Re: [Qemu-devel] [PATCH v3 2/5] util: introduce threaded workqueue

[Dr. David Alan Gilbert]

* address@hidden (address@hidden) wrote:
> From: Xiao Guangrong <address@hidden>
> 
> This modules implements the lockless and efficient threaded workqueue.
> 
> Three abstracted objects are used in this module:
> - Request.
>      It not only contains the data that the workqueue fetches out
>     to finish the request but also offers the space to save the result
>     after the workqueue handles the request.
> 
>     It's flowed between user and workqueue. The user fills the request
>     data into it when it is owned by user. After it is submitted to the
>     workqueue, the workqueue fetched data out and save the result into
>     it after the request is handled.
> 
>     All the requests are pre-allocated and carefully partitioned between
>     threads so there is no contention on the request, that make threads
>     be parallel as much as possible.
> 
> - User, i.e, the submitter
>     It's the one fills the request and submits it to the workqueue,
>     the result will be collected after it is handled by the work queue.
> 
>     The user can consecutively submit requests without waiting the previous
>     requests been handled.
>     It only supports one submitter, you should do serial submission by
>     yourself if you want more, e.g, use lock on you side.
> 
> - Workqueue, i.e, thread
>     Each workqueue is represented by a running thread that fetches
>     the request submitted by the user, do the specified work and save
>     the result to the request.
> 
> Signed-off-by: Xiao Guangrong <address@hidden>
> ---
>  include/qemu/threaded-workqueue.h | 106 +++++++++
>  util/Makefile.objs                |   1 +
>  util/threaded-workqueue.c         | 463 
> ++++++++++++++++++++++++++++++++++++++
>  3 files changed, 570 insertions(+)
>  create mode 100644 include/qemu/threaded-workqueue.h
>  create mode 100644 util/threaded-workqueue.c
> 
> diff --git a/include/qemu/threaded-workqueue.h 
> b/include/qemu/threaded-workqueue.h
> new file mode 100644
> index 0000000000..e0ede496d0
> --- /dev/null
> +++ b/include/qemu/threaded-workqueue.h
> @@ -0,0 +1,106 @@
> +/*
> + * Lockless and Efficient Threaded Workqueue Abstraction
> + *
> + * Author:
> + *   Xiao Guangrong <address@hidden>
> + *
> + * Copyright(C) 2018 Tencent Corporation.
> + *
> + * This work is licensed under the terms of the GNU LGPL, version 2.1 or 
> later.
> + * See the COPYING.LIB file in the top-level directory.
> + */
> +
> +#ifndef QEMU_THREADED_WORKQUEUE_H
> +#define QEMU_THREADED_WORKQUEUE_H
> +
> +#include "qemu/queue.h"
> +#include "qemu/thread.h"
> +
> +/*
> + * This modules implements the lockless and efficient threaded workqueue.
> + *
> + * Three abstracted objects are used in this module:
> + * - Request.
> + *   It not only contains the data that the workqueue fetches out
> + *   to finish the request but also offers the space to save the result
> + *   after the workqueue handles the request.
> + *
> + *   It's flowed between user and workqueue. The user fills the request
> + *   data into it when it is owned by user. After it is submitted to the
> + *   workqueue, the workqueue fetched data out and save the result into
> + *   it after the request is handled.
> + *
> + *   All the requests are pre-allocated and carefully partitioned between
> + *   threads so there is no contention on the request, that make threads
> + *   be parallel as much as possible.
> + *
> + * - User, i.e, the submitter
> + *   It's the one fills the request and submits it to the workqueue,
> + *   the result will be collected after it is handled by the work queue.
> + *
> + *   The user can consecutively submit requests without waiting the previous
> + *   requests been handled.
> + *   It only supports one submitter, you should do serial submission by
> + *   yourself if you want more, e.g, use lock on you side.
> + *
> + * - Workqueue, i.e, thread
> + *   Each workqueue is represented by a running thread that fetches
> + *   the request submitted by the user, do the specified work and save
> + *   the result to the request.
> + */
> +
> +typedef struct Threads Threads;
> +
> +struct ThreadedWorkqueueOps {
> +    /* constructor of the request */
> +    int (*thread_request_init)(void *request);
> +    /*  destructor of the request */
> +    void (*thread_request_uninit)(void *request);
> +
> +    /* the handler of the request that is called by the thread */
> +    void (*thread_request_handler)(void *request);
> +    /* called by the user after the request has been handled */
> +    void (*thread_request_done)(void *request);
> +
> +    size_t request_size;
> +};
> +typedef struct ThreadedWorkqueueOps ThreadedWorkqueueOps;
> +
> +/* the default number of requests that thread need handle */
> +#define DEFAULT_THREAD_REQUEST_NR 4
> +/* the max number of requests that thread need handle */
> +#define MAX_THREAD_REQUEST_NR     (sizeof(uint64_t) * BITS_PER_BYTE)
> +
> +/*
> + * create a threaded queue. Other APIs will work on the Threads it returned
> + *
> + * @name: the identity of the workqueue which is used to construct the name
> + *    of threads only
> + * @threads_nr: the number of threads that the workqueue will create
> + * @thread_requests_nr: the number of requests that each single thread will
> + *    handle
> + * @ops: the handlers of the request
> + *
> + * Return NULL if it failed
> + */
> +Threads *threaded_workqueue_create(const char *name, unsigned int threads_nr,
> +                                   unsigned int thread_requests_nr,
> +                                   const ThreadedWorkqueueOps *ops);
> +void threaded_workqueue_destroy(Threads *threads);
> +
> +/*
> + * find a free request where the user can store the data that is needed to
> + * finish the request
> + *
> + * If all requests are used up, return NULL
> + */
> +void *threaded_workqueue_get_request(Threads *threads);
> +/* submit the request and notify the thread */
> +void threaded_workqueue_submit_request(Threads *threads, void *request);
> +
> +/*
> + * wait all threads to complete the request to make sure there is no
> + * previous request exists
> + */
> +void threaded_workqueue_wait_for_requests(Threads *threads);
> +#endif
> diff --git a/util/Makefile.objs b/util/Makefile.objs
> index 0820923c18..f26dfe5182 100644
> --- a/util/Makefile.objs
> +++ b/util/Makefile.objs
> @@ -50,5 +50,6 @@ util-obj-y += range.o
>  util-obj-y += stats64.o
>  util-obj-y += systemd.o
>  util-obj-y += iova-tree.o
> +util-obj-y += threaded-workqueue.o
>  util-obj-$(CONFIG_LINUX) += vfio-helpers.o
>  util-obj-$(CONFIG_OPENGL) += drm.o
> diff --git a/util/threaded-workqueue.c b/util/threaded-workqueue.c
> new file mode 100644
> index 0000000000..2ab37cee8d
> --- /dev/null
> +++ b/util/threaded-workqueue.c
> @@ -0,0 +1,463 @@
> +/*
> + * Lockless and Efficient Threaded Workqueue Abstraction
> + *
> + * Author:
> + *   Xiao Guangrong <address@hidden>
> + *
> + * Copyright(C) 2018 Tencent Corporation.
> + *
> + * This work is licensed under the terms of the GNU LGPL, version 2.1 or 
> later.
> + * See the COPYING.LIB file in the top-level directory.
> + */
> +
> +#include "qemu/osdep.h"
> +#include "qemu/bitmap.h"
> +#include "qemu/threaded-workqueue.h"
> +
> +#define SMP_CACHE_BYTES 64

That's architecture dependent isn't it?

> +
> +/*
> + * the request representation which contains the internally used mete data,
> + * it is the header of user-defined data.
> + *
> + * It should be aligned to the nature size of CPU.
