT_DECLARE_TYPE()
 - Fix build dependencies

 event-loop-base.c                | 13 ++++++++
 include/qemu/main-loop.h         | 10 ++++++
 include/sysemu/event-loop-base.h |  1 +
 meson.build                      |  3 +-
 qapi/qom.json                    | 32 +++++++++++++++---
 util/main-loop.c                 | 56 ++++++++++++++++++++++++++++++++
 6 files changed, 109 insertions(+), 6 deletions(-)

diff --git a/event-loop-base.c b/event-loop-base.c
index a924c73a7c..e7f99a6ec8 100644
--- a/event-loop-base.c
+++ b/event-loop-base.c
@@ -73,10 +73,23 @@ static void event_loop_base_complete(UserCreatable *uc, Error **errp)
     }
 }
 
+static bool event_loop_base_can_be_deleted(UserCreatable *uc)
+{
+    EventLoopBaseClass *bc = EVENT_LOOP_BASE_GET_CLASS(uc);
+    EventLoopBase *backend = EVENT_LOOP_BASE(uc);
+
+    if (bc->can_be_deleted) {
+        return bc->can_be_deleted(backend);
+    }
+
+    return true;
+}
+
 static void event_loop_base_class_init(ObjectClass *klass, void *class_data)
 {
     UserCreatableClass *ucc = USER_CREATABLE_CLASS(klass);
     ucc->complete = event_loop_base_complete;
+    ucc->can_be_deleted = event_loop_base_can_be_deleted;
 
     object_class_property_add(klass, "aio-max-batch", "int",
                               event_loop_base_get_param,
diff --git a/include/qemu/main-loop.h b/include/qemu/main-loop.h
index d3750c8e76..20c9387654 100644
--- a/include/qemu/main-loop.h
+++ b/include/qemu/main-loop.h
@@ -26,9 +26,19 @@
 #define QEMU_MAIN_LOOP_H
 
 #include "block/aio.h"
+#include "qom/object.h"
+#include "sysemu/event-loop-base.h"
 
 #define SIG_IPI SIGUSR1
 
+#define TYPE_MAIN_LOOP  "main-loop"
+OBJECT_DECLARE_TYPE(MainLoop, MainLoopClass, MAIN_LOOP)
+
+struct MainLoop {
+    EventLoopBase parent_obj;
+};
+typedef struct MainLoop MainLoop;
+
 /**
  * qemu_init_main_loop: Set up the process so that it can run the main loop.
  *
diff --git a/include/sysemu/event-loop-base.h b/include/sysemu/event-loop-base.h
index 8e77d8b69f..fced4c9fea 100644
--- a/include/sysemu/event-loop-base.h
+++ b/include/sysemu/event-loop-base.h
@@ -25,6 +25,7 @@ struct EventLoopBaseClass {
 
     void (*init)(EventLoopBase *base, Error **errp);
     void (*update_params)(EventLoopBase *base, Error **errp);
+    bool (*can_be_deleted)(EventLoopBase *base);
 };
 
 struct EventLoopBase {
diff --git a/meson.build b/meson.build
index 89aa3a2518..b8046b5b35 100644
--- a/meson.build
+++ b/meson.build
@@ -2830,7 +2830,8 @@ libqemuutil = static_library('qemuutil',
                              sources: util_ss.sources() + stub_ss.sources() + genh,
                              dependencies: [util_ss.dependencies(), libm, threads, glib, socket, malloc, pixman])
 qemuutil = declare_dependency(link_with: libqemuutil,
-                              sources: genh + version_res)
+                              sources: genh + version_res,
+                              dependencies: [event_loop_base])
 
 if have_system or have_user
   decodetree = generator(find_program('scripts/decodetree.py'),
diff --git a/qapi/qom.json b/qapi/qom.json
index eeb5395ff3..e5f31c4469 100644
--- a/qapi/qom.json
+++ b/qapi/qom.json
@@ -499,6 +499,17 @@
             '*repeat': 'bool',
             '*grab-toggle': 'GrabToggleKeys' } }
 
+##
+# @EventLoopBaseProperties:
+#
+# Common properties for objects derived from EventLoopBase
+#
+# @aio-max-batch: maximum number of requests in a batch for the AIO engine,
+#                 0 means that the engine will use its default.
+##
+{ 'struct': 'EventLoopBaseProperties',
+  'data': { '*aio-max-batch': 'int' } }
+
 ##
 # @IothreadProperties:
 #
@@ -516,17 +527,26 @@
 #               algorithm detects it is spending too long polling without
 #               encountering events. 0 selects a default behaviour (default: 0)
 #
-# @aio-max-batch: maximum number of requests in a batch for the AIO engine,
-#                 0 means that the engine will use its default
-#                 (default:0, since 6.1)
+# The @aio-max-batch option is available since 6.1.
 #
 # Since: 2.0
 ##
 { 'struct': 'IothreadProperties',
+  'base': 'EventLoopBaseProperties',
   'data': { '*poll-max-ns': 'int',
             '*poll-grow': 'int',
-            '*poll-shrink': 'int',
-            '*aio-max-batch': 'int' } }
+            '*poll-shrink': 'int' } }
+
+##
+# @MainLoopProperties:
+#
+# Properties for the main-loop object.
+#
+# Since: 7.1
+##
+{ 'struct': 'MainLoopProperties',
+  'base': 'EventLoopBaseProperties',
+  'data': {} }
 
