On 27.11.19 19:08, Vladimir Sementsov-Ogievskiy wrote: > Currently, block_copy operation lock the whole requested region. But > there is no reason to lock clusters, which are already copied, it will > disturb other parallel block_copy requests for no reason. > > Let's instead do the following: > > Lock only sub-region, which we are going to operate on. Then, after > copying all dirty sub-regions, we should wait for intersecting > requests block-copy, if they failed, we should retry these new dirty > clusters.
Just a thought spoken aloud:
I would expect the number of intersecting CBW requests to be low in
general, so I don’t know how useful this change is in practice. OTOH,
it makes block_copy call the existing implementation in a loop, which
seems just worse.
But then again, in the common case, block_copy_dirty_clusters() won’t
copy anything because it’s all been copied already, so there is no
change; and even if something is copied, the second call will just
re-check the dirty bitmap to see that the area’s clean (which will be
quick compared to the copy operation). So there’s probably nothing to
worry about.
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
> ---
> block/block-copy.c | 116 +++++++++++++++++++++++++++++++++++++--------
> 1 file changed, 95 insertions(+), 21 deletions(-)
>
> diff --git a/block/block-copy.c b/block/block-copy.c
> index 20068cd699..aca44b13fb 100644
> --- a/block/block-copy.c
> +++ b/block/block-copy.c
> @@ -39,29 +39,62 @@ static BlockCopyInFlightReq
> *block_copy_find_inflight_req(BlockCopyState *s,
> return NULL;
> }
>
> -static void coroutine_fn block_copy_wait_inflight_reqs(BlockCopyState *s,
> - int64_t offset,
> - int64_t bytes)
> +/*
> + * If there are no intersecting requests return false. Otherwise, wait for
> the
> + * first found intersecting request to finish and return true.
> + */
> +static bool coroutine_fn block_copy_wait_one(BlockCopyState *s, int64_t
> start,
> + int64_t end)
s/end/bytes/?
(And maybe s/start/offset/, too)
> {
> - BlockCopyInFlightReq *req;
> + BlockCopyInFlightReq *req = block_copy_find_inflight_req(s, start, end);
>
> - while ((req = block_copy_find_inflight_req(s, offset, bytes))) {
> - qemu_co_queue_wait(&req->wait_queue, NULL);
> + if (!req) {
> + return false;
> }
> +
> + qemu_co_queue_wait(&req->wait_queue, NULL);
> +
> + return true;
> }
>
> +/* Called only on full-dirty region */
> static void block_copy_inflight_req_begin(BlockCopyState *s,
> BlockCopyInFlightReq *req,
> int64_t offset, int64_t bytes)
> {
> + assert(!block_copy_find_inflight_req(s, offset, bytes));
> +
> + bdrv_reset_dirty_bitmap(s->copy_bitmap, offset, bytes);
> +
> req->offset = offset;
> req->bytes = bytes;
> qemu_co_queue_init(&req->wait_queue);
> QLIST_INSERT_HEAD(&s->inflight_reqs, req, list);
> }
>
> -static void coroutine_fn block_copy_inflight_req_end(BlockCopyInFlightReq
> *req)
> +static void coroutine_fn block_copy_inflight_req_shrink(BlockCopyState *s,
> + BlockCopyInFlightReq *req, int64_t new_bytes)
It took me a while to understand that this is operation drops the tail
of the request. I think there should be a comment on this.
(I thought it would successively drop the head after each copy, and so I
was wondering why the code didn’t match that.)
> {
> + if (new_bytes == req->bytes) {
> + return;
> + }
> +
> + assert(new_bytes > 0 && new_bytes < req->bytes);
> +
> + bdrv_set_dirty_bitmap(s->copy_bitmap,
> + req->offset + new_bytes, req->bytes - new_bytes);>
> +
> + req->bytes = new_bytes;
> + qemu_co_queue_restart_all(&req->wait_queue);
> +}
> +
> +static void coroutine_fn block_copy_inflight_req_end(BlockCopyState *s,
> + BlockCopyInFlightReq
> *req,
> + int ret)
> +{
> + if (ret < 0) {
> + bdrv_set_dirty_bitmap(s->copy_bitmap, req->offset, req->bytes);
> + }
> QLIST_REMOVE(req, list);
> qemu_co_queue_restart_all(&req->wait_queue);
> }
> @@ -344,12 +377,19 @@ int64_t block_copy_reset_unallocated(BlockCopyState *s,
> return ret;
> }
>
> -int coroutine_fn block_copy(BlockCopyState *s,
> - int64_t offset, uint64_t bytes,
> - bool *error_is_read)
> +/*
> + * block_copy_dirty_clusters
> + *
> + * Copy dirty clusters in @start/@bytes range.
> + * Returns 1 if dirty clusters found and successfully copied, 0 if no dirty
> + * clusters found and -errno on failure.
> + */
> +static int coroutine_fn block_copy_dirty_clusters(BlockCopyState *s,
> + int64_t offset, int64_t
> bytes,
> + bool *error_is_read)
> {
> int ret = 0;
> - BlockCopyInFlightReq req;
> + bool found_dirty = false;
>
> /*
> * block_copy() user is responsible for keeping source and target in same
> @@ -361,10 +401,8 @@ int coroutine_fn block_copy(BlockCopyState *s,
> assert(QEMU_IS_ALIGNED(offset, s->cluster_size));
> assert(QEMU_IS_ALIGNED(bytes, s->cluster_size));
>
> - block_copy_wait_inflight_reqs(s, offset, bytes);
> - block_copy_inflight_req_begin(s, &req, offset, bytes);
> -
> while (bytes) {
> + BlockCopyInFlightReq req;
> int64_t next_zero, cur_bytes, status_bytes;
>
> if (!bdrv_dirty_bitmap_get(s->copy_bitmap, offset)) {
> @@ -374,6 +412,8 @@ int coroutine_fn block_copy(BlockCopyState *s,
> continue; /* already copied */
> }
>
> + found_dirty = true;
> +
> cur_bytes = MIN(bytes, s->copy_size);
>
> next_zero = bdrv_dirty_bitmap_next_zero(s->copy_bitmap, offset,
> @@ -383,10 +423,12 @@ int coroutine_fn block_copy(BlockCopyState *s,
> assert(next_zero < offset + cur_bytes); /* no need to do MIN() */
> cur_bytes = next_zero - offset;
> }
> + block_copy_inflight_req_begin(s, &req, offset, cur_bytes);
>
> ret = block_copy_block_status(s, offset, cur_bytes, &status_bytes);
> + block_copy_inflight_req_shrink(s, &req, status_bytes);
block_copy_inflight_req_shrink() asserts that status_bytes <= cur_bytes.
That isn’t necessarily correct, as block_copy_block_status() rounds up
on the last cluster. So this should use the same MIN() as for the
cur_bytes update after the next block.
Would it make sense to move the block_copy_inflight_req_shrink() there
and pass the updated cur_bytes to it?
> if (s->skip_unallocated && !(ret & BDRV_BLOCK_ALLOCATED)) {
> - bdrv_reset_dirty_bitmap(s->copy_bitmap, offset, status_bytes);
> + block_copy_inflight_req_end(s, &req, 0);
> s->progress_reset_callback(s->progress_opaque);
> trace_block_copy_skip_range(s, offset, status_bytes);
> offset += status_bytes;
> @@ -398,15 +440,13 @@ int coroutine_fn block_copy(BlockCopyState *s,
>
> trace_block_copy_process(s, offset);
>
> - bdrv_reset_dirty_bitmap(s->copy_bitmap, offset, cur_bytes);
> -
> co_get_from_shres(s->mem, cur_bytes);
> ret = block_copy_do_copy(s, offset, cur_bytes, ret & BDRV_BLOCK_ZERO,
> error_is_read);
> co_put_to_shres(s->mem, cur_bytes);
> + block_copy_inflight_req_end(s, &req, ret);
> if (ret < 0) {
> - bdrv_set_dirty_bitmap(s->copy_bitmap, offset, cur_bytes);
> - break;
> + return ret;
> }
>
> s->progress_bytes_callback(cur_bytes, s->progress_opaque);
> @@ -414,7 +454,41 @@ int coroutine_fn block_copy(BlockCopyState *s,
> bytes -= cur_bytes;
> }
>
> - block_copy_inflight_req_end(&req);
> + return found_dirty;
> +}
>
> - return ret;
> +int coroutine_fn block_copy(BlockCopyState *s, int64_t start, uint64_t bytes,
> + bool *error_is_read)
> +{
> + while (true) {
> + int ret = block_copy_dirty_clusters(s, start, bytes, error_is_read);
> +
> + if (ret < 0) {
> + /*
> + * IO operation failed, which means the whole block_copy request
> + * failed.
> + */
> + return ret;
> + }
> + if (ret) {
> + /*
> + * Something was copied, which means that there were yield points
> + * and some new dirty bits may appered (due to failed parallel
s/appered/have appeared/
> + * block-copy requests).
> + */
> + continue;
> + }
> +
> + /*
> + * Here ret == 0, which means that there is no dirty clusters in
> + * requested region.
> + */
> +
> + if (!block_copy_wait_one(s, start, bytes)) {
> + /* No dirty bits and nothing to wait: the whole request is done
> */
Wouldn’t it make more sense to keep block_copy_wait_one() a loop (i.e.,
keep it as block_copy_wait_inflight_reqs()) that returns whether it
waited or not? Because I suppose if we had to wait for anything, we
might as well wait for everything in the range.
> + break;
> + }
> + }
Continuing my loud thought from the beginning, I would have written this
as a tail-recursive function to stress that this isn’t really a
(potentially expensive) loop but more of a re-check to be sure.
(i.e.
int ret = block_copy_dirty...();
if (ret < 0) {
return ret;
}
if (ret || block_copy_wait_one()) {
/* Something might have changed, re-check */
return block_copy();
}
/* Done */
return 0;
)
But who cares.
Max
> +
> + return 0;
> }
>
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