> + */
> +struct ThreadRequest {
> +    /*
> +     * the request has been handled by the thread and need the user
> +     * to fetch result out.
> +     */
> +    uint8_t done;
> +
> +    /*
> +     * the index to Thread::requests.
> +     * Save it to the padding space although it can be calculated at runtime.
> +     */
> +    uint8_t request_index;
> +
> +    /* the index to Threads::per_thread_data */
> +    unsigned int thread_index;
> +} QEMU_ALIGNED(sizeof(unsigned long));
> +typedef struct ThreadRequest ThreadRequest;
> +
> +struct ThreadLocal {
> +    struct Threads *threads;
> +
> +    /* the index of the thread */
> +    int self;
> +
> +    /* thread is useless and needs to exit */
> +    bool quit;
> +
> +    QemuThread thread;
> +
> +    void *requests;
> +
> +   /*
> +     * the bit in these two bitmaps indicates the index of the ＠requests
> +     * respectively. If it's the same, the corresponding request is free
> +     * and owned by the user, i.e, where the user fills a request. Otherwise,
> +     * it is valid and owned by the thread, i.e, where the thread fetches
> +     * the request and write the result.
> +     */
> +
> +    /* after the user fills the request, the bit is flipped. */
> +    uint64_t request_fill_bitmap QEMU_ALIGNED(SMP_CACHE_BYTES);
> +    /* after handles the request, the thread flips the bit. */
> +    uint64_t request_done_bitmap QEMU_ALIGNED(SMP_CACHE_BYTES);

Patchew complained about some type mismatches; I think those are because
you're using the bitmap_* functions on these; those functions always
operate on 'long' not on uint64_t - and on some platforms they're
unfortunately not the same.


Dave

> +    /*
> +     * the event used to wake up the thread whenever a valid request has
> +     * been submitted
> +     */
> +    QemuEvent request_valid_ev QEMU_ALIGNED(SMP_CACHE_BYTES);
> +
> +    /*
> +     * the event is notified whenever a request has been completed
> +     * (i.e, become free), which is used to wake up the user
> +     */
> +    QemuEvent request_free_ev QEMU_ALIGNED(SMP_CACHE_BYTES);
> +};
> +typedef struct ThreadLocal ThreadLocal;
> +
> +/*
> + * the main data struct represents multithreads which is shared by
> + * all threads
> + */
> +struct Threads {
> +    /* the request header, ThreadRequest, is contained */
> +    unsigned int request_size;
> +    unsigned int thread_requests_nr;
> +    unsigned int threads_nr;
> +
> +    /* the request is pushed to the thread with round-robin manner */
> +    unsigned int current_thread_index;
> +
> +    const ThreadedWorkqueueOps *ops;
> +
> +    ThreadLocal per_thread_data[0];
> +};
> +typedef struct Threads Threads;
> +
> +static ThreadRequest *index_to_request(ThreadLocal *thread, int 
> request_index)
> +{
> +    ThreadRequest *request;
> +
> +    request = thread->requests + request_index * 
> thread->threads->request_size;
> +    assert(request->request_index == request_index);
> +    assert(request->thread_index == thread->self);
> +    return request;
> +}
> +
> +static int request_to_index(ThreadRequest *request)
> +{
> +    return request->request_index;
> +}
> +
> +static int request_to_thread_index(ThreadRequest *request)
> +{
> +    return request->thread_index;
> +}
> +
> +/*
> + * free request: the request is not used by any thread, however, it might
> + *   contain the result need the user to call thread_request_done()
> + *
> + * valid request: the request contains the request data and it's committed
> + *   to the thread, i,e. it's owned by thread.
