Re: [Qemu-devel] [PATCH v3 3/5] migration: use threaded workqueue for compression

[Paolo Bonzini <pbonzini@redhat.com>]

On 23/11/18 19:17, Dr. David Alan Gilbert wrote:
> * guangrong.xiao@gmail.com (guangrong.xiao@gmail.com) wrote:
>> From: Xiao Guangrong <xiaoguangrong@tencent.com>
>>
>> Adapt the compression code to the threaded workqueue
>>
>> Signed-off-by: Xiao Guangrong <xiaoguangrong@tencent.com>
>> ---
>>  migration/ram.c | 308 ++++++++++++++++++++------------------------------------
>>  1 file changed, 110 insertions(+), 198 deletions(-)
>>
>> diff --git a/migration/ram.c b/migration/ram.c
>> index 7e7deec4d8..254c08f27b 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -57,6 +57,7 @@
>>  #include "qemu/uuid.h"
>>  #include "savevm.h"
>>  #include "qemu/iov.h"
>> +#include "qemu/threaded-workqueue.h"
>>  
>>  /***********************************************************/
>>  /* ram save/restore */
>> @@ -349,22 +350,6 @@ typedef struct PageSearchStatus PageSearchStatus;
>>  
>>  CompressionStats compression_counters;
>>  
>> -struct CompressParam {
>> -    bool done;
>> -    bool quit;
>> -    bool zero_page;
>> -    QEMUFile *file;
>> -    QemuMutex mutex;
>> -    QemuCond cond;
>> -    RAMBlock *block;
>> -    ram_addr_t offset;
>> -
>> -    /* internally used fields */
>> -    z_stream stream;
>> -    uint8_t *originbuf;
>> -};
>> -typedef struct CompressParam CompressParam;
>> -
>>  struct DecompressParam {
>>      bool done;
>>      bool quit;
>> @@ -377,15 +362,6 @@ struct DecompressParam {
>>  };
>>  typedef struct DecompressParam DecompressParam;
>>  
>> -static CompressParam *comp_param;
>> -static QemuThread *compress_threads;
>> -/* comp_done_cond is used to wake up the migration thread when
>> - * one of the compression threads has finished the compression.
>> - * comp_done_lock is used to co-work with comp_done_cond.
>> - */
>> -static QemuMutex comp_done_lock;
>> -static QemuCond comp_done_cond;
>> -/* The empty QEMUFileOps will be used by file in CompressParam */
>>  static const QEMUFileOps empty_ops = { };
>>  
>>  static QEMUFile *decomp_file;
>> @@ -394,125 +370,6 @@ static QemuThread *decompress_threads;
>>  static QemuMutex decomp_done_lock;
>>  static QemuCond decomp_done_cond;
>>  
>> -static bool do_compress_ram_page(QEMUFile *f, z_stream *stream, RAMBlock *block,
>> -                                 ram_addr_t offset, uint8_t *source_buf);
>> -
>> -static void *do_data_compress(void *opaque)
>> -{
>> -    CompressParam *param = opaque;
>> -    RAMBlock *block;
>> -    ram_addr_t offset;
>> -    bool zero_page;
>> -
>> -    qemu_mutex_lock(&param->mutex);
>> -    while (!param->quit) {
>> -        if (param->block) {
>> -            block = param->block;
>> -            offset = param->offset;
>> -            param->block = NULL;
>> -            qemu_mutex_unlock(&param->mutex);
>> -
>> -            zero_page = do_compress_ram_page(param->file, &param->stream,
>> -                                             block, offset, param->originbuf);
>> -
>> -            qemu_mutex_lock(&comp_done_lock);
>> -            param->done = true;
>> -            param->zero_page = zero_page;
>> -            qemu_cond_signal(&comp_done_cond);
>> -            qemu_mutex_unlock(&comp_done_lock);
>> -
>> -            qemu_mutex_lock(&param->mutex);
>> -        } else {
>> -            qemu_cond_wait(&param->cond, &param->mutex);
>> -        }
>> -    }
>> -    qemu_mutex_unlock(&param->mutex);
>> -
>> -    return NULL;
>> -}
>> -
>> -static void compress_threads_save_cleanup(void)
>> -{
>> -    int i, thread_count;
>> -
>> -    if (!migrate_use_compression() || !comp_param) {
>> -        return;
>> -    }
>> -
>> -    thread_count = migrate_compress_threads();
>> -    for (i = 0; i < thread_count; i++) {
>> -        /*
>> -         * we use it as a indicator which shows if the thread is
>> -         * properly init'd or not
>> -         */
>> -        if (!comp_param[i].file) {
>> -            break;
>> -        }
>> -
>> -        qemu_mutex_lock(&comp_param[i].mutex);
>> -        comp_param[i].quit = true;
