* Arun R B <a...@linux.vnet.ibm.com> [2010-10-19 23:12:45]:
> From: Aneesh Kumar K.V <aneesh.ku...@linux.vnet.ibm.com>
>
> This patch creates a generic asynchronous-task-offloading infrastructure named
> threadlets. The core idea has been borrowed from the threading framework that
> is being used by paio.
>
> The reason for creating this generic infrastructure is so that other
> subsystems,
> such as virtio-9p could make use of it for offloading tasks that could block.
>
> The patch creates a global queue on-to which subsystems can queue their tasks
> to
> be executed asynchronously.
>
> The patch also provides API's that allow a subsystem to create a private queue
> with an associated pool of threads.
>
> [...@in.ibm.com: Facelift of the code, Documentation, cancel_threadlet
> and other helpers]
>
> Signed-off-by: Aneesh Kumar K.V <aneesh.ku...@linux.vnet.ibm.com>
> Signed-off-by: Gautham R Shenoy <e...@in.ibm.com>
> Signed-off-by: Sripathi Kodi <sripat...@in.ibm.com>
> Signed-off-by: Arun R Bharadwaj <a...@linux.vnet.ibm.com>
> ---
> Makefile.objs | 3 +
> docs/async-support.txt | 141 +++++++++++++++++++++++++++++++++++++++++
> qemu-threadlets.c | 165
> ++++++++++++++++++++++++++++++++++++++++++++++++
> qemu-threadlets.h | 48 ++++++++++++++
> 4 files changed, 356 insertions(+), 1 deletions(-)
> create mode 100644 docs/async-support.txt
> create mode 100644 qemu-threadlets.c
> create mode 100644 qemu-threadlets.h
>
> diff --git a/Makefile.objs b/Makefile.objs
> index cd5a24b..2cf8aba 100644
> --- a/Makefile.objs
> +++ b/Makefile.objs
> @@ -9,6 +9,8 @@ qobject-obj-y += qerror.o
>
> block-obj-y = cutils.o cache-utils.o qemu-malloc.o qemu-option.o module.o
> block-obj-y += nbd.o block.o aio.o aes.o osdep.o qemu-config.o
> +block-obj-$(CONFIG_POSIX) += qemu-thread.o
> +block-obj-$(CONFIG_POSIX) += qemu-threadlets.o
> block-obj-$(CONFIG_POSIX) += posix-aio-compat.o
> block-obj-$(CONFIG_LINUX_AIO) += linux-aio.o
>
> @@ -124,7 +126,6 @@ endif
> common-obj-y += $(addprefix ui/, $(ui-obj-y))
>
> common-obj-y += iov.o acl.o
> -common-obj-$(CONFIG_THREAD) += qemu-thread.o
> common-obj-y += notify.o event_notifier.o
> common-obj-y += qemu-timer.o
>
> diff --git a/docs/async-support.txt b/docs/async-support.txt
> new file mode 100644
> index 0000000..9f22b9a
> --- /dev/null
> +++ b/docs/async-support.txt
> @@ -0,0 +1,141 @@
> +== How to use the threadlets infrastructure supported in Qemu ==
> +
> +== Threadlets ==
> +
> +Q.1: What are threadlets ?
> +A.1: Threadlets is an infrastructure within QEMU that allows other subsystems
> + to offload possibly blocking work to a queue to be processed by a pool
> + of threads asynchronously.
> +
> +Q.2: When would one want to use threadlets ?
> +A.2: Threadlets are useful when there are operations that can be performed
> + outside the context of the VCPU/IO threads inorder to free these latter
> + to service any other guest requests.
> +
> +Q.3: I have some work that can be executed in an asynchronous context. How
> + should I go about it ?
> +A.3: One could follow the steps listed below:
> +
> + - Define a function which would do the asynchronous work.
> + static void my_threadlet_func(ThreadletWork *work)
> + {
> + }
> +
> + - Declare an object of type ThreadletWork;
> + ThreadletWork work;
> +
> +
> + - Assign a value to the "func" member of ThreadletWork object.
> + work.func = my_threadlet_func;
> +
> + - Submit the threadlet to the global queue.
> + submit_threadletwork(&work);
> +
> + - Continue servicing some other guest operations.
> +
> +Q.4: I want to my_threadlet_func to access some non-global data. How do I do
> + that ?