 ##
 # @MemoryBackendProperties:
@@ -818,6 +838,7 @@
     { 'name': 'input-linux',
       'if': 'CONFIG_LINUX' },
     'iothread',
+    'main-loop',
     { 'name': 'memory-backend-epc',
       'if': 'CONFIG_LINUX' },
     'memory-backend-file',
@@ -883,6 +904,7 @@
       'input-linux':                { 'type': 'InputLinuxProperties',
                                       'if': 'CONFIG_LINUX' },
       'iothread':                   'IothreadProperties',
+      'main-loop':                  'MainLoopProperties',
       'memory-backend-epc':         { 'type': 'MemoryBackendEpcProperties',
                                       'if': 'CONFIG_LINUX' },
       'memory-backend-file':        'MemoryBackendFileProperties',
diff --git a/util/main-loop.c b/util/main-loop.c
index b7b0ce4ca0..5b13f456fa 100644
--- a/util/main-loop.c
+++ b/util/main-loop.c
@@ -33,6 +33,7 @@
 #include "qemu/error-report.h"
 #include "qemu/queue.h"
 #include "qemu/compiler.h"
+#include "qom/object.h"
 
 #ifndef _WIN32
 #include <sys/wait.h>
@@ -184,6 +185,61 @@ int qemu_init_main_loop(Error **errp)
     return 0;
 }
 
+static void main_loop_update_params(EventLoopBase *base, Error **errp)
+{
+    if (!qemu_aio_context) {
+        error_setg(errp, "qemu aio context not ready");
+        return;
+    }
+
+    aio_context_set_aio_params(qemu_aio_context, base->aio_max_batch, errp);
+}
+
+MainLoop *mloop;
+
+static void main_loop_init(EventLoopBase *base, Error **errp)
+{
+    MainLoop *m = MAIN_LOOP(base);
+
+    if (mloop) {
+        error_setg(errp, "only one main-loop instance allowed");
+        return;
+    }
+
+    main_loop_update_params(base, errp);
+
+    mloop = m;
+    return;
+}
+
+static bool main_loop_can_be_deleted(EventLoopBase *base)
+{
+    return false;
+}
+
+static void main_loop_class_init(ObjectClass *oc, void *class_data)
+{
+    EventLoopBaseClass *bc = EVENT_LOOP_BASE_CLASS(oc);
+
+    bc->init = main_loop_init;
+    bc->update_params = main_loop_update_params;
+    bc->can_be_deleted = main_loop_can_be_deleted;
+}
+
+static const TypeInfo main_loop_info = {
+    .name = TYPE_MAIN_LOOP,
+    .parent = TYPE_EVENT_LOOP_BASE,
+    .class_init = main_loop_class_init,
+    .instance_size = sizeof(MainLoop),
+};
+
+static void main_loop_register_types(void)
+{
+    type_register_static(&main_loop_info);
+}
+
+type_init(main_loop_register_types)
+
 static int max_priority;
 
 #ifndef _WIN32
-- 
2.35.1

[PATCH v4 3/3] util/event-loop-base: Introduce options to set the thread pool size

[Nicolas Saenz Julienne]

The thread pool regulates itself: when idle, it kills threads until
empty, when in demand, it creates new threads until full. This behaviour
doesn't play well with latency sensitive workloads where the price of
creating a new thread is too high. For example, when paired with qemu's
'-mlock', or using safety features like SafeStack, creating a new thread
has been measured take multiple milliseconds.

In order to mitigate this let's introduce a new 'EventLoopBase'
property to set the thread pool size. The threads will be created during
the pool's initialization or upon updating the property's value, remain
available during its lifetime regardless of demand, and destroyed upon
freeing it. A properly characterized workload will then be able to
configure the pool to avoid any latency spikes.

Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
---

Changes since v3:
 - Rework qom.json to avoid duplication

Changes since v2:
 - Don't wait when decreasing pool size
 - Fix qapi versioning

Changes since v1:
 - Add INT_MAX check
 - Have copy of thread pool sizes in AioContext to properly decouple
   both instances
 - More coherent variable naming
 - Handle case where max_threads decreases
 - Code comments

 event-loop-base.c                | 23 +++++++++++++
 include/block/aio.h              | 10 ++++++
 include/block/thread-pool.h      |  3 ++
 include/sysemu/event-loop-base.h |  4 +++
 iothread.c                       |  3 ++
 qapi/qom.json                    | 10 +++++-
 util/aio-posix.c                 |  1 +
 util/async.c                     | 20 ++++++++++++
 util/main-loop.c                 |  9 ++++++
 util/thread-pool.c               | 55 +++++++++++++++++++++++++++++---
 10 files changed, 133 insertions(+), 5 deletions(-)

diff --git a/event-loop-base.c b/event-loop-base.c
index e7f99a6ec8..d5be4dc6fc 100644
--- a/event-loop-base.c
+++ b/event-loop-base.c
@@ -14,6 +14,7 @@
 #include "qemu/osdep.h"
 #include "qom/object_interfaces.h"
 #include "qapi/error.h"
+#include "block/thread-pool.h"
 #include "sysemu/event-loop-base.h"
 
 typedef struct {
@@ -21,9 +22,22 @@ typedef struct {
     ptrdiff_t offset; /* field's byte offset in EventLoopBase struct */
 } EventLoopBaseParamInfo;
 
