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migration: Use atomic ops properly for page accountings
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To prepare for thread-safety on page accountings, at least below counters
need to be accessed only atomically, they are:

        ram_counters.transferred
        ram_counters.duplicate
        ram_counters.normal
        ram_counters.postcopy_bytes

There are a lot of other counters but they won't be accessed outside
migration thread, then they're still safe to be accessed without atomic
ops.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
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@xzpeter
xzpeter authored and Juan Quintela committed Nov 15, 2022
1 parent 7f88420 commit e7cf488
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Showing 4 changed files with 51 additions and 23 deletions.
10 changes: 5 additions & 5 deletions migration/migration.c
View file
Expand Up @@ -1069,13 +1069,13 @@ static void populate_ram_info(MigrationInfo *info, MigrationState *s)

info->has_ram = true;
info->ram = g_malloc0(sizeof(*info->ram));
info->ram->transferred = ram_counters.transferred;
info->ram->transferred = stat64_get(&ram_atomic_counters.transferred);
info->ram->total = ram_bytes_total();
info->ram->duplicate = ram_counters.duplicate;
info->ram->duplicate = stat64_get(&ram_atomic_counters.duplicate);
/* legacy value. It is not used anymore */
info->ram->skipped = 0;
info->ram->normal = ram_counters.normal;
info->ram->normal_bytes = ram_counters.normal * page_size;
info->ram->normal = stat64_get(&ram_atomic_counters.normal);
info->ram->normal_bytes = info->ram->normal * page_size;
info->ram->mbps = s->mbps;
info->ram->dirty_sync_count = ram_counters.dirty_sync_count;
info->ram->dirty_sync_missed_zero_copy =
Expand All @@ -1086,7 +1086,7 @@ static void populate_ram_info(MigrationInfo *info, MigrationState *s)
info->ram->pages_per_second = s->pages_per_second;
info->ram->precopy_bytes = ram_counters.precopy_bytes;
info->ram->downtime_bytes = ram_counters.downtime_bytes;
info->ram->postcopy_bytes = ram_counters.postcopy_bytes;
info->ram->postcopy_bytes = stat64_get(&ram_atomic_counters.postcopy_bytes);

if (migrate_use_xbzrle()) {
info->has_xbzrle_cache = true;
Expand Down
4 changes: 2 additions & 2 deletions migration/multifd.c
View file
Expand Up @@ -432,7 +432,7 @@ static int multifd_send_pages(QEMUFile *f)
transferred = ((uint64_t) pages->num) * p->page_size + p->packet_len;
qemu_file_acct_rate_limit(f, transferred);
ram_counters.multifd_bytes += transferred;
ram_counters.transferred += transferred;
stat64_add(&ram_atomic_counters.transferred, transferred);
qemu_mutex_unlock(&p->mutex);
qemu_sem_post(&p->sem);

Expand Down Expand Up @@ -624,7 +624,7 @@ int multifd_send_sync_main(QEMUFile *f)
p->pending_job++;
qemu_file_acct_rate_limit(f, p->packet_len);
ram_counters.multifd_bytes += p->packet_len;
ram_counters.transferred += p->packet_len;
stat64_add(&ram_atomic_counters.transferred, p->packet_len);
qemu_mutex_unlock(&p->mutex);
qemu_sem_post(&p->sem);

Expand Down
40 changes: 24 additions & 16 deletions migration/ram.c
View file
Expand Up @@ -453,18 +453,25 @@ uint64_t ram_bytes_remaining(void)
0;
}

/*
* NOTE: not all stats in ram_counters are used in reality. See comments
* for struct MigrationAtomicStats. The ultimate result of ram migration
* counters will be a merged version with both ram_counters and the atomic
* fields in ram_atomic_counters.
*/
MigrationStats ram_counters;
MigrationAtomicStats ram_atomic_counters;

void ram_transferred_add(uint64_t bytes)
{
if (runstate_is_running()) {
ram_counters.precopy_bytes += bytes;
} else if (migration_in_postcopy()) {
ram_counters.postcopy_bytes += bytes;
stat64_add(&ram_atomic_counters.postcopy_bytes, bytes);
} else {
ram_counters.downtime_bytes += bytes;
}
ram_counters.transferred += bytes;
stat64_add(&ram_atomic_counters.transferred, bytes);
}

void dirty_sync_missed_zero_copy(void)
Expand Down Expand Up @@ -753,7 +760,7 @@ void mig_throttle_counter_reset(void)

rs->time_last_bitmap_sync = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
rs->num_dirty_pages_period = 0;
rs->bytes_xfer_prev = ram_counters.transferred;
rs->bytes_xfer_prev = stat64_get(&ram_atomic_counters.transferred);
}