> + */
> +static uint64_t get_free_request_bitmap(Threads *threads, ThreadLocal 
> *thread)
> +{
> +    uint64_t request_fill_bitmap, request_done_bitmap, result_bitmap;
> +
> +    request_fill_bitmap = atomic_rcu_read(&thread->request_fill_bitmap);
> +    request_done_bitmap = atomic_rcu_read(&thread->request_done_bitmap);
> +    bitmap_xor(&result_bitmap, &request_fill_bitmap, &request_done_bitmap,
> +               threads->thread_requests_nr);
> +
> +    /*
> +     * paired with smp_wmb() in mark_request_free() to make sure that we
> +     * read request_done_bitmap before fetching the result out.
> +     */
> +    smp_rmb();
> +
> +    return result_bitmap;
> +}
> +
> +static ThreadRequest
> +*find_thread_free_request(Threads *threads, ThreadLocal *thread)
> +{
> +    uint64_t result_bitmap = get_free_request_bitmap(threads, thread);
> +    int index;
> +
> +    index  = find_first_zero_bit(&result_bitmap, 
> threads->thread_requests_nr);
> +    if (index >= threads->thread_requests_nr) {
> +        return NULL;
> +    }
> +
> +    return index_to_request(thread, index);
> +}
> +
> +static ThreadRequest *threads_find_free_request(Threads *threads)
> +{
> +    ThreadLocal *thread;
> +    ThreadRequest *request;
> +    int cur_thread, thread_index;
> +
> +    cur_thread = threads->current_thread_index % threads->threads_nr;
> +    thread_index = cur_thread;
> +    do {
> +        thread = threads->per_thread_data + thread_index++;
> +        request = find_thread_free_request(threads, thread);
> +        if (request) {
> +            break;
> +        }
> +        thread_index %= threads->threads_nr;
> +    } while (thread_index != cur_thread);
> +
> +    return request;
> +}
> +
> +/*
> + * the change bit operation combined with READ_ONCE and WRITE_ONCE which
> + * only works on single uint64_t width
> + */
> +static void change_bit_once(long nr, uint64_t *addr)
> +{
> +    uint64_t value = atomic_rcu_read(addr) ^ BIT_MASK(nr);
> +
> +    atomic_rcu_set(addr, value);
> +}
> +
> +static void mark_request_valid(Threads *threads, ThreadRequest *request)
> +{
> +    int thread_index = request_to_thread_index(request);
> +    int request_index = request_to_index(request);
> +    ThreadLocal *thread = threads->per_thread_data + thread_index;
> +
> +    /*
> +     * paired with smp_rmb() in find_first_valid_request_index() to make
> +     * sure the request has been filled before the bit is flipped that
> +     * will make the request be visible to the thread
> +     */
> +    smp_wmb();
> +
> +    change_bit_once(request_index, &thread->request_fill_bitmap);
> +    qemu_event_set(&thread->request_valid_ev);
> +}
> +
> +static int thread_find_first_valid_request_index(ThreadLocal *thread)
> +{
> +    Threads *threads = thread->threads;
> +    uint64_t request_fill_bitmap, request_done_bitmap, result_bitmap;
> +    int index;
> +
> +    request_fill_bitmap = atomic_rcu_read(&thread->request_fill_bitmap);
> +    request_done_bitmap = atomic_rcu_read(&thread->request_done_bitmap);
> +    bitmap_xor(&result_bitmap, &request_fill_bitmap, &request_done_bitmap,
> +               threads->thread_requests_nr);
> +    /*
> +     * paired with smp_wmb() in mark_request_valid() to make sure that
> +     * we read request_fill_bitmap before fetch the request out.
> +     */
> +    smp_rmb();
> +
> +    index = find_first_bit(&result_bitmap, threads->thread_requests_nr);
> +    return index >= threads->thread_requests_nr ? -1 : index;
> +}
> +
> +static void mark_request_free(ThreadLocal *thread, ThreadRequest *request)
> +{
> +    int index = request_to_index(request);
> +
> +    /*
> +     * smp_wmb() is implied in change_bit_atomic() that is paired with
> +     * smp_rmb() in get_free_request_bitmap() to make sure the result
> +     * has been saved before the bit is flipped.