+static void event_loop_base_instance_init(Object *obj)
+{
+    EventLoopBase *base = EVENT_LOOP_BASE(obj);
+
+    base->thread_pool_max = THREAD_POOL_MAX_THREADS_DEFAULT;
+}
+
 static EventLoopBaseParamInfo aio_max_batch_info = {
     "aio-max-batch", offsetof(EventLoopBase, aio_max_batch),
 };
+static EventLoopBaseParamInfo thread_pool_min_info = {
+    "thread-pool-min", offsetof(EventLoopBase, thread_pool_min),
+};
+static EventLoopBaseParamInfo thread_pool_max_info = {
+    "thread-pool-max", offsetof(EventLoopBase, thread_pool_max),
+};
 
 static void event_loop_base_get_param(Object *obj, Visitor *v,
         const char *name, void *opaque, Error **errp)
@@ -95,12 +109,21 @@ static void event_loop_base_class_init(ObjectClass *klass, void *class_data)
                               event_loop_base_get_param,
                               event_loop_base_set_param,
                               NULL, &aio_max_batch_info);
+    object_class_property_add(klass, "thread-pool-min", "int",
+                              event_loop_base_get_param,
+                              event_loop_base_set_param,
+                              NULL, &thread_pool_min_info);
+    object_class_property_add(klass, "thread-pool-max", "int",
+                              event_loop_base_get_param,
+                              event_loop_base_set_param,
+                              NULL, &thread_pool_max_info);
 }
 
 static const TypeInfo event_loop_base_info = {
     .name = TYPE_EVENT_LOOP_BASE,
     .parent = TYPE_OBJECT,
     .instance_size = sizeof(EventLoopBase),
+    .instance_init = event_loop_base_instance_init,
     .class_size = sizeof(EventLoopBaseClass),
     .class_init = event_loop_base_class_init,
     .abstract = true,
diff --git a/include/block/aio.h b/include/block/aio.h
index 5634173b12..d128558f1d 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -192,6 +192,8 @@ struct AioContext {
     QSLIST_HEAD(, Coroutine) scheduled_coroutines;
     QEMUBH *co_schedule_bh;
 
+    int thread_pool_min;
+    int thread_pool_max;
     /* Thread pool for performing work and receiving completion callbacks.
      * Has its own locking.
      */
@@ -769,4 +771,12 @@ void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
 void aio_context_set_aio_params(AioContext *ctx, int64_t max_batch,
                                 Error **errp);
 
+/**
+ * aio_context_set_thread_pool_params:
+ * @ctx: the aio context
+ * @min: min number of threads to have readily available in the thread pool
+ * @min: max number of threads the thread pool can contain
+ */
+void aio_context_set_thread_pool_params(AioContext *ctx, int64_t min,
+                                        int64_t max, Error **errp);
 #endif
diff --git a/include/block/thread-pool.h b/include/block/thread-pool.h
index 7dd7d730a0..2020bcc92d 100644
--- a/include/block/thread-pool.h
+++ b/include/block/thread-pool.h
@@ -20,6 +20,8 @@
 
 #include "block/block.h"
 
+#define THREAD_POOL_MAX_THREADS_DEFAULT         64
+
 typedef int ThreadPoolFunc(void *opaque);
 
 typedef struct ThreadPool ThreadPool;
@@ -33,5 +35,6 @@ BlockAIOCB *thread_pool_submit_aio(ThreadPool *pool,
 int coroutine_fn thread_pool_submit_co(ThreadPool *pool,
         ThreadPoolFunc *func, void *arg);
 void thread_pool_submit(ThreadPool *pool, ThreadPoolFunc *func, void *arg);
+void thread_pool_update_params(ThreadPool *pool, struct AioContext *ctx);
 
 #endif
diff --git a/include/sysemu/event-loop-base.h b/include/sysemu/event-loop-base.h
index fced4c9fea..2748bf6ae1 100644
--- a/include/sysemu/event-loop-base.h
+++ b/include/sysemu/event-loop-base.h
@@ -33,5 +33,9 @@ struct EventLoopBase {
 
     /* AioContext AIO engine parameters */
     int64_t aio_max_batch;
+
+    /* AioContext thread pool parameters */
+    int64_t thread_pool_min;
+    int64_t thread_pool_max;
 };
 #endif
diff --git a/iothread.c b/iothread.c
index 8fa2f3bfb8..529194a566 100644
--- a/iothread.c
+++ b/iothread.c
@@ -174,6 +174,9 @@ static void iothread_set_aio_context_params(EventLoopBase *base, Error **errp)
     aio_context_set_aio_params(iothread->ctx,
                                iothread->parent_obj.aio_max_batch,
                                errp);
+
+    aio_context_set_thread_pool_params(iothread->ctx, base->thread_pool_min,
+                                       base->thread_pool_max, errp);
 }
 
 
diff --git a/qapi/qom.json b/qapi/qom.json
index e5f31c4469..06b8c3d10b 100644
--- a/qapi/qom.json
+++ b/qapi/qom.json
@@ -506,9 +506,17 @@
 #
 # @aio-max-batch: maximum number of requests in a batch for the AIO engine,
 #                 0 means that the engine will use its default.
+#
+# @thread-pool-min: minimum number of threads readily available in the thread
+#                   pool (default:0, since 7.1)
+#
+# @thread-pool-max: maximum number of threads the thread pool can contain
+#                   (default:64, since 7.1)
 ##
 { 'struct': 'EventLoopBaseProperties',
-  'data': { '*aio-max-batch': 'int' } }
+  'data': { '*aio-max-batch': 'int',
+            '*thread-pool-min': 'int',
+            '*thread-pool-max': 'int' } }
 
 ##
 # @IothreadProperties:
diff --git a/util/aio-posix.c b/util/aio-posix.c
index be0182a3c6..731f3826c0 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -15,6 +15,7 @@
 
 #include "qemu/osdep.h"
 #include "block/block.h"
+#include "block/thread-pool.h"
 #include "qemu/main-loop.h"
 #include "qemu/rcu.h"
 #include "qemu/rcu_queue.h"
diff --git a/util/async.c b/util/async.c
index 2ea1172f3e..554ba70cca 100644
--- a/util/async.c
+++ b/util/async.c
@@ -563,6 +563,9 @@ AioContext *aio_context_new(Error **errp)
 
     ctx->aio_max_batch = 0;
 
