Really a great idea, though I have some remarks on the implementation below.
On 19/02/20 11:00, Stefan Hajnoczi wrote:
> + * Each aio_bh_poll() call carves off a slice of the BH list. This way newly
> + * scheduled BHs are not processed until the next aio_bh_poll() call. This
> + * concept extends to nested aio_bh_poll() calls because slices are chained
> + * together.
This is the tricky part so I would expand a bit on why it's needed:
/*
* Each aio_bh_poll() call carves off a slice of the BH list, so that
* newly scheduled BHs are not processed until the next aio_bh_poll()
* call. All active aio_bh_poll() calls chained their slices together
* in a list, so that nested aio_bh_poll() calls process all scheduled
* bottom halves.
*/
> +struct BHListSlice {
> + QEMUBH *first_bh;
> + BHListSlice *next;
> +};
> +
Using QLIST and QSLIST removes the need to create your own lists, since
you can use QSLIST_MOVE_ATOMIC and QSLIST_INSERT_HEAD_ATOMIC. For example:
struct BHListSlice {
QSLIST_HEAD(, QEMUBH) first_bh;
QLIST_ENTRY(BHListSlice) next;
};
...
QSLIST_HEAD(, QEMUBH) active_bh;
QLIST_HEAD(, BHListSlice) bh_list;
Related to this, in the aio_bh_poll() loop:
for (s = ctx->bh_list.next; s; s = s->next) {
}
You can actually do the removal inside the loop. This is slightly more
efficient since you can remove slices early from the nested
aio_bh_poll(). Not that it's relevant for performance, but I think the
FIFO order for slices is also more intuitive than LIFO.
Putting this idea together with the QLIST one, you would get:
/*
* If a bottom half causes a recursive call, this slice will be
* removed by the nested aio_bh_poll().
*/
QSLIST_MOVE_ATOMIC(&slice.first_bh, ctx->active_bh);
QLIST_INSERT_TAIL(&ctx->bh_list, slice);
while ((s = QLIST_FIRST(&ctx->bh_list)) {
while ((bh = aio_bh_dequeue(&s, &flags))) {
}
QLIST_REMOVE_HEAD(s, next);
}
> /* Multiple occurrences of aio_bh_poll cannot be called concurrently.
> * The count in ctx->list_lock is incremented before the call, and is
> * not affected by the call.
The second sentence is now stale.
Paolo
> */
> int aio_bh_poll(AioContext *ctx)
> {
> - QEMUBH *bh, **bhp, *next;
> + BHListSlice slice;
> + BHListSlice **old_tail;
> + BHListSlice *s;
> + QEMUBH *bh;
> + unsigned flags;
> int ret;
> - bool deleted = false;
> +
> + old_tail = push_bh_list_slice(ctx, &slice);
>