> +     */
> +    change_bit_atomic(index, &thread->request_done_bitmap);
> +    qemu_event_set(&thread->request_free_ev);
> +}
> +
> +/* retry to see if there is available request before actually go to wait. */
> +#define BUSY_WAIT_COUNT 1000
> +
> +static ThreadRequest *
> +thread_busy_wait_for_request(ThreadLocal *thread)
> +{
> +    int index, count = 0;
> +
> +    for (count = 0; count < BUSY_WAIT_COUNT; count++) {
> +        index = thread_find_first_valid_request_index(thread);
> +        if (index >= 0) {
> +            return index_to_request(thread, index);
> +        }
> +
> +        cpu_relax();
> +    }
> +
> +    return NULL;
> +}
> +
> +static void *thread_run(void *opaque)
> +{
> +    ThreadLocal *self_data = (ThreadLocal *)opaque;
> +    Threads *threads = self_data->threads;
> +    void (*handler)(void *request) = threads->ops->thread_request_handler;
> +    ThreadRequest *request;
> +
> +    for ( ; !atomic_read(&self_data->quit); ) {
> +        qemu_event_reset(&self_data->request_valid_ev);
> +
> +        request = thread_busy_wait_for_request(self_data);
> +        if (!request) {
> +            qemu_event_wait(&self_data->request_valid_ev);
> +            continue;
> +        }
> +
> +        assert(!request->done);
> +
> +        handler(request + 1);
> +        request->done = true;
> +        mark_request_free(self_data, request);
> +    }
> +
> +    return NULL;
> +}
> +
> +static void uninit_thread_requests(ThreadLocal *thread, int free_nr)
> +{
> +    Threads *threads = thread->threads;
> +    ThreadRequest *request = thread->requests;
> +    int i;
> +
> +    for (i = 0; i < free_nr; i++) {
> +        threads->ops->thread_request_uninit(request + 1);
> +        request = (void *)request + threads->request_size;
> +    }
> +    g_free(thread->requests);
> +}
> +
> +static int init_thread_requests(ThreadLocal *thread)
> +{
> +    Threads *threads = thread->threads;
> +    ThreadRequest *request;
> +    int ret, i, thread_reqs_size;
> +
> +    thread_reqs_size = threads->thread_requests_nr * threads->request_size;
> +    thread_reqs_size = QEMU_ALIGN_UP(thread_reqs_size, SMP_CACHE_BYTES);
> +    thread->requests = g_malloc0(thread_reqs_size);
> +
> +    request = thread->requests;
> +    for (i = 0; i < threads->thread_requests_nr; i++) {
> +        ret = threads->ops->thread_request_init(request + 1);
> +        if (ret < 0) {
> +            goto exit;
> +        }
> +
> +        request->request_index = i;
> +        request->thread_index = thread->self;
> +        request = (void *)request + threads->request_size;
> +    }
> +    return 0;
> +
> +exit:
> +    uninit_thread_requests(thread, i);
> +    return -1;
> +}
> +
> +static void uninit_thread_data(Threads *threads, int free_nr)
> +{
> +    ThreadLocal *thread_local = threads->per_thread_data;
> +    int i;
> +
> +    for (i = 0; i < free_nr; i++) {
> +        thread_local[i].quit = true;
> +        qemu_event_set(&thread_local[i].request_valid_ev);
> +        qemu_thread_join(&thread_local[i].thread);
> +        qemu_event_destroy(&thread_local[i].request_valid_ev);
> +        qemu_event_destroy(&thread_local[i].request_free_ev);
> +        uninit_thread_requests(&thread_local[i], 
> threads->thread_requests_nr);
> +    }
> +}
> +
> +static int
> +init_thread_data(Threads *threads, const char *thread_name, int thread_nr)
> +{
> +    ThreadLocal *thread_local = threads->per_thread_data;
> +    char *name;
> +    int i;
> +
> +    for (i = 0; i < thread_nr; i++) {
> +        thread_local[i].threads = threads;