>> -        qemu_cond_signal(&comp_param[i].cond);
>> -        qemu_mutex_unlock(&comp_param[i].mutex);
>> -
>> -        qemu_thread_join(compress_threads + i);
>> -        qemu_mutex_destroy(&comp_param[i].mutex);
>> -        qemu_cond_destroy(&comp_param[i].cond);
>> -        deflateEnd(&comp_param[i].stream);
>> -        g_free(comp_param[i].originbuf);
>> -        qemu_fclose(comp_param[i].file);
>> -        comp_param[i].file = NULL;
>> -    }
>> -    qemu_mutex_destroy(&comp_done_lock);
>> -    qemu_cond_destroy(&comp_done_cond);
>> -    g_free(compress_threads);
>> -    g_free(comp_param);
>> -    compress_threads = NULL;
>> -    comp_param = NULL;
>> -}
>> -
>> -static int compress_threads_save_setup(void)
>> -{
>> -    int i, thread_count;
>> -
>> -    if (!migrate_use_compression()) {
>> -        return 0;
>> -    }
>> -    thread_count = migrate_compress_threads();
>> -    compress_threads = g_new0(QemuThread, thread_count);
>> -    comp_param = g_new0(CompressParam, thread_count);
>> -    qemu_cond_init(&comp_done_cond);
>> -    qemu_mutex_init(&comp_done_lock);
>> -    for (i = 0; i < thread_count; i++) {
>> -        comp_param[i].originbuf = g_try_malloc(TARGET_PAGE_SIZE);
>> -        if (!comp_param[i].originbuf) {
>> -            goto exit;
>> -        }
>> -
>> -        if (deflateInit(&comp_param[i].stream,
>> -                        migrate_compress_level()) != Z_OK) {
>> -            g_free(comp_param[i].originbuf);
>> -            goto exit;
>> -        }
>> -
>> -        /* comp_param[i].file is just used as a dummy buffer to save data,
>> -         * set its ops to empty.
>> -         */
>> -        comp_param[i].file = qemu_fopen_ops(NULL, &empty_ops);
>> -        comp_param[i].done = true;
>> -        comp_param[i].quit = false;
>> -        qemu_mutex_init(&comp_param[i].mutex);
>> -        qemu_cond_init(&comp_param[i].cond);
>> -        qemu_thread_create(compress_threads + i, "compress",
>> -                           do_data_compress, comp_param + i,
>> -                           QEMU_THREAD_JOINABLE);
>> -    }
>> -    return 0;
>> -
>> -exit:
>> -    compress_threads_save_cleanup();
>> -    return -1;
>> -}
>> -
>>  /* Multiple fd's */
>>  
>>  #define MULTIFD_MAGIC 0x11223344U
>> @@ -1909,12 +1766,25 @@ exit:
>>      return zero_page;
>>  }
>>  
>> +struct CompressData {
>> +    /* filled by migration thread.*/
>> +    RAMBlock *block;
>> +    ram_addr_t offset;
>> +
>> +    /* filled by compress thread. */
>> +    QEMUFile *file;
>> +    z_stream stream;
>> +    uint8_t *originbuf;
>> +    bool zero_page;
>> +};
>> +typedef struct CompressData CompressData;
>> +
>>  static void
>> -update_compress_thread_counts(const CompressParam *param, int bytes_xmit)
>> +update_compress_thread_counts(CompressData *cd, int bytes_xmit)
> 
> Keep the const?
>>  {
>>      ram_counters.transferred += bytes_xmit;
>>  
>> -    if (param->zero_page) {
>> +    if (cd->zero_page) {
>>          ram_counters.duplicate++;
>>          return;
>>      }
>> @@ -1924,81 +1794,123 @@ update_compress_thread_counts(const CompressParam *param, int bytes_xmit)
>>      compression_counters.pages++;
>>  }
>>  
>> +static int compress_thread_data_init(void *request)
>> +{
>> +    CompressData *cd = request;
>> +
>> +    cd->originbuf = g_try_malloc(TARGET_PAGE_SIZE);
>> +    if (!cd->originbuf) {
>> +        return -1;
>> +    }
>> +
>> +    if (deflateInit(&cd->stream, migrate_compress_level()) != Z_OK) {
>> +        g_free(cd->originbuf);
>> +        return -1;
>> +    }
> 
> Please print errors if you fail in any case so we can easily tell what
> happened.
> 
>> +    cd->file = qemu_fopen_ops(NULL, &empty_ops);
>> +    return 0;
>> +}
>> +
>> +static void compress_thread_data_fini(void *request)
>> +{
>> +    CompressData *cd = request;
>> +
>> +    qemu_fclose(cd->file);
>> +    deflateEnd(&cd->stream);
>> +    g_free(cd->originbuf);
>> +}
>> +
>> +static void compress_thread_data_handler(void *request)
>> +{
>> +    CompressData *cd = request;
>> +
>> +    /*
>> +     * if compression fails, it will be indicated by
>> +     * migrate_get_current()->to_dst_file.
