Re: [Qemu-devel] [PATCH 1/3] Introduce threadlets

["Venkateswararao Jujjuri (JV)" <jvrao@linux.vnet.ibm.com>]

On 10/14/2010 2:02 AM, Stefan Hajnoczi wrote:
> On Wed, Oct 13, 2010 at 4:31 PM, Arun R Bharadwaj
> <arun@linux.vnet.ibm.com> wrote:
>> From: Aneesh Kumar K.V <aneesh.kumar@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.
>>
>> [ego@in.ibm.com: Facelift of the code, Documentation, cancel_threadlet
>> and other helpers]
>>
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
>> Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
>> Signed-off-by: Sripathi Kodi <sripathik@in.ibm.com>
>> Signed-off-by: Arun R Bharadwaj <arun@linux.vnet.ibm.com>
>> ---
>>  Makefile.objs          |    3 +
>>  docs/async-support.txt |  141 ++++++++++++++++++++++++++++++++++++++++
>>  qemu-threadlets.c      |  169 ++++++++++++++++++++++++++++++++++++++++++++++++
>>  qemu-threadlets.h      |   48 ++++++++++++++
>>  4 files changed, 360 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..2e8adc9
>> --- /dev/null
>> +++ b/docs/async-support.txt
> 
> Why not call it docs/threadlets.txt?
> 
>> @@ -0,0 +1,141 @@
>> +== How to use the various asynchronous models supported in Qemu ==
> 
> This only describes threadlets.
> 
>> +
>> +== 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 asynchrnonously.
> 
> asynchronously typo
> 
>> +
>> +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 asynchrnous work.
> 
> asynchronous typo
> 
>> +       void my_threadlet_func(ThreadletWork *work)
> 
> Usually these functions will be static.
> 
>> +       {
>> +       }
>> +
>> +     - 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.
>> +       typdef myPrivateData {
> 
> typedef typo, struct missing
> 
> Also, the QEMU coding style usually requires CapsNames for struct types...
> 
>> +               ...;
>> +               ...;
>> +               ...;
>> +               ThreadletWork work;
>> +       } myPrivateData;
>> +
>> +       myPrivateData myData;
> 
> ...and myData would be 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
>> +       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
>> +     Asynchrnous thread-pool ?
> 
> asynchronous typo
> 
>> +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..1442122
>> --- /dev/null
>> +++ b/qemu-threadlets.c
>> @@ -0,0 +1,169 @@
>> +/*
>> + * Threadlet support for offloading tasks to be executed asynchronously
>> + *
>> + * Copyright IBM, Corp. 2008
>> + * Copyright IBM, Corp. 2010
>> + *
>> + * Authors:
>> + *  Anthony Liguori     <aliguori@us.ibm.com>
>> + *  Aneesh Kumar K.V    <aneesh.kumar@linux.vnet.ibm.com>
>> + *  Gautham R Shenoy    <ego@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
>> +ThreadletQueue globalqueue;
> 
> This should be static.
> 
>> +static int globalqueue_init;
>> +
>> +static void *threadlet_worker(void *data)
>> +{
>> +    ThreadletQueue *queue = data;
>> +
>> +    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);
>> +        }
>> +
>> +        assert(queue->idle_threads != 0);
>> +        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;
>> +}
>> +
>> +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);
>> +    }
>> +    QTAILQ_INSERT_TAIL(&(queue->request_list), work, node);
>> +    qemu_mutex_unlock(&(queue->lock));
>> +    qemu_cond_signal(&(queue->cond));
> 
> Worker thread signalling and spawning has race conditions.  See my
> previous email:
> 
> "There are race conditions here:
> 
> 1. When a new threadlet is started because there are no idle threads,
> qemu_cond_signal() may fire a blank because the threadlet isn't inside
> qemu_cond_timedwait() yet.  The result, the work item is deadlocked
> until another thread grabs more work off the queue.  If I'm reading
> the code correctly this bug is currently present!

Moving QTAILQ_INSERT_TAIL() ahead of spawn_threadlet() should take care of this
issue
right?

If not the new work should be blocked until the new thread wait is timed out
or another threadlet done with its previous job..which ever occurs first.
As the worker_thread go and recheck the queue at the end of each timeout.


> 2. Moving qemu_cond_signal() outside queue->lock is dangerous for the
> same reason: you need to be careful not to qemu_cond_signal() when the
> thread isn't inside qemu_cond_timedwait()."

Again here the worst case scenario is timedwait duration if the work is not flowing
continuously. If we have to employ another synchronization technique, with
another set of locks etc..we need to be sure that the additional cost is justified.

Thanks,
JV

> 
>> +}
>> +
>> +/**
>> + * 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;
>> +    }
>> +
>> +    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 found = 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);
>> +            found = 1;
>> +            break;
>> +        }
>> +    }
>> +    qemu_mutex_unlock(&(queue->lock));
>> +
>> +    if (found) {
>> +        return 0;
>> +    }
>> +
>> +    return 1;
> 
> Perhaps invert the logic of "found" and just call it "ret", then you
> don't need this if statement that flips its value.
> 
>> +}
>> +
>> +/**
>> + * cancel_threadletwork: Cancel a task queued on the global queue.
>> + * @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..3df9b10
>> --- /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     <aliguori@us.ibm.com>
>> + *  Aneesh Kumar K.V    <aneesh.kumar@linux.vnet.ibm.com>
>> + *  Gautham R Shenoy    <ego@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
> 
> QEMU_THREADLETS_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(, threadlet_work) request_list;
>> +} ThreadletQueue;
>> +
>> +typedef struct threadlet_work
> 
> struct ThreadletWork follows coding style more closely.
> 
> Stefan
>