+    ctx->thread_pool_min = 0;
+    ctx->thread_pool_max = THREAD_POOL_MAX_THREADS_DEFAULT;
+
     return ctx;
 fail:
     g_source_destroy(&ctx->source);
@@ -696,3 +699,20 @@ void qemu_set_current_aio_context(AioContext *ctx)
     assert(!get_my_aiocontext());
     set_my_aiocontext(ctx);
 }
+
+void aio_context_set_thread_pool_params(AioContext *ctx, int64_t min,
+                                        int64_t max, Error **errp)
+{
+
+    if (min > max || !max || min > INT_MAX || max > INT_MAX) {
+        error_setg(errp, "bad thread-pool-min/thread-pool-max values");
+        return;
+    }
+
+    ctx->thread_pool_min = min;
+    ctx->thread_pool_max = max;
+
+    if (ctx->thread_pool) {
+        thread_pool_update_params(ctx->thread_pool, ctx);
+    }
+}
diff --git a/util/main-loop.c b/util/main-loop.c
index 5b13f456fa..a0f48186ab 100644
--- a/util/main-loop.c
+++ b/util/main-loop.c
@@ -30,6 +30,7 @@
 #include "sysemu/replay.h"
 #include "qemu/main-loop.h"
 #include "block/aio.h"
+#include "block/thread-pool.h"
 #include "qemu/error-report.h"
 #include "qemu/queue.h"
 #include "qemu/compiler.h"
@@ -187,12 +188,20 @@ int qemu_init_main_loop(Error **errp)
 
 static void main_loop_update_params(EventLoopBase *base, Error **errp)
 {
+    ERRP_GUARD();
+
     if (!qemu_aio_context) {
         error_setg(errp, "qemu aio context not ready");
         return;
     }
 
     aio_context_set_aio_params(qemu_aio_context, base->aio_max_batch, errp);
+    if (*errp) {
+        return;
+    }
+
+    aio_context_set_thread_pool_params(qemu_aio_context, base->thread_pool_min,
+                                       base->thread_pool_max, errp);
 }
 
 MainLoop *mloop;
diff --git a/util/thread-pool.c b/util/thread-pool.c
index d763cea505..196835b4d3 100644
--- a/util/thread-pool.c
+++ b/util/thread-pool.c
@@ -58,7 +58,6 @@ struct ThreadPool {
     QemuMutex lock;
     QemuCond worker_stopped;
     QemuSemaphore sem;
-    int max_threads;
     QEMUBH *new_thread_bh;
 
     /* The following variables are only accessed from one AioContext. */
@@ -71,8 +70,27 @@ struct ThreadPool {
     int new_threads;     /* backlog of threads we need to create */
     int pending_threads; /* threads created but not running yet */
     bool stopping;
+    int min_threads;
+    int max_threads;
 };
 
+static inline bool back_to_sleep(ThreadPool *pool, int ret)
+{
+    /*
+     * The semaphore timed out, we should exit the loop except when:
+     *  - There is work to do, we raced with the signal.
+     *  - The max threads threshold just changed, we raced with the signal.
+     *  - The thread pool forces a minimum number of readily available threads.
+     */
+    if (ret == -1 && (!QTAILQ_EMPTY(&pool->request_list) ||
+            pool->cur_threads > pool->max_threads ||
+            pool->cur_threads <= pool->min_threads)) {
+            return true;
+    }
+
+    return false;
+}
+
 static void *worker_thread(void *opaque)
 {
     ThreadPool *pool = opaque;
@@ -91,8 +109,9 @@ static void *worker_thread(void *opaque)
             ret = qemu_sem_timedwait(&pool->sem, 10000);
             qemu_mutex_lock(&pool->lock);
             pool->idle_threads--;
-        } while (ret == -1 && !QTAILQ_EMPTY(&pool->request_list));
-        if (ret == -1 || pool->stopping) {
+        } while (back_to_sleep(pool, ret));
+        if (ret == -1 || pool->stopping ||
+            pool->cur_threads > pool->max_threads) {
             break;
         }
 
@@ -294,6 +313,33 @@ void thread_pool_submit(ThreadPool *pool, ThreadPoolFunc *func, void *arg)
     thread_pool_submit_aio(pool, func, arg, NULL, NULL);
 }
 
+void thread_pool_update_params(ThreadPool *pool, AioContext *ctx)
+{
+    qemu_mutex_lock(&pool->lock);
+
+    pool->min_threads = ctx->thread_pool_min;
+    pool->max_threads = ctx->thread_pool_max;
+
+    /*
+     * We either have to:
+     *  - Increase the number available of threads until over the min_threads
+     *    threshold.
+     *  - Decrease the number of available threads until under the max_threads
+     *    threshold.
+     *  - Do nothing. The current number of threads fall in between the min and
+     *    max thresholds. We'll let the pool manage itself.
+     */
+    for (int i = pool->cur_threads; i < pool->min_threads; i++) {
+        spawn_thread(pool);
+    }
+
+    for (int i = pool->cur_threads; i > pool->max_threads; i--) {
+        qemu_sem_post(&pool->sem);
+    }
+
+    qemu_mutex_unlock(&pool->lock);
+}
+
 static void thread_pool_init_one(ThreadPool *pool, AioContext *ctx)
 {
     if (!ctx) {
@@ -306,11 +352,12 @@ static void thread_pool_init_one(ThreadPool *pool, AioContext *ctx)
     qemu_mutex_init(&pool->lock);
     qemu_cond_init(&pool->worker_stopped);
     qemu_sem_init(&pool->sem, 0);
-    pool->max_threads = 64;
     pool->new_thread_bh = aio_bh_new(ctx, spawn_thread_bh_fn, pool);
 
     QLIST_INIT(&pool->head);
     QTAILQ_INIT(&pool->request_list);
+
+    thread_pool_update_params(pool, ctx);
 }
 
 ThreadPool *thread_pool_new(AioContext *ctx)
-- 
2.35.1

Re: [PATCH v4 0/4] util/thread-pool: Expose minimun and maximum size

[Nicolas Saenz Julienne]

On Fri, 2022-04-01 at 11:35 +0200, Nicolas Saenz Julienne wrote:

Subject says 0/4 where is should've been 0/3.