> +A.4: Suppose you want my_threadlet_func to access some non-global data-object
> + of type myPrivateData. In that case one could follow the following
> steps.
> +
> + - Define a member of the type ThreadletWork within myPrivateData.
> + typedef struct MyPrivateData {
> + ...;
> + ...;
> + ...;
> + ThreadletWork work;
> + } MyPrivateData;
> +
> + MyPrivateData my_data;
> +
> + - Initialize myData.work as described in A.3
> + myData.work.func = my_threadlet_func;
> + submit_threadletwork(&myData.work);
> +
> + - Access the myData object inside my_threadlet_func() using container_of
> + primitive
> + static void my_threadlet_func(ThreadletWork *work)
> + {
> + myPrivateData *mydata_ptr;
> + mydata_ptr = container_of(work, myPrivateData, work);
> +
> + /* mydata_ptr now points to myData object */
> + }
> +
> +Q.5: Are there any precautions one must take while sharing data with the
> + Asynchronous thread-pool ?
> +A.5: Yes, make sure that the helper function of the type my_threadlet_func()
> + does not access/modify data when it can be accessed or modified in the
> + context of VCPU thread or IO thread. This is because the asynchronous
> + threads in the pool can run in parallel with the VCPU/IOThreads as shown
> + in the figure.
> +
> + A typical workflow is as follows:
> +
> + VCPU/IOThread
> + |
> + | (1)
> + |
> + V
> + Offload work (2)
> + |-------> to threadlets -----------------------------> Helper thread
> + | | |
> + | | |
> + | | (3) | (4)
> + | | |
> + | Handle other Guest requests |
> + | | |
> + | | V
> + | | (3) Signal the I/O
> Thread
> + |(6) | |
> + | | /
> + | | /
> + | V /
> + | Do the post <---------------------------------/
> + | processing (5)
> + | |
> + | | (6)
> + | V
> + |-Yes------ More async work?
> + |
> + | (7)
> + No
> + |
> + |
> + .
> + .
> +
> + Hence one needs to make sure that in the steps (3) and (4) which run in
> + parallel, any global data is accessed within only one context.
> +
> +Q.6: I have queued a threadlet which I want to cancel. How do I do that ?
> +A.6: Threadlets framework provides the API cancel_threadlet:
> + - int cancel_threadletwork(ThreadletWork *work)
> +
> + The API scans the ThreadletQueue to see if (work) is present. If it
> finds
> + work, it'll dequeue work and return 0.
> +
> + On the other hand, if it does not find the (work) in the ThreadletQueue,
> + then it'll return 1. This can imply two things. Either the work is being
> + processed by one of the helper threads or it has been processed. The
> + threadlet infrastructure currently _does_not_ distinguish between these
> + two and the onus is on the caller to do that.
> +
> +Q.7: Apart from the global pool of threads, can I have my own private Queue ?
> +A.7: Yes, the threadlets framework allows subsystems to create their own
> private
> + queues with associated pools of threads.
> +
> + - Define a PrivateQueue
> + ThreadletQueue myQueue;
> +
> + - Initialize it:
> + threadlet_queue_init(&myQueue, my_max_threads, my_min_threads);
> + where my_max_threads is the maximum number of threads that can be in
> the
> + thread pool and my_min_threads is the minimum number of active threads
> + that can be in the thread-pool.
> +
> + - Submit work:
> + submit_threadletwork_to_queue(&myQueue, &my_work);
> +
> + - Cancel work:
> + cancel_threadletwork_on_queue(&myQueue, &my_work);
> diff --git a/qemu-threadlets.c b/qemu-threadlets.c
> new file mode 100644
> index 0000000..fd33752
> --- /dev/null
> +++ b/qemu-threadlets.c
> @@ -0,0 +1,165 @@
> +/*
> + * Threadlet support for offloading tasks to be executed asynchronously
> + *
> + * Copyright IBM, Corp. 2008
> + * Copyright IBM, Corp. 2010
> + *
> + * Authors:
> + * Anthony Liguori <aligu...@us.ibm.com>
> + * Aneesh Kumar K.V <aneesh.ku...@linux.vnet.ibm.com>
> + * Gautham R Shenoy <e...@in.ibm.com>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2. See
> + * the COPYING file in the top-level directory.
