Re: [RFC PATCH 4/8] migration/dirtyrate: Record hash results for each ramblock

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

* Chuan Zheng (zhengchuan@huawei.com) wrote:
> From: Zheng Chuan <zhengchuan@huawei.com>
> 
> Record hash results for each ramblock.

Please be careful when talking about 'ramblock' since we already use
that for a chunk of memory in QEMU.

> Signed-off-by: Zheng Chuan <zhengchuan@huawei.com>
> Signed-off-by: YanYing Zhang <ann.zhuangyanying@huawei.com>
> ---
>  migration/dirtyrate.c | 157 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  migration/dirtyrate.h |   1 +
>  2 files changed, 158 insertions(+)
> 
> diff --git a/migration/dirtyrate.c b/migration/dirtyrate.c
> index 6baf674..45cfc91 100644
> --- a/migration/dirtyrate.c
> +++ b/migration/dirtyrate.c
> @@ -10,12 +10,27 @@
>   * See the COPYING file in the top-level directory.
>   */
>  
> +#include "qemu/osdep.h"
> +#include "qapi/error.h"
> +#include "crypto/hash.h"
> +#include "crypto/random.h"
> +#include "qemu/config-file.h"
> +#include "exec/memory.h"
> +#include "exec/ramblock.h"
> +#include "exec/target_page.h"
> +#include "qemu/rcu_queue.h"
> +#include "qapi/qapi-commands-migration.h"
> +#include "migration.h"
>  #include "dirtyrate.h"
>  
>  static uint64_t sample_pages_per_gigabytes = DIRTYRATE_DEFAULT_SAMPLE_PAGES;
>  static struct dirtyrate_statistics dirty_stat;
>  CalculatingDirtyRateStage calculating_dirty_rate_stage = CAL_DIRTY_RATE_INIT;
>  
> +#define RAMBLOCK_FOREACH_MIGRATABLE(block)             \
> +        INTERNAL_RAMBLOCK_FOREACH(block)                   \
> +        if (!qemu_ram_is_migratable(block)) {} else
> +

Instead of redefining this here, please move the existing definition up
into migration/ram.h ?

>  static void reset_dirtyrate_stat(void)
>  {
>      dirty_stat.total_dirty_samples = 0;
> @@ -44,6 +59,148 @@ static void update_dirtyrate(int64_t msec)
>      dirty_stat.dirty_rate = dirty_rate;
>  }
>  
> +static int get_block_vfn_hash(struct block_dirty_info *info, unsigned long vfn,
> +                              uint8_t **md, size_t *hash_len)
> +{
> +    struct iovec iov_array;
> +    int ret = 0;
> +    int nkey = 1;
> +
> +    iov_array.iov_base = info->block_addr +
> +                         vfn * DIRTYRATE_SAMPLE_PAGE_SIZE;
> +    iov_array.iov_len = DIRTYRATE_SAMPLE_PAGE_SIZE;

I'm a bit confused by how this is working; is 'vfn' an index
in SAMPLE_PAGE_SIZE's rather than individual pages? So this is
going to hash over something the size of a 'DIRTYRATE_SAMPLE_PAGE_SIZE'?

> +    if (qcrypto_hash_bytesv(QCRYPTO_HASH_ALG_MD5,
> +                            &iov_array, nkey,
> +                            md, hash_len, NULL) < 0) {
> +        ret = -1;
> +    }
> +
> +    return ret;
> +}
> +
> +static int save_block_hash(struct block_dirty_info *info)
> +{
> +    unsigned long *rand_buf = NULL;
> +    unsigned int sample_pages_count;
> +    uint8_t *md = NULL;
> +    size_t hash_len;
> +    int i;
> +    int ret = -1;
> +
> +    sample_pages_count = info->sample_pages_count;
> +    /* block size less than one page, return success to skip this block */
> +    if (unlikely(info->block_pages == 0 || sample_pages_count == 0)) {
> +        ret = 0;
> +        goto out;
> +    }
> +
> +    /* use random bytes to pick sample page vfn */
> +    rand_buf = g_malloc0_n(sample_pages_count, sizeof(unsigned long));
> +    /* DEFAULT_READ_RANDOM_MAX_LIMIT 32M,
> +     * can support 4T vm 1024 sample_pages_per_gigabytes
> +     */
> +    ret = qcrypto_random_bytes((unsigned char *)rand_buf,
> +                               sample_pages_count * sizeof(unsigned long),
> +                               NULL);

I think there may be a different way to do this without having to store
the rand_buf.
We already link to glib, and glib has a PRNG; https://developer.gnome.org/glib/stable/glib-Random-Numbers.html
If you create a new prng is g_rand_new() at the start of day, and take a
copy with g_rand_copy(), then you can replay the sequence of random
numbers it generates without actually storing the sequence.
So g_rand_new(), g_rand_copy() and then before you traverse the set of
pages you go and take a copy again and use the copy; it'll give the same
sequence every time.

Note also, because you're allocating a potentially large array,
for this and the hash_result please use g_try_malloc0_n (or g_try_new0)
and fail properly if it returns NULL.