> As discussed on the previous RFC[1] the thread-pool's dynamic thread
> management doesn't play well with real-time and latency sensitive
> systems. This series introduces a set of controls that'll permit
> achieving more deterministic behaviours, for example by fixing the
> pool's size.
> 
> We first introduce a new common interface to event loop configuration by
> moving iothread's already available properties into an abstract class
> called 'EventLooopBackend' and have both 'IOThread' and the newly
> created 'MainLoop' inherit the properties from that class.
> 
> With this new configuration interface in place it's relatively simple to
> introduce new options to fix the even loop's thread pool sizes. The
> resulting QAPI looks like this:
> 
>     -object main-loop,id=main-loop,thread-pool-min=1,thread-pool-max=1
> 
> Note that all patches are bisect friendly and pass all the tests.
> 
> [1] https://patchwork.ozlabs.org/project/qemu-devel/patch/20220202175234.656711-1-nsaenzju@redhat.com/
> 
> @Stefan I kept your Signed-off-by, since the changes trivial/not
> thread-pool related
> 
> ---
> Changes since v3:
>  - Avoid duplication in qom.json by creating EventLoopBaseProperties.
>  - Fix failures on first compilation due to race between
>    event-loop-base.o and qapi header generation.
> 
> Changes since v2:
>  - Get rid of wrong locking/waiting
>  - Fix qapi versioning
>  - Better commit messages
> 
> Changes since v1:
>  - Address all Stefan's comments
>  - Introduce new fix
> 
> Nicolas Saenz Julienne (3):
>   Introduce event-loop-base abstract class
>   util/main-loop: Introduce the main loop into QOM
>   util/event-loop-base: Introduce options to set the thread pool size
> 
>  event-loop-base.c                | 140 +++++++++++++++++++++++++++++++
>  include/block/aio.h              |  10 +++
>  include/block/thread-pool.h      |   3 +
>  include/qemu/main-loop.h         |  10 +++
>  include/sysemu/event-loop-base.h |  41 +++++++++
>  include/sysemu/iothread.h        |   6 +-
>  iothread.c                       |  68 +++++----------
>  meson.build                      |  26 +++---
>  qapi/qom.json                    |  40 +++++++--
>  util/aio-posix.c                 |   1 +
>  util/async.c                     |  20 +++++
>  util/main-loop.c                 |  65 ++++++++++++++
>  util/thread-pool.c               |  55 +++++++++++-
>  13 files changed, 416 insertions(+), 69 deletions(-)
>  create mode 100644 event-loop-base.c
>  create mode 100644 include/sysemu/event-loop-base.h
> 

-- 
Nicolás Sáenz

Re: [PATCH v4 0/4] util/thread-pool: Expose minimun and maximum size

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 1283 bytes --]

On Fri, Apr 01, 2022 at 11:35:20AM +0200, Nicolas Saenz Julienne wrote:
> As discussed on the previous RFC[1] the thread-pool's dynamic thread
> management doesn't play well with real-time and latency sensitive
> systems. This series introduces a set of controls that'll permit
> achieving more deterministic behaviours, for example by fixing the
> pool's size.
> 
> We first introduce a new common interface to event loop configuration by
> moving iothread's already available properties into an abstract class
> called 'EventLooopBackend' and have both 'IOThread' and the newly
> created 'MainLoop' inherit the properties from that class.
> 
> With this new configuration interface in place it's relatively simple to
> introduce new options to fix the even loop's thread pool sizes. The
> resulting QAPI looks like this:
> 
>     -object main-loop,id=main-loop,thread-pool-min=1,thread-pool-max=1
> 
> Note that all patches are bisect friendly and pass all the tests.
> 
> [1] https://patchwork.ozlabs.org/project/qemu-devel/patch/20220202175234.656711-1-nsaenzju@redhat.com/
> 
> @Stefan I kept your Signed-off-by, since the changes trivial/not
> thread-pool related

Looks good to me. I will wait for Markus to review the QAPI schema changes.

Stefan

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 488 bytes --]

Re: [PATCH v4 0/4] util/thread-pool: Expose minimun and maximum size

[Nicolas Saenz Julienne]

On Mon, 2022-04-04 at 10:29 +0100, Stefan Hajnoczi wrote:
> On Fri, Apr 01, 2022 at 11:35:20AM +0200, Nicolas Saenz Julienne wrote:
> > As discussed on the previous RFC[1] the thread-pool's dynamic thread
> > management doesn't play well with real-time and latency sensitive
> > systems. This series introduces a set of controls that'll permit
> > achieving more deterministic behaviours, for example by fixing the
> > pool's size.
> > 
> > We first introduce a new common interface to event loop configuration by
> > moving iothread's already available properties into an abstract class
> > called 'EventLooopBackend' and have both 'IOThread' and the newly
> > created 'MainLoop' inherit the properties from that class.
> > 
> > With this new configuration interface in place it's relatively simple to
> > introduce new options to fix the even loop's thread pool sizes. The
> > resulting QAPI looks like this:
> > 
> >     -object main-loop,id=main-loop,thread-pool-min=1,thread-pool-max=1
> > 
> > Note that all patches are bisect friendly and pass all the tests.
> > 
> > [1] https://patchwork.ozlabs.org/project/qemu-devel/patch/20220202175234.656711-1-nsaenzju@redhat.com/
> > 
> > @Stefan I kept your Signed-off-by, since the changes trivial/not
> > thread-pool related
> 
> Looks good to me. I will wait for Markus to review the QAPI schema changes.

ping :)

-- 
Nicolás Sáenz

Re: [PATCH v4 2/3] util/main-loop: Introduce the main loop into QOM

[Markus Armbruster]

Nicolas Saenz Julienne <nsaenzju@redhat.com> writes:

> 'event-loop-base' provides basic property handling for all 'AioContext'
> based event loops. So let's define a new 'MainLoopClass' that inherits
> from it. This will permit tweaking the main loop's properties through
> qapi as well as through the command line using the '-object' keyword[1].
> Only one instance of 'MainLoopClass' might be created at any time.
>
> 'EventLoopBaseClass' learns a new callback, 'can_be_deleted()' so as to
> mark 'MainLoop' as non-deletable.
>
> [1] For example:
>       -object main-loop,id=main-loop,aio-max-batch=<value>
>
> Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

[...]

> diff --git a/qapi/qom.json b/qapi/qom.json
> index eeb5395ff3..e5f31c4469 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -499,6 +499,17 @@
>              '*repeat': 'bool',
>              '*grab-toggle': 'GrabToggleKeys' } }
>  
> +##
> +# @EventLoopBaseProperties:
> +#
> +# Common properties for objects derived from EventLoopBase

This makes sense as internal documentation: QAPI type
EventLoopBaseProperties is associated with C type EventLoopBase.  Doc
comments are *external* documentation: they go into the QEMU QMP
Reference Manual.

What about "Common properties for event loops"?

> +#
> +# @aio-max-batch: maximum number of requests in a batch for the AIO engine,
> +#                 0 means that the engine will use its default.

Missing:

   # Since: 7.1

Permit me a short digression.  We add these unthinkingly, because
thinking is expensive.  Even when the type isn't really part of the
external interface.  The deeper problem is that we're trying to generate
documentation of the external interface from doc comments that are
written as documentation of the internal QAPI data structures.  Here,
for example, we document EventLoopBaseProperties even though it is a
purely internal thing: whether we factor out common members into a base
type or not is not visible in QMP.

> +##
> +{ 'struct': 'EventLoopBaseProperties',
> +  'data': { '*aio-max-batch': 'int' } }
> +
>  ##
>  # @IothreadProperties:
>  #
> @@ -516,17 +527,26 @@
>  #               algorithm detects it is spending too long polling without
>  #               encountering events. 0 selects a default behaviour (default: 0)
>  #
> -# @aio-max-batch: maximum number of requests in a batch for the AIO engine,
> -#                 0 means that the engine will use its default
> -#                 (default:0, since 6.1)
> +# The @aio-max-batch option is available since 6.1.

This separates the member's "since" information from its defintion.
Can't be helped, because its defined in the base type, but the since
only applies here.  Okay.

>  #
>  # Since: 2.0
>  ##
>  { 'struct': 'IothreadProperties',
> +  'base': 'EventLoopBaseProperties',
>    'data': { '*poll-max-ns': 'int',
>              '*poll-grow': 'int',
> -            '*poll-shrink': 'int',
> -            '*aio-max-batch': 'int' } }
> +            '*poll-shrink': 'int' } }
> +
> +##
> +# @MainLoopProperties:
> +#
> +# Properties for the main-loop object.
> +#
> +# Since: 7.1
> +##
> +{ 'struct': 'MainLoopProperties',
> +  'base': 'EventLoopBaseProperties',
> +  'data': {} }

The patch does two things:

1. Factor EventLoopBaseProperties out of IothreadProperties.

2. Create MainLoopProperties.

I'd split it.  This is not a demand.

>  
>  ##
>  # @MemoryBackendProperties:
> @@ -818,6 +838,7 @@
>      { 'name': 'input-linux',
>        'if': 'CONFIG_LINUX' },
>      'iothread',
> +    'main-loop',
>      { 'name': 'memory-backend-epc',
>        'if': 'CONFIG_LINUX' },
>      'memory-backend-file',
> @@ -883,6 +904,7 @@
>        'input-linux':                { 'type': 'InputLinuxProperties',
>                                        'if': 'CONFIG_LINUX' },
>        'iothread':                   'IothreadProperties',
> +      'main-loop':                  'MainLoopProperties',
>        'memory-backend-epc':         { 'type': 'MemoryBackendEpcProperties',
>                                        'if': 'CONFIG_LINUX' },
>        'memory-backend-file':        'MemoryBackendFileProperties',

[...]

Re: [PATCH v4 3/3] util/event-loop-base: Introduce options to set the thread pool size

[Markus Armbruster]

Nicolas Saenz Julienne <nsaenzju@redhat.com> writes:

> The thread pool regulates itself: when idle, it kills threads until
> empty, when in demand, it creates new threads until full. This behaviour
> doesn't play well with latency sensitive workloads where the price of
> creating a new thread is too high. For example, when paired with qemu's
> '-mlock', or using safety features like SafeStack, creating a new thread
> has been measured take multiple milliseconds.
>
> In order to mitigate this let's introduce a new 'EventLoopBase'
> property to set the thread pool size. The threads will be created during
> the pool's initialization or upon updating the property's value, remain
> available during its lifetime regardless of demand, and destroyed upon
> freeing it. A properly characterized workload will then be able to
> configure the pool to avoid any latency spikes.
>
> Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

[...]

> diff --git a/qapi/qom.json b/qapi/qom.json
> index e5f31c4469..06b8c3d10b 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -506,9 +506,17 @@
>  #
>  # @aio-max-batch: maximum number of requests in a batch for the AIO engine,
>  #                 0 means that the engine will use its default.
> +#
> +# @thread-pool-min: minimum number of threads readily available in the thread
> +#                   pool (default:0, since 7.1)

What do you mean by "readily available in the thread pool"?

> +#
> +# @thread-pool-max: maximum number of threads the thread pool can contain
> +#                   (default:64, since 7.1)
>  ##

If you add "Since: 7.1" in the previous patch, then the "since 7.1" here
need to go.

>  { 'struct': 'EventLoopBaseProperties',
> -  'data': { '*aio-max-batch': 'int' } }
> +  'data': { '*aio-max-batch': 'int',
> +            '*thread-pool-min': 'int',
> +            '*thread-pool-max': 'int' } }
>  
>  ##
>  # @IothreadProperties:

[...]

Re: [PATCH v4 2/3] util/main-loop: Introduce the main loop into QOM

[Nicolas Saenz Julienne]

On Fri, 2022-04-22 at 13:13 +0200, Markus Armbruster wrote:
> Nicolas Saenz Julienne <nsaenzju@redhat.com> writes:
> 
> > 'event-loop-base' provides basic property handling for all 'AioContext'
> > based event loops. So let's define a new 'MainLoopClass' that inherits
> > from it. This will permit tweaking the main loop's properties through
> > qapi as well as through the command line using the '-object' keyword[1].
> > Only one instance of 'MainLoopClass' might be created at any time.
> > 
> > 'EventLoopBaseClass' learns a new callback, 'can_be_deleted()' so as to
> > mark 'MainLoop' as non-deletable.
> > 
> > [1] For example:
> >       -object main-loop,id=main-loop,aio-max-batch=<value>
> > 
> > Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
> > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> 
> [...]
> 
> > diff --git a/qapi/qom.json b/qapi/qom.json
> > index eeb5395ff3..e5f31c4469 100644
> > --- a/qapi/qom.json
> > +++ b/qapi/qom.json
> > @@ -499,6 +499,17 @@
> >              '*repeat': 'bool',
> >              '*grab-toggle': 'GrabToggleKeys' } }
> >  
> > +##
> > +# @EventLoopBaseProperties:
> > +#
> > +# Common properties for objects derived from EventLoopBase
> 
> This makes sense as internal documentation: QAPI type
> EventLoopBaseProperties is associated with C type EventLoopBase.  Doc
> comments are *external* documentation: they go into the QEMU QMP
> Reference Manual.
> 
> What about "Common properties for event loops"?

Sounds better, yes. I'll change it.

> > +#
> > +# @aio-max-batch: maximum number of requests in a batch for the AIO engine,
> > +#                 0 means that the engine will use its default.
> 
> Missing:
> 
>    # Since: 7.1
> 
> Permit me a short digression.  We add these unthinkingly, because
> thinking is expensive.  Even when the type isn't really part of the
> external interface.  The deeper problem is that we're trying to generate
> documentation of the external interface from doc comments that are
> written as documentation of the internal QAPI data structures.  Here,
> for example, we document EventLoopBaseProperties even though it is a
> purely internal thing: whether we factor out common members into a base
> type or not is not visible in QMP.

Thanks for the explanation.

> > +##
> > +{ 'struct': 'EventLoopBaseProperties',
> > +  'data': { '*aio-max-batch': 'int' } }
> > +
> >  ##
> >  # @IothreadProperties:
> >  #
> > @@ -516,17 +527,26 @@
> >  #               algorithm detects it is spending too long polling without
> >  #               encountering events. 0 selects a default behaviour (default: 0)
> >  #
> > -# @aio-max-batch: maximum number of requests in a batch for the AIO engine,
> > -#                 0 means that the engine will use its default
> > -#                 (default:0, since 6.1)
> > +# The @aio-max-batch option is available since 6.1.
> 
> This separates the member's "since" information from its defintion.
> Can't be helped, because its defined in the base type, but the since
> only applies here.  Okay.

IIUC my compromise of having the 'since version' annotated on each externally
visible type is good, right? No need to add the info in
EventLoopBaseProperties.

> >  #
> >  # Since: 2.0
> >  ##
> >  { 'struct': 'IothreadProperties',
> > +  'base': 'EventLoopBaseProperties',
> >    'data': { '*poll-max-ns': 'int',
> >              '*poll-grow': 'int',
> > -            '*poll-shrink': 'int',
> > -            '*aio-max-batch': 'int' } }
> > +            '*poll-shrink': 'int' } }
> > +
> > +##
> > +# @MainLoopProperties:
> > +#
> > +# Properties for the main-loop object.
> > +#
> > +# Since: 7.1
> > +##
> > +{ 'struct': 'MainLoopProperties',
> > +  'base': 'EventLoopBaseProperties',
> > +  'data': {} }
> 
> The patch does two things:
> 
> 1. Factor EventLoopBaseProperties out of IothreadProperties.
> 
> 2. Create MainLoopProperties.
> 
> I'd split it.  This is not a demand.

Since I'm preparing a v5 of the series, I agree it makes sense to move 1. to
the fist patch.

Thanks!