> + */
> +
> +#include "qemu-threadlets.h"
> +#include "osdep.h"
> +
> +#define MAX_GLOBAL_THREADS 64
> +#define MIN_GLOBAL_THREADS 64
> +static ThreadletQueue globalqueue;
> +static int globalqueue_init;
> +
> +static void *threadlet_worker(void *data)
> +{
> + ThreadletQueue *queue = data;
> +
Ideally you need
s = pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);
But qemu will need to wrap this around as well.
> + qemu_mutex_lock(&(queue->lock));
> + while (1) {
> + ThreadletWork *work;
> + int ret = 0;
> +
> + while (QTAILQ_EMPTY(&(queue->request_list)) &&
> + (ret != ETIMEDOUT)) {
> + ret = qemu_cond_timedwait(&(queue->cond),
> + &(queue->lock), 10*100000);
Ewww... what is 10*100000, can we use something more meaningful
please?
> + }
> +
> + assert(queue->idle_threads != 0);
This assertion holds because we believe one of the idle_threads
actually did the dequeuing, right?
> + if (QTAILQ_EMPTY(&(queue->request_list))) {
> + if (queue->cur_threads > queue->min_threads) {
> + /* We retain the minimum number of threads */
> + break;
> + }
> + } else {
> + work = QTAILQ_FIRST(&(queue->request_list));
> + QTAILQ_REMOVE(&(queue->request_list), work, node);
> +
> + queue->idle_threads--;
> + qemu_mutex_unlock(&(queue->lock));
> +
> + /* execute the work function */
> + work->func(work);
> +
> + qemu_mutex_lock(&(queue->lock));
> + queue->idle_threads++;
> + }
> + }
> +
> + queue->idle_threads--;
> + queue->cur_threads--;
> + qemu_mutex_unlock(&(queue->lock));
> +
> + return NULL;
Does anybody do a join on the exiting thread from the pool?
> +}
> +
> +static void spawn_threadlet(ThreadletQueue *queue)
> +{
> + QemuThread thread;
> +
> + queue->cur_threads++;
> + queue->idle_threads++;
> +
> + qemu_thread_create(&thread, threadlet_worker, queue);
> +}
> +
> +/**
> + * submit_threadletwork_to_queue: Submit a new task to a private queue to be
> + * executed asynchronously.
> + * @queue: Per-subsystem private queue to which the new task needs
> + * to be submitted.
> + * @work: Contains information about the task that needs to be submitted.
> + */
> +void submit_threadletwork_to_queue(ThreadletQueue *queue, ThreadletWork
> *work)
> +{
> + qemu_mutex_lock(&(queue->lock));
> + if (queue->idle_threads == 0 && queue->cur_threads < queue->max_threads)
> {
> + spawn_threadlet(queue);
So we hold queue->lock, spawn the thread, the spawned thread tries to
acquire queue->lock
> + }
> + QTAILQ_INSERT_TAIL(&(queue->request_list), work, node);
> + qemu_mutex_unlock(&(queue->lock));
> + qemu_cond_signal(&(queue->cond));
In the case that we just spawned the threadlet, the cond_signal is
spurious. If we need predictable scheduling behaviour,
qemu_cond_signal needs to happen with queue->lock held.
I'd rewrite the function as
/**
* submit_threadletwork_to_queue: Submit a new task to a private queue to be
* executed asynchronously.
* @queue: Per-subsystem private queue to which the new task needs
* to be submitted.
* @work: Contains information about the task that needs to be submitted.
*/
void submit_threadletwork_to_queue(ThreadletQueue *queue, ThreadletWork *work)
{
qemu_mutex_lock(&(queue->lock));
if (queue->idle_threads == 0 && (queue->cur_threads < queue->max_threads)) {
spawn_threadlet(queue);
} else {
qemu_cond_signal(&(queue->cond));
}
QTAILQ_INSERT_TAIL(&(queue->request_list), work, node);
qemu_mutex_unlock(&(queue->lock));
}
> +/**
> + * submit_threadletwork: Submit to the global queue a new task to be executed
> + * asynchronously.
> + * @work: Contains information about the task that needs to be submitted.
> + */
> +void submit_threadletwork(ThreadletWork *work)
> +{
> + if (unlikely(!globalqueue_init)) {
> + threadlet_queue_init(&globalqueue, MAX_GLOBAL_THREADS,
> + MIN_GLOBAL_THREADS);
> + globalqueue_init = 1;
> + }
What protects globalqueue_init?
> +
> + submit_threadletwork_to_queue(&globalqueue, work);
> +}
> +
> +/**
> + * cancel_threadletwork_on_queue: Cancel a task queued on a Queue.
> + * @queue: The queue containing the task to be cancelled.
> + * @work: Contains the information of the task that needs to be cancelled.
> + *
> + * Returns: 0 if the task is successfully cancelled.
> + * 1 otherwise.
> + */
> +int cancel_threadletwork_on_queue(ThreadletQueue *queue, ThreadletWork *work)
> +{
> + ThreadletWork *ret_work;
> + int ret = 0;
> +
> + qemu_mutex_lock(&(queue->lock));
> + QTAILQ_FOREACH(ret_work, &(queue->request_list), node) {
> + if (ret_work == work) {
> + QTAILQ_REMOVE(&(queue->request_list), ret_work, node);
> + ret = 1;
> + break;
> + }
> + }
> + qemu_mutex_unlock(&(queue->lock));
> +
> + return ret;
> +}
> +
> +/**
> + * cancel_threadletwork: Cancel a task queued on the global queue.
NOTE: cancel is a confusing term, thread cancel is different from
cancelling a job on the global queue, I'd preferrably call this
dequeue_threadletwork
Generic question, is thread a reason to use threadletwork as one word,
instead of threadlet_work? Specially since the data structure is
called ThreadletWork.
> + * @work: Contains the information of the task that needs to be cancelled.
> + *
> + * Returns: 0 if the task is successfully cancelled.
> + * 1 otherwise.
> + */
> +int cancel_threadletwork(ThreadletWork *work)
> +{
> + return cancel_threadletwork_on_queue(&globalqueue, work);
> +}
> +
> +/**
> + * threadlet_queue_init: Initialize a threadlet queue.
> + * @queue: The threadlet queue to be initialized.
> + * @max_threads: Maximum number of threads processing the queue.
> + * @min_threads: Minimum number of threads processing the queue.
> + */
> +void threadlet_queue_init(ThreadletQueue *queue,
> + int max_threads, int min_threads)
> +{
> + queue->cur_threads = 0;
> + queue->idle_threads = 0;
> + queue->max_threads = max_threads;
> + queue->min_threads = min_threads;
> + QTAILQ_INIT(&(queue->request_list));
> + qemu_mutex_init(&(queue->lock));
> + qemu_cond_init(&(queue->cond));
> +}
> diff --git a/qemu-threadlets.h b/qemu-threadlets.h
> new file mode 100644
> index 0000000..9c8f9e5
> --- /dev/null
> +++ b/qemu-threadlets.h
> @@ -0,0 +1,48 @@
> +/*
> + * Threadlet support for offloading tasks to be executed asynchronously
> + *
> + * Copyright IBM, Corp. 2008
> + * Copyright IBM, Corp. 2010
> + *
> + * Authors:
> + * Anthony Liguori <aligu...@us.ibm.com>
> + * Aneesh Kumar K.V <aneesh.ku...@linux.vnet.ibm.com>
> + * Gautham R Shenoy <e...@in.ibm.com>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2. See
> + * the COPYING file in the top-level directory.
> + */
> +
> +#ifndef QEMU_ASYNC_WORK_H
> +#define QEMU_ASYNC_WORK_H
> +
> +#include "qemu-queue.h"
> +#include "qemu-common.h"
> +#include "qemu-thread.h"
> +
> +typedef struct ThreadletQueue
> +{
> + QemuMutex lock;
> + QemuCond cond;
> + int max_threads;
> + int min_threads;
> + int cur_threads;
> + int idle_threads;
> + QTAILQ_HEAD(, ThreadletWork) request_list;
> +} ThreadletQueue;
> +
> +typedef struct ThreadletWork
> +{
> + QTAILQ_ENTRY(ThreadletWork) node;
> + void (*func)(struct ThreadletWork *work);
> +} ThreadletWork;
> +
> +extern void submit_threadletwork_to_queue(ThreadletQueue *queue,
> + ThreadletWork *work);
> +extern void submit_threadletwork(ThreadletWork *work);
> +extern int cancel_threadletwork_on_queue(ThreadletQueue *queue,
> + ThreadletWork *work);
> +extern int cancel_threadletwork(ThreadletWork *work);
> +extern void threadlet_queue_init(ThreadletQueue *queue, int max_threads,
> + int min_threads);
> +#endif
>
>
--
Three Cheers,
Balbir