Note we have users with more than 4T of RAM in their VMs, although I
doubt in a single RAMBlock

> +    if (ret) {
> +        ret = -1;
> +        goto out;
> +    }
> +
> +    hash_len = qcrypto_hash_digest_len(QCRYPTO_HASH_ALG_MD5);
> +    info->hash_result = g_malloc0_n(sample_pages_count, sizeof(uint8_t) * hash_len);
> +    info->sample_page_vfn = g_malloc0_n(sample_pages_count, sizeof(unsigned long));
> +
> +    for (i = 0; i < sample_pages_count; i++) {
> +        md = info->hash_result + i * hash_len;
> +        info->sample_page_vfn[i] = rand_buf[i] % info->block_pages;
> +        ret = get_block_vfn_hash(info, info->sample_page_vfn[i], &md, &hash_len);
> +        if (ret < 0) {
> +            goto out;
> +        }
> +    }
> +    ret = 0;
> +out:
> +    g_free(rand_buf);
> +    return ret;
> +}
> +
> +static void get_block_dirty_info(RAMBlock *block, struct block_dirty_info *info,
> +                                 struct dirtyrate_config *config)
> +{
> +    uint64_t sample_pages_per_gigabytes = config->sample_pages_per_gigabytes;
> +
> +    /* Right shift 30 bits to calc block size in GB */
> +    info->sample_pages_count = (qemu_ram_get_used_length(block) * sample_pages_per_gigabytes) >> 30;
> +
> +    info->block_pages = qemu_ram_get_used_length(block) >> DIRTYRATE_PAGE_SIZE_SHIFT;
> +    info->block_addr = qemu_ram_get_host_addr(block);
> +    strcpy(info->idstr, qemu_ram_get_idstr(block));
> +}
> +
> +static struct block_dirty_info *
> +alloc_block_dirty_info(int *block_index,
> +                       struct block_dirty_info *block_dinfo)
> +{
> +    struct block_dirty_info *info = NULL;
> +    int index = *block_index;
> +
> +    if (!block_dinfo) {
> +        block_dinfo = g_new(struct block_dirty_info, 1);
> +        index = 0;
> +    } else {
> +        block_dinfo = g_realloc(block_dinfo, (index + 1) *
> +                                sizeof(struct block_dirty_info));
> +        index++;

I think g_realloc works on a NULL pointer, so you might be able to
simplify.

> +    }
> +    info = &block_dinfo[index];
> +    memset(info, 0, sizeof(struct block_dirty_info));
> +
> +    *block_index = index;
> +    return block_dinfo;
> +}
> +
> +static int ram_block_skip(RAMBlock *block)
> +{
> +    if (!strstr(qemu_ram_get_idstr(block), "ram-node") &&
> +        !strstr(qemu_ram_get_idstr(block), "memdimm")) {
> +        if (strcmp(qemu_ram_get_idstr(block), "mach-virt.ram") ||
> +            strcmp(block->idstr, "pc.ram")) {
> +            return -1;
> +        }
> +    }

We can't tie this to the names you guess that RAMBlocks might have - 
things like 'memdimm' are settable by the caller and can be anything,
so you need to picka  different way of chosing which RAMBlocks to
use; I suggest anything larger than some cutoff size, that's really RAM.

> +    return 0;
> +}
> +
> +static int record_block_hash_info(struct dirtyrate_config config,
> +                                  struct block_dirty_info **block_dinfo, int *block_index)
> +{
> +    struct block_dirty_info *info = NULL;
> +    struct block_dirty_info *dinfo = NULL;
> +    RAMBlock *block = NULL;
> +    int index = 0;
> +
> +    RAMBLOCK_FOREACH_MIGRATABLE(block) {
> +        if (ram_block_skip(block) < 0) {
> +            continue;
> +        }
> +        dinfo = alloc_block_dirty_info(&index, dinfo);
> +        info = &dinfo[index];
> +        get_block_dirty_info(block, info, &config);
> +        if (save_block_hash(info) < 0) {
> +            *block_dinfo = dinfo;
> +            *block_index = index;
> +            return -1;
> +        }
> +    }
> +
> +    *block_dinfo = dinfo;
> +    *block_index = index;
> +
> +    return 0;
> +}
>  
>  static void calculate_dirtyrate(struct dirtyrate_config config, int64_t time)
>  {
> diff --git a/migration/dirtyrate.h b/migration/dirtyrate.h
> index 2994535..4d9b3b8 100644
> --- a/migration/dirtyrate.h
> +++ b/migration/dirtyrate.h
> @@ -15,6 +15,7 @@
>  
>  /* take 256 pages per GB for cal dirty rate */
>  #define DIRTYRATE_DEFAULT_SAMPLE_PAGES    256
> +#define DIRTYRATE_SAMPLE_PAGE_SIZE      4096
>  #define DIRTYRATE_PAGE_SIZE_SHIFT       12
>  #define BLOCK_INFO_MAX_LEN              256
>  #define PAGE_SIZE_SHIFT                 20
> -- 
> 1.8.3.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK