Re: [Qemu-devel] [PATCH V7 RESEND 09/17] COLO: Flush memory data from ram cache

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Zhang Chen (zhangckid@gmail.com) wrote:
> During the time of VM's running, PVM may dirty some pages, we will transfer
> PVM's dirty pages to SVM and store them into SVM's RAM cache at next checkpoint
> time. So, the content of SVM's RAM cache will always be same with PVM's memory
> after checkpoint.
> 
> Instead of flushing all content of PVM's RAM cache into SVM's MEMORY,
> we do this in a more efficient way:
> Only flush any page that dirtied by PVM since last checkpoint.
> In this way, we can ensure SVM's memory same with PVM's.
> 
> Besides, we must ensure flush RAM cache before load device state.
> 
> Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
> Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>  migration/ram.c        | 39 +++++++++++++++++++++++++++++++++++++++
>  migration/trace-events |  2 ++
>  2 files changed, 41 insertions(+)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index e35dfee06e..4235a8f24d 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -3031,6 +3031,40 @@ static bool postcopy_is_running(void)
>      return ps >= POSTCOPY_INCOMING_LISTENING && ps < POSTCOPY_INCOMING_END;
>  }
>  
> +/*
> + * Flush content of RAM cache into SVM's memory.
> + * Only flush the pages that be dirtied by PVM or SVM or both.
> + */
> +static void colo_flush_ram_cache(void)
> +{
> +    RAMBlock *block = NULL;
> +    void *dst_host;
> +    void *src_host;
> +    unsigned long offset = 0;
> +
> +    trace_colo_flush_ram_cache_begin(ram_state->migration_dirty_pages);
> +    rcu_read_lock();
> +    block = QLIST_FIRST_RCU(&ram_list.blocks);
> +
> +    while (block) {
> +        offset = migration_bitmap_find_dirty(ram_state, block, offset);
> +        migration_bitmap_clear_dirty(ram_state, block, offset);

That looks suspicious to me; shouldn't that be inside the else block
below?   If the find_dirty returns a bit after or equal to used_length
(i.e. there's nothing dirty in this block), then you don't want to clear
that bit yet because it really means there's a dirty page at the start
of the next block?

Dave

> +        if (offset << TARGET_PAGE_BITS >= block->used_length) {
> +            offset = 0;
> +            block = QLIST_NEXT_RCU(block, next);
> +        } else {
> +            dst_host = block->host + (offset << TARGET_PAGE_BITS);
> +            src_host = block->colo_cache + (offset << TARGET_PAGE_BITS);
> +            memcpy(dst_host, src_host, TARGET_PAGE_SIZE);
> +        }
> +    }
> +
> +    rcu_read_unlock();
> +    trace_colo_flush_ram_cache_end();
> +    assert(ram_state->migration_dirty_pages == 0);
> +}
> +
>  static int ram_load(QEMUFile *f, void *opaque, int version_id)
>  {
>      int flags = 0, ret = 0, invalid_flags = 0;
> @@ -3043,6 +3077,7 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
>      bool postcopy_running = postcopy_is_running();
>      /* ADVISE is earlier, it shows the source has the postcopy capability on */
>      bool postcopy_advised = postcopy_is_advised();
> +    bool need_flush = false;
>  
>      seq_iter++;
>  
> @@ -3218,6 +3253,10 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
>      ret |= wait_for_decompress_done();
>      rcu_read_unlock();
>      trace_ram_load_complete(ret, seq_iter);
> +
> +    if (!ret  && migration_incoming_in_colo_state() && need_flush) {
> +        colo_flush_ram_cache();
> +    }
>      return ret;
>  }
>  
> diff --git a/migration/trace-events b/migration/trace-events
> index 9295b4cf40..8e2f9749e0 100644
> --- a/migration/trace-events
> +++ b/migration/trace-events
> @@ -78,6 +78,8 @@ ram_load_postcopy_loop(uint64_t addr, int flags) "@%" PRIx64 " %x"
>  ram_postcopy_send_discard_bitmap(void) ""
>  ram_save_page(const char *rbname, uint64_t offset, void *host) "%s: offset: 0x%" PRIx64 " host: %p"
>  ram_save_queue_pages(const char *rbname, size_t start, size_t len) "%s: start: 0x%zx len: 0x%zx"
> +colo_flush_ram_cache_begin(uint64_t dirty_pages) "dirty_pages %" PRIu64
> +colo_flush_ram_cache_end(void) ""
>  
>  # migration/migration.c
>  await_return_path_close_on_source_close(void) ""
> -- 
> 2.17.0
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK