Re: [PATCH 10/16] migration/multifd: Enable DSA offloading in multifd sender path.

[Fabiano Rosas]

Hao Xiang <hao.xiang@bytedance.com> writes:

> Multifd sender path gets an array of pages queued by the migration
> thread. It performs zero page checking on every page in the array.
> The pages are classfied as either a zero page or a normal page. This
> change uses Intel DSA to offload the zero page checking from CPU to
> the DSA accelerator. The sender thread submits a batch of pages to DSA
> hardware and waits for the DSA completion thread to signal for work
> completion.
>
> Signed-off-by: Hao Xiang <hao.xiang@bytedance.com>
> ---
>  migration/multifd.c | 101 +++++++++++++++++++++++++++++++++++++-------
>  migration/multifd.h |   3 ++
>  2 files changed, 89 insertions(+), 15 deletions(-)
>
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 452fb158b8..79fecbd3ae 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -13,6 +13,8 @@
>  #include "qemu/osdep.h"
>  #include "qemu/rcu.h"
>  #include "qemu/cutils.h"
> +#include "qemu/dsa.h"
> +#include "qemu/memalign.h"
>  #include "exec/target_page.h"
>  #include "sysemu/sysemu.h"
>  #include "exec/ramblock.h"
> @@ -555,6 +557,8 @@ void multifd_save_cleanup(void)
>              qemu_thread_join(&p->thread);
>          }
>      }
> +    dsa_stop();
> +    dsa_cleanup();
>      for (i = 0; i < migrate_multifd_channels(); i++) {
>          MultiFDSendParams *p = &multifd_send_state->params[i];
>          Error *local_err = NULL;
> @@ -571,6 +575,11 @@ void multifd_save_cleanup(void)
>          p->name = NULL;
>          multifd_pages_clear(p->pages);
>          p->pages = NULL;
> +        g_free(p->addr);
> +        p->addr = NULL;
> +        buffer_zero_batch_task_destroy(p->dsa_batch_task);
> +        qemu_vfree(p->dsa_batch_task);
> +        p->dsa_batch_task = NULL;
>          p->packet_len = 0;
>          g_free(p->packet);
>          p->packet = NULL;
> @@ -675,13 +684,71 @@ int multifd_send_sync_main(QEMUFile *f)
>      return 0;
>  }
>  
> +static void set_page(MultiFDSendParams *p, bool zero_page, uint64_t offset)
> +{
> +    RAMBlock *rb = p->pages->block;
> +    if (zero_page) {
> +        p->zero[p->zero_num] = offset;
> +        p->zero_num++;
> +        ram_release_page(rb->idstr, offset);
> +    } else {
> +        p->normal[p->normal_num] = offset;
> +        p->normal_num++;
> +    }
> +}
> +
> +static void buffer_is_zero_use_cpu(MultiFDSendParams *p)
> +{
> +    const void **buf = (const void **)p->addr;
> +    assert(!migrate_use_main_zero_page());
> +    assert(!dsa_is_running());
> +
> +    for (int i = 0; i < p->pages->num; i++) {
> +        p->dsa_batch_task->results[i] = buffer_is_zero(buf[i], p->page_size);
> +    }
> +}
> +
> +static void buffer_is_zero_use_dsa(MultiFDSendParams *p)
> +{
> +    assert(!migrate_use_main_zero_page());
> +    assert(dsa_is_running());
> +
> +    buffer_is_zero_dsa_batch_async(p->dsa_batch_task,
> +                                   (const void **)p->addr,
> +                                   p->pages->num,
> +                                   p->page_size);
> +}
> +
> +static void multifd_zero_page_check(MultiFDSendParams *p)
> +{
> +    /* older qemu don't understand zero page on multifd channel */
> +    bool use_multifd_zero_page = !migrate_use_main_zero_page();
> +    bool use_multifd_dsa_accel = dsa_is_running();
> +
> +    RAMBlock *rb = p->pages->block;
> +
> +    for (int i = 0; i < p->pages->num; i++) {
> +        p->addr[i] = (ram_addr_t)(rb->host + p->pages->offset[i]);
> +    }
> +
> +    if (!use_multifd_zero_page || !use_multifd_dsa_accel) {
> +        buffer_is_zero_use_cpu(p);
> +    } else {
> +        buffer_is_zero_use_dsa(p);
> +    }
> +
> +    for (int i = 0; i < p->pages->num; i++) {
> +        uint64_t offset = p->pages->offset[i];
> +        bool zero_page = p->dsa_batch_task->results[i];
> +        set_page(p, zero_page, offset);
> +    }
> +}

You're moving existing (not really, but ok) code and adding dsa support
at the same time. The introduction of this function needs to be in a
separate patch. That would be a preliminary patch that isolates all of
the use_cpu code and a subsequent one that adds the use_dsa part.