> +        thread_local[i].self = i;
> +
> +        if (init_thread_requests(&thread_local[i]) < 0) {
> +            goto exit;
> +        }
> +
> +        qemu_event_init(&thread_local[i].request_free_ev, false);
> +        qemu_event_init(&thread_local[i].request_valid_ev, false);
> +
> +        name = g_strdup_printf("%s/%d", thread_name, thread_local[i].self);
> +        qemu_thread_create(&thread_local[i].thread, name,
> +                           thread_run, &thread_local[i], 
> QEMU_THREAD_JOINABLE);
> +        g_free(name);
> +    }
> +    return 0;
> +
> +exit:
> +    uninit_thread_data(threads, i);
> +    return -1;
> +}
> +
> +Threads *threaded_workqueue_create(const char *name, unsigned int threads_nr,
> +                                   unsigned int thread_requests_nr,
> +                                   const ThreadedWorkqueueOps *ops)
> +{
> +    Threads *threads;
> +
> +    if (threads_nr > MAX_THREAD_REQUEST_NR) {
> +        return NULL;
> +    }
> +
> +    threads = g_malloc0(sizeof(*threads) + threads_nr * sizeof(ThreadLocal));
> +    threads->ops = ops;
> +    threads->threads_nr = threads_nr;
> +    threads->thread_requests_nr = thread_requests_nr;
> +
> +    QEMU_BUILD_BUG_ON(!QEMU_IS_ALIGNED(sizeof(ThreadRequest), sizeof(long)));
> +    threads->request_size = threads->ops->request_size;
> +    threads->request_size = QEMU_ALIGN_UP(threads->request_size, 
> sizeof(long));
> +    threads->request_size += sizeof(ThreadRequest);
> +
> +    if (init_thread_data(threads, name, threads_nr) < 0) {
> +        g_free(threads);
> +        return NULL;
> +    }
> +
> +    return threads;
> +}
> +
> +void threaded_workqueue_destroy(Threads *threads)
> +{
> +    uninit_thread_data(threads, threads->threads_nr);
> +    g_free(threads);
> +}
> +
> +static void request_done(Threads *threads, ThreadRequest *request)
> +{
> +    if (!request->done) {
> +        return;
> +    }
> +
> +    threads->ops->thread_request_done(request + 1);
> +    request->done = false;
> +}
> +
> +void *threaded_workqueue_get_request(Threads *threads)
> +{
> +    ThreadRequest *request;
> +
> +    request = threads_find_free_request(threads);
> +    if (!request) {
> +        return NULL;
> +    }
> +
> +    request_done(threads, request);
> +    return request + 1;
> +}
> +
> +void threaded_workqueue_submit_request(Threads *threads, void *request)
> +{
> +    ThreadRequest *req = request - sizeof(ThreadRequest);
> +    int thread_index = request_to_thread_index(request);
> +
> +    assert(!req->done);
> +    mark_request_valid(threads, req);
> +    threads->current_thread_index = thread_index  + 1;
> +}
> +
> +void threaded_workqueue_wait_for_requests(Threads *threads)
> +{
> +    ThreadLocal *thread;
> +    uint64_t result_bitmap;
> +    int thread_index, index = 0;
> +
> +    for (thread_index = 0; thread_index < threads->threads_nr; 
> thread_index++) {
> +        thread = threads->per_thread_data + thread_index;
> +        index = 0;
> +retry:
> +        qemu_event_reset(&thread->request_free_ev);
> +        result_bitmap = get_free_request_bitmap(threads, thread);
> +
> +        for (; index < threads->thread_requests_nr; index++) {
> +            if (test_bit(index, &result_bitmap)) {
> +                qemu_event_wait(&thread->request_free_ev);
> +                goto retry;
> +            }
> +
> +            request_done(threads, index_to_request(thread, index));
> +        }
> +    }
> +}
> -- 
> 2.14.5
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK