On Mon, Apr 10, 2017 at 9:52 PM, Stefan Hajnoczi <stefa...@gmail.com> wrote:
> On Sat, Apr 08, 2017 at 09:17:58PM +0800, 858585 jemmy wrote:
>> On Fri, Apr 7, 2017 at 7:34 PM, Stefan Hajnoczi <stefa...@redhat.com> wrote:
>> > On Fri, Apr 07, 2017 at 09:30:33AM +0800, 858585 jemmy wrote:
>> >> On Thu, Apr 6, 2017 at 10:02 PM, Stefan Hajnoczi <stefa...@redhat.com>
>> >> wrote:
>> >> > On Wed, Apr 05, 2017 at 05:27:58PM +0800, jemmy858...@gmail.com wrote:
>> >> >
>> >> > A proper solution is to refactor the synchronous code to make it
>> >> > asynchronous. This might require invoking the system call from a
>> >> > thread pool worker.
>> >> >
>> >>
>> >> yes, i agree with you, but this is a big change.
>> >> I will try to find how to optimize this code, maybe need a long time.
>> >>
>> >> this patch is not a perfect solution, but can alleviate the problem.
>> >
>> > Let's try to understand the problem fully first.
>> >
>>
>> when migrate the vm with high speed, i find vnc response slowly sometime.
>> not only vnc response slowly, virsh console aslo response slowly sometime.
>> and the guest os block io performance is also reduce.
>>
>> the bug can be reproduce by this command:
>> virsh migrate-setspeed 165cf436-312f-47e7-90f2-f8aa63f34893 900
>> virsh migrate --live 165cf436-312f-47e7-90f2-f8aa63f34893
>> --copy-storage-inc qemu+ssh://10.59.163.38/system
>>
>> and --copy-storage-all have no problem.
>> virsh migrate --live 165cf436-312f-47e7-90f2-f8aa63f34893
>> --copy-storage-all qemu+ssh://10.59.163.38/system
>>
>> compare the difference between --copy-storage-inc and
>> --copy-storage-all. i find out the reason is
>> mig_save_device_bulk invoke bdrv_is_allocated, but bdrv_is_allocated
>> is synchronous and maybe wait
>> for a long time.
>>
>> i write this code to measure the time used by brdrv_is_allocated()
>>
>> 279 static int max_time = 0;
>> 280 int tmp;
>>
>> 288 clock_gettime(CLOCK_MONOTONIC_RAW, &ts1);
>> 289 ret = bdrv_is_allocated(blk_bs(bb), cur_sector,
>> 290 MAX_IS_ALLOCATED_SEARCH,
>> &nr_sectors);
>> 291 clock_gettime(CLOCK_MONOTONIC_RAW, &ts2);
>> 292
>> 293
>> 294 tmp = (ts2.tv_sec - ts1.tv_sec)*1000000000L
>> 295 + (ts2.tv_nsec - ts1.tv_nsec);
>> 296 if (tmp > max_time) {
>> 297 max_time=tmp;
>> 298 fprintf(stderr, "max_time is %d\n", max_time);
>> 299 }
>>
>> the test result is below:
>>
>> max_time is 37014
>> max_time is 1075534
>> max_time is 17180913
>> max_time is 28586762
>> max_time is 49563584
>> max_time is 103085447
>> max_time is 110836833
>> max_time is 120331438
>>
>> bdrv_is_allocated is called after qemu_mutex_lock_iothread.
>> and the main thread is also call qemu_mutex_lock_iothread.
>> so cause the main thread maybe wait for a long time.
>>
>> if (bmds->shared_base) {
>> qemu_mutex_lock_iothread();
>> aio_context_acquire(blk_get_aio_context(bb));
>> /* Skip unallocated sectors; intentionally treats failure as
>> * an allocated sector */
>> while (cur_sector < total_sectors &&
>> !bdrv_is_allocated(blk_bs(bb), cur_sector,
>> MAX_IS_ALLOCATED_SEARCH, &nr_sectors)) {
>> cur_sector += nr_sectors;
>> }
>> aio_context_release(blk_get_aio_context(bb));
>> qemu_mutex_unlock_iothread();
>> }
>>
>> #0 0x00007f107322f264 in __lll_lock_wait () from /lib64/libpthread.so.0
>> #1 0x00007f107322a508 in _L_lock_854 () from /lib64/libpthread.so.0
>> #2 0x00007f107322a3d7 in pthread_mutex_lock () from /lib64/libpthread.so.0
>> #3 0x0000000000949ecb in qemu_mutex_lock (mutex=0xfc51a0) at
>> util/qemu-thread-posix.c:60
>> #4 0x0000000000459e58 in qemu_mutex_lock_iothread () at
>> /root/qemu/cpus.c:1516
>> #5 0x0000000000945322 in os_host_main_loop_wait (timeout=28911939) at
>> util/main-loop.c:258
>> #6 0x00000000009453f2 in main_loop_wait (nonblocking=0) at
>> util/main-loop.c:517
>> #7 0x00000000005c76b4 in main_loop () at vl.c:1898
>> #8 0x00000000005ceb77 in main (argc=49, argv=0x7fff921182b8,
>> envp=0x7fff92118448) at vl.c:4709
>
> The following patch moves bdrv_is_allocated() into bb's AioContext. It
> will execute without blocking other I/O activity.
>
> Compile-tested only.
i will try this patch.
>
> diff --git a/migration/block.c b/migration/block.c
> index 7734ff7..a5572a4 100644
> --- a/migration/block.c
> +++ b/migration/block.c
> @@ -263,6 +263,29 @@ static void blk_mig_read_cb(void *opaque, int ret)
> blk_mig_unlock();
> }
>
> +typedef struct {
> + int64_t *total_sectors;
> + int64_t *cur_sector;
> + BlockBackend *bb;
> + QemuEvent event;
> +} MigNextAllocatedClusterData;
> +
> +static void coroutine_fn mig_next_allocated_cluster(void *opaque)
> +{
> + MigNextAllocatedClusterData *data = opaque;
> + int nr_sectors;
> +
> + /* Skip unallocated sectors; intentionally treats failure as
> + * an allocated sector */
> + while (*data->cur_sector < *data->total_sectors &&
> + !bdrv_is_allocated(blk_bs(data->bb), *data->cur_sector,
> + MAX_IS_ALLOCATED_SEARCH, &nr_sectors)) {
> + *data->cur_sector += nr_sectors;
> + }
> +
> + qemu_event_set(&data->event);
> +}
> +
> /* Called with no lock taken. */
>
> static int mig_save_device_bulk(QEMUFile *f, BlkMigDevState *bmds)
> @@ -274,17 +297,27 @@ static int mig_save_device_bulk(QEMUFile *f,
> BlkMigDevState *bmds)
> int nr_sectors;
>
> if (bmds->shared_base) {
> + /* Searching for the next allocated cluster can block. Do it in a
> + * coroutine inside bb's AioContext. That way we don't need to hold
> + * the global mutex while blocked.
> + */
> + AioContext *bb_ctx;
> + Coroutine *co;
> + MigNextAllocatedClusterData data = {
> + .cur_sector = &cur_sector,
> + .total_sectors = &total_sectors,
> + .bb = bb,
> + };
> +
> + qemu_event_init(&data.event, false);
> +
> qemu_mutex_lock_iothread();
> - aio_context_acquire(blk_get_aio_context(bb));
> - /* Skip unallocated sectors; intentionally treats failure as
> - * an allocated sector */
> - while (cur_sector < total_sectors &&
> - !bdrv_is_allocated(blk_bs(bb), cur_sector,
> - MAX_IS_ALLOCATED_SEARCH, &nr_sectors)) {
> - cur_sector += nr_sectors;
> - }
> - aio_context_release(blk_get_aio_context(bb));
> + bb_ctx = blk_get_aio_context(bb);
> qemu_mutex_unlock_iothread();
> +
> + co = qemu_coroutine_create(mig_next_allocated_cluster, &data);
> + aio_co_schedule(bb_ctx, co);
> + qemu_event_wait(&data.event);
> }
>
> if (cur_sector >= total_sectors) {