/**
Expand Down Expand Up @@ -1113,8 +1120,9 @@ uint64_t ram_pagesize_summary(void)

uint64_t ram_get_total_transferred_pages(void)
{
return ram_counters.normal + ram_counters.duplicate +
compression_counters.pages + xbzrle_counters.pages;
return stat64_get(&ram_atomic_counters.normal) +
stat64_get(&ram_atomic_counters.duplicate) +
compression_counters.pages + xbzrle_counters.pages;
}

static void migration_update_rates(RAMState *rs, int64_t end_time)
Expand Down Expand Up @@ -1173,8 +1181,8 @@ static void migration_trigger_throttle(RAMState *rs)
{
MigrationState *s = migrate_get_current();
uint64_t threshold = s->parameters.throttle_trigger_threshold;

uint64_t bytes_xfer_period = ram_counters.transferred - rs->bytes_xfer_prev;
uint64_t bytes_xfer_period =
stat64_get(&ram_atomic_counters.transferred) - rs->bytes_xfer_prev;
uint64_t bytes_dirty_period = rs->num_dirty_pages_period * TARGET_PAGE_SIZE;
uint64_t bytes_dirty_threshold = bytes_xfer_period * threshold / 100;

Expand Down Expand Up @@ -1237,7 +1245,7 @@ static void migration_bitmap_sync(RAMState *rs)
/* reset period counters */
rs->time_last_bitmap_sync = end_time;
rs->num_dirty_pages_period = 0;
rs->bytes_xfer_prev = ram_counters.transferred;
rs->bytes_xfer_prev = stat64_get(&ram_atomic_counters.transferred);
}
if (migrate_use_events()) {
qapi_event_send_migration_pass(ram_counters.dirty_sync_count);
Expand Down Expand Up @@ -1313,7 +1321,7 @@ static int save_zero_page(RAMState *rs, RAMBlock *block, ram_addr_t offset)
int len = save_zero_page_to_file(rs, rs->f, block, offset);

if (len) {
ram_counters.duplicate++;
stat64_add(&ram_atomic_counters.duplicate, 1);
ram_transferred_add(len);
return 1;
}
Expand Down Expand Up @@ -1350,9 +1358,9 @@ static bool control_save_page(RAMState *rs, RAMBlock *block, ram_addr_t offset,
}

if (bytes_xmit > 0) {
ram_counters.normal++;
stat64_add(&ram_atomic_counters.normal, 1);
} else if (bytes_xmit == 0) {
ram_counters.duplicate++;
stat64_add(&ram_atomic_counters.duplicate, 1);
}

return true;
Expand Down Expand Up @@ -1382,7 +1390,7 @@ static int save_normal_page(RAMState *rs, RAMBlock *block, ram_addr_t offset,
qemu_put_buffer(rs->f, buf, TARGET_PAGE_SIZE);
}
ram_transferred_add(TARGET_PAGE_SIZE);
ram_counters.normal++;
stat64_add(&ram_atomic_counters.normal, 1);
return 1;
}

Expand Down Expand Up @@ -1438,7 +1446,7 @@ static int ram_save_multifd_page(RAMState *rs, RAMBlock *block,
if (multifd_queue_page(rs->f, block, offset) < 0) {
return -1;
}
ram_counters.normal++;
stat64_add(&ram_atomic_counters.normal, 1);

return 1;
}
Expand Down Expand Up @@ -1476,7 +1484,7 @@ update_compress_thread_counts(const CompressParam *param, int bytes_xmit)
ram_transferred_add(bytes_xmit);

if (param->zero_page) {
ram_counters.duplicate++;
stat64_add(&ram_atomic_counters.duplicate, 1);
return;
}

Expand Down Expand Up @@ -2651,9 +2659,9 @@ void acct_update_position(QEMUFile *f, size_t size, bool zero)
uint64_t pages = size / TARGET_PAGE_SIZE;

if (zero) {
ram_counters.duplicate += pages;
stat64_add(&ram_atomic_counters.duplicate, pages);
} else {
ram_counters.normal += pages;
stat64_add(&ram_atomic_counters.normal, pages);
ram_transferred_add(size);
qemu_file_credit_transfer(f, size);
}
Expand Down
20 changes: 20 additions & 0 deletions migration/ram.h
View file
Expand Up @@ -32,7 +32,27 @@
#include "qapi/qapi-types-migration.h"
#include "exec/cpu-common.h"
#include "io/channel.h"
#include "qemu/stats64.h"

/*
* These are the migration statistic counters that need to be updated using
* atomic ops (can be accessed by more than one thread). Here since we
* cannot modify MigrationStats directly to use Stat64 as it was defined in
* the QAPI scheme, we define an internal structure to hold them, and we
* propagate the real values when QMP queries happen.
*
* IOW, the corresponding fields within ram_counters on these specific
* fields will be always zero and not being used at all; they're just
* placeholders to make it QAPI-compatible.
*/
typedef struct {
Stat64 transferred;
Stat64 duplicate;
Stat64 normal;
Stat64 postcopy_bytes;
} MigrationAtomicStats;

extern MigrationAtomicStats ram_atomic_counters;
extern MigrationStats ram_counters;
extern XBZRLECacheStats xbzrle_counters;
extern CompressionStats compression_counters;
Expand Down

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