-- 
Nicolás Sáenz

Re: [PATCH v4 3/3] util/event-loop-base: Introduce options to set the thread pool size

[Nicolas Saenz Julienne]

On Fri, 2022-04-22 at 13:15 +0200, Markus Armbruster wrote:
> Nicolas Saenz Julienne <nsaenzju@redhat.com> writes:
> 
> > The thread pool regulates itself: when idle, it kills threads until
> > empty, when in demand, it creates new threads until full. This behaviour
> > doesn't play well with latency sensitive workloads where the price of
> > creating a new thread is too high. For example, when paired with qemu's
> > '-mlock', or using safety features like SafeStack, creating a new thread
> > has been measured take multiple milliseconds.
> > 
> > In order to mitigate this let's introduce a new 'EventLoopBase'
> > property to set the thread pool size. The threads will be created during
> > the pool's initialization or upon updating the property's value, remain
> > available during its lifetime regardless of demand, and destroyed upon
> > freeing it. A properly characterized workload will then be able to
> > configure the pool to avoid any latency spikes.
> > 
> > Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
> > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> 
> [...]
> 
> > diff --git a/qapi/qom.json b/qapi/qom.json
> > index e5f31c4469..06b8c3d10b 100644
> > --- a/qapi/qom.json
> > +++ b/qapi/qom.json
> > @@ -506,9 +506,17 @@
> >  #
> >  # @aio-max-batch: maximum number of requests in a batch for the AIO engine,
> >  #                 0 means that the engine will use its default.
> > +#
> > +# @thread-pool-min: minimum number of threads readily available in the thread
> > +#                   pool (default:0, since 7.1)
> 
> What do you mean by "readily available in the thread pool"?

How about "minimum number of threads reserved in the thread pool"?

> > +#
> > +# @thread-pool-max: maximum number of threads the thread pool can contain
> > +#                   (default:64, since 7.1)
> >  ##
> 
> If you add "Since: 7.1" in the previous patch, then the "since 7.1" here
> need to go.

Noted.

-- 
Nicolás Sáenz

Re: [PATCH v4 2/3] util/main-loop: Introduce the main loop into QOM

[Nicolas Saenz Julienne]

On Fri, 2022-04-22 at 13:40 +0200, Nicolas Saenz Julienne wrote:
> > > +##
> > > +{ 'struct': 'EventLoopBaseProperties',
> > > +  'data': { '*aio-max-batch': 'int' } }
> > > +
> > >  ##
> > >  # @IothreadProperties:
> > >  #
> > > @@ -516,17 +527,26 @@
> > >  #               algorithm detects it is spending too long polling without
> > >  #               encountering events. 0 selects a default behaviour (default: 0)
> > >  #
> > > -# @aio-max-batch: maximum number of requests in a batch for the AIO engine,
> > > -#                 0 means that the engine will use its default
> > > -#                 (default:0, since 6.1)
> > > +# The @aio-max-batch option is available since 6.1.
> > 
> > This separates the member's "since" information from its defintion.
> > Can't be helped, because its defined in the base type, but the since
> > only applies here.  Okay.
> 
> IIUC my compromise of having the 'since version' annotated on each externally
> visible type is good, right? No need to add the info in
> EventLoopBaseProperties.

OK, nevermind this reply. I misunderstood you. I'll:
 - Add a "since 7.1" in EventLoopBaseProperties.
 - Add a special comment in IothreadProperties mentioning aio-max-batch is
   available since 6.1.
 - Remove version info from MainLoopProperties.

Thanks!

-- 
Nicolás Sáenz

Re: [PATCH v4 3/3] util/event-loop-base: Introduce options to set the thread pool size

[Markus Armbruster]

Nicolas Saenz Julienne <nsaenzju@redhat.com> writes:

> On Fri, 2022-04-22 at 13:15 +0200, Markus Armbruster wrote:
>> Nicolas Saenz Julienne <nsaenzju@redhat.com> writes:
>> 
>> > The thread pool regulates itself: when idle, it kills threads until
>> > empty, when in demand, it creates new threads until full. This behaviour
>> > doesn't play well with latency sensitive workloads where the price of
>> > creating a new thread is too high. For example, when paired with qemu's
>> > '-mlock', or using safety features like SafeStack, creating a new thread
>> > has been measured take multiple milliseconds.
>> > 
>> > In order to mitigate this let's introduce a new 'EventLoopBase'
>> > property to set the thread pool size. The threads will be created during
>> > the pool's initialization or upon updating the property's value, remain
>> > available during its lifetime regardless of demand, and destroyed upon
>> > freeing it. A properly characterized workload will then be able to
>> > configure the pool to avoid any latency spikes.
>> > 
>> > Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
>> > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
>> 
>> [...]
>> 
>> > diff --git a/qapi/qom.json b/qapi/qom.json
>> > index e5f31c4469..06b8c3d10b 100644
>> > --- a/qapi/qom.json
>> > +++ b/qapi/qom.json
>> > @@ -506,9 +506,17 @@
>> >  #
>> >  # @aio-max-batch: maximum number of requests in a batch for the AIO engine,
>> >  #                 0 means that the engine will use its default.
>> > +#
>> > +# @thread-pool-min: minimum number of threads readily available in the thread
>> > +#                   pool (default:0, since 7.1)
>> 
>> What do you mean by "readily available in the thread pool"?
>
> How about "minimum number of threads reserved in the thread pool"?

Works for me, thanks!

>> > +#
>> > +# @thread-pool-max: maximum number of threads the thread pool can contain
>> > +#                   (default:64, since 7.1)
>> >  ##
>> 
>> If you add "Since: 7.1" in the previous patch, then the "since 7.1" here
>> need to go.
>
> Noted.