>> +     */
>> +    cd->zero_page = do_compress_ram_page(cd->file, &cd->stream, cd->block,
>> +                                         cd->offset, cd->originbuf);
>> +}
>> +
>> +static void compress_thread_data_done(void *request)
>> +{
>> +    CompressData *cd = request;
>> +    RAMState *rs = ram_state;
>> +    int bytes_xmit;
>> +
>> +    bytes_xmit = qemu_put_qemu_file(rs->f, cd->file);
>> +    update_compress_thread_counts(cd, bytes_xmit);
>> +}
>> +
>> +static const ThreadedWorkqueueOps compress_ops = {
>> +    .thread_request_init = compress_thread_data_init,
>> +    .thread_request_uninit = compress_thread_data_fini,
>> +    .thread_request_handler = compress_thread_data_handler,
>> +    .thread_request_done = compress_thread_data_done,
>> +    .request_size = sizeof(CompressData),
>> +};
>> +
>> +static Threads *compress_threads;
>> +
>>  static bool save_page_use_compression(RAMState *rs);
>>  
>>  static void flush_compressed_data(RAMState *rs)
>>  {
>> -    int idx, len, thread_count;
>> -
>>      if (!save_page_use_compression(rs)) {
>>          return;
>>      }
>> -    thread_count = migrate_compress_threads();
>>  
>> -    qemu_mutex_lock(&comp_done_lock);
>> -    for (idx = 0; idx < thread_count; idx++) {
>> -        while (!comp_param[idx].done) {
>> -            qemu_cond_wait(&comp_done_cond, &comp_done_lock);
>> -        }
>> -    }
>> -    qemu_mutex_unlock(&comp_done_lock);
>> +    threaded_workqueue_wait_for_requests(compress_threads);
>> +}
>>  
>> -    for (idx = 0; idx < thread_count; idx++) {
>> -        qemu_mutex_lock(&comp_param[idx].mutex);
>> -        if (!comp_param[idx].quit) {
>> -            len = qemu_put_qemu_file(rs->f, comp_param[idx].file);
>> -            /*
>> -             * it's safe to fetch zero_page without holding comp_done_lock
>> -             * as there is no further request submitted to the thread,
>> -             * i.e, the thread should be waiting for a request at this point.
>> -             */
>> -            update_compress_thread_counts(&comp_param[idx], len);
>> -        }
>> -        qemu_mutex_unlock(&comp_param[idx].mutex);
>> +static void compress_threads_save_cleanup(void)
>> +{
>> +    if (!compress_threads) {
>> +        return;
>>      }
>> +
>> +    threaded_workqueue_destroy(compress_threads);
>> +    compress_threads = NULL;
>>  }
>>  
>> -static inline void set_compress_params(CompressParam *param, RAMBlock *block,
>> -                                       ram_addr_t offset)
>> +static int compress_threads_save_setup(void)
>>  {
>> -    param->block = block;
>> -    param->offset = offset;
>> +    if (!migrate_use_compression()) {
>> +        return 0;
>> +    }
>> +
>> +    compress_threads = threaded_workqueue_create("compress",
>> +                                migrate_compress_threads(),
>> +                                DEFAULT_THREAD_REQUEST_NR, &compress_ops);
>> +    return compress_threads ? 0 : -1;
>>  }
>>  
>>  static int compress_page_with_multi_thread(RAMState *rs, RAMBlock *block,
>>                                             ram_addr_t offset)
>>  {
>> -    int idx, thread_count, bytes_xmit = -1, pages = -1;
>> +    CompressData *cd;
>>      bool wait = migrate_compress_wait_thread();
>>  
>> -    thread_count = migrate_compress_threads();
>> -    qemu_mutex_lock(&comp_done_lock);
>>  retry:
>> -    for (idx = 0; idx < thread_count; idx++) {
>> -        if (comp_param[idx].done) {
>> -            comp_param[idx].done = false;
>> -            bytes_xmit = qemu_put_qemu_file(rs->f, comp_param[idx].file);
>> -            qemu_mutex_lock(&comp_param[idx].mutex);
>> -            set_compress_params(&comp_param[idx], block, offset);
>> -            qemu_cond_signal(&comp_param[idx].cond);
>> -            qemu_mutex_unlock(&comp_param[idx].mutex);
>> -            pages = 1;
>> -            update_compress_thread_counts(&comp_param[idx], bytes_xmit);
>> -            break;
>> +    cd = threaded_workqueue_get_request(compress_threads);
>> +    if (!cd) {
>> +        /*
>> +         * wait for the free thread if the user specifies
>> +         * 'compress-wait-thread', otherwise we will post
>> +         *  the page out in the main thread as normal page.
>> +         */
>> +        if (wait) {
>> +            cpu_relax();
>> +            goto retry;
> 
> Is there nothing better we can use to wait without eating CPU time?

There is a mechanism to wait without eating CPU time in the data
structure, but it makes sense to busy wait.  There are 4 threads in the
workqueue, so you have to compare 1/4th of the time spent compressing a
page, with the trip into the kernel to wake you up.  You're adding 20%
CPU usage, but I'm not surprised it's worthwhile.

Paolo