Re: [PATCH RFC} io_uring: io_kiocb alloc cache

Jens Axboe
2020-05-14T15:15:32Z

On 5/14/20 8:53 AM, Pavel Begunkov wrote:
> On 14/05/2020 17:33, Jens Axboe wrote:
>> On 5/14/20 8:22 AM, Jens Axboe wrote:
>>>> I still use my previous io_uring_nop_stress tool to evaluate the improvement
>>>> in a physical machine. Memory 250GB and cpu is "Intel(R) Xeon(R) CPU E5-2682 v4 @ 2.50GHz".
>>>> Before this patch:
>>>> $sudo taskset -c 60 ./io_uring_nop_stress -r 300
>>>> total ios: 1608773840
>>>> IOPS: 5362579
>>>>
>>>> With this patch:
>>>> sudo taskset -c 60 ./io_uring_nop_stress -r 300
>>>> total ios: 1676910736
>>>> IOPS: 5589702
>>>> About 4.2% improvement.
>>>
>>> That's not bad. Can you try the patch from Pekka as well, just to see if
>>> that helps for you?
>>>
>>> I also had another idea... We basically have two types of request life
>>> times:
>>>
>>> 1) io_kiocb can get queued up internally
>>> 2) io_kiocb completes inline
>>>
>>> For the latter, it's totally feasible to just have the io_kiocb on
>>> stack. The downside is if we need to go the slower path, then we need to
>>> alloc an io_kiocb then and copy it. But maybe that's OK... I'll play
>>> with it.
>
> Does it differ from having one pre-allocated req? Like fallback_req,
> but without atomics and returned only under uring_mutex (i.e. in
> __io_queue_sqe()). Putting aside its usefulness, at least it will have
> a chance to work with reads/writes.

But then you need atomics. I actually think the bigger win here is not
having to use atomic refcounts for this particular part, since we know
the request can't get shared.

Modified below with poor-man-dec-and-test, pretty sure that part is a
bigger win than avoiding the allocator. So maybe that's a better
directiont to take, for things we can't get irq completions on.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index d2e37215d05a..27c1c8f4d2f4 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -525,6 +525,7 @@ enum {
REQ_F_POLLED_BIT,
REQ_F_BUFFER_SELECTED_BIT,
REQ_F_NO_FILE_TABLE_BIT,
+ REQ_F_STACK_REQ_BIT,

/* not a real bit, just to check we're not overflowing the space */
__REQ_F_LAST_BIT,
@@ -580,6 +581,8 @@ enum {
REQ_F_BUFFER_SELECTED = BIT(REQ_F_BUFFER_SELECTED_BIT),
/* doesn't need file table for this request */
REQ_F_NO_FILE_TABLE = BIT(REQ_F_NO_FILE_TABLE_BIT),
+ /* on-stack req */
+ REQ_F_STACK_REQ = BIT(REQ_F_STACK_REQ_BIT),
};

struct async_poll {
@@ -695,10 +698,14 @@ struct io_op_def {
unsigned pollout : 1;
/* op supports buffer selection */
unsigned buffer_select : 1;
+ /* op can use stack req */
+ unsigned stack_req : 1;
};

static const struct io_op_def io_op_defs[] = {
- [IORING_OP_NOP] = {},
+ [IORING_OP_NOP] = {
+ .stack_req = 1,
+ },
[IORING_OP_READV] = {
.async_ctx = 1,
.needs_mm = 1,
@@ -1345,7 +1352,8 @@ static void __io_req_aux_free(struct io_kiocb *req)
if (req->flags & REQ_F_NEED_CLEANUP)
io_cleanup_req(req);

- kfree(req->io);
+ if (req->io)
+ kfree(req->io);
if (req->file)
io_put_file(req, req->file, (req->flags & REQ_F_FIXED_FILE));
if (req->task)
@@ -1370,6 +1378,8 @@ static void __io_free_req(struct io_kiocb *req)
}

percpu_ref_put(&req->ctx->refs);
+ if (req->flags & REQ_F_STACK_REQ)
+ return;
if (likely(!io_is_fallback_req(req)))
kmem_cache_free(req_cachep, req);
else
@@ -1585,7 +1595,14 @@ static void io_wq_assign_next(struct io_wq_work **workptr, struct io_kiocb *nxt)
__attribute__((nonnull))
static void io_put_req_find_next(struct io_kiocb *req, struct io_kiocb **nxtptr)
{
- if (refcount_dec_and_test(&req->refs)) {
+ bool free_it;
+
+ if (req->flags & REQ_F_STACK_REQ)
+ free_it = __refcount_dec_and_test(&req->refs);
+ else
+ free_it = refcount_dec_and_test(&req->refs);
+
+ if (free_it) {
io_req_find_next(req, nxtptr);
__io_free_req(req);
}
@@ -1593,7 +1610,13 @@ static void io_put_req_find_next(struct io_kiocb *req, struct io_kiocb **nxtptr)

static void io_put_req(struct io_kiocb *req)
{
- if (refcount_dec_and_test(&req->refs))
+ bool free_it;
+
+ if (req->flags & REQ_F_STACK_REQ)
+ free_it = __refcount_dec_and_test(&req->refs);
+ else
+ free_it = refcount_dec_and_test(&req->refs);
+ if (free_it)
io_free_req(req);
}

@@ -5784,12 +5807,10 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
* link list.
*/
req->sequence = ctx->cached_sq_head - ctx->cached_sq_dropped;
- req->opcode = READ_ONCE(sqe->opcode);
req->user_data = READ_ONCE(sqe->user_data);
req->io = NULL;
req->file = NULL;
req->ctx = ctx;
- req->flags = 0;
/* one is dropped after submission, the other at completion */
refcount_set(&req->refs, 2);
req->task = NULL;
@@ -5839,6 +5860,7 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
{
struct io_submit_state state, *statep = NULL;
struct io_kiocb *link = NULL;
+ struct io_kiocb stack_req;
int i, submitted = 0;

/* if we have a backlog and couldn't flush it all, return BUSY */
@@ -5865,20 +5887,31 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
for (i = 0; i < nr; i++) {
const struct io_uring_sqe *sqe;
struct io_kiocb *req;
- int err;
+ int err, op;

sqe = io_get_sqe(ctx);
if (unlikely(!sqe)) {
io_consume_sqe(ctx);
break;
}
- req = io_alloc_req(ctx, statep);
- if (unlikely(!req)) {
- if (!submitted)
- submitted = -EAGAIN;
- break;
+
+ op = READ_ONCE(sqe->opcode);
+
+ if (io_op_defs[op].stack_req) {
+ req = &stack_req;
+ req->flags = REQ_F_STACK_REQ;
+ } else {
+ req = io_alloc_req(ctx, statep);
+ if (unlikely(!req)) {
+ if (!submitted)
+ submitted = -EAGAIN;
+ break;
+ }
+ req->flags = 0;
}

+ req->opcode = op;
+
err = io_init_req(ctx, req, sqe, statep, async);
io_consume_sqe(ctx);
/* will complete beyond this point, count as submitted */
diff --git a/include/linux/refcount.h b/include/linux/refcount.h
index 0e3ee25eb156..1afaf73c0984 100644
--- a/include/linux/refcount.h
+++ b/include/linux/refcount.h
@@ -294,6 +294,13 @@ static inline __must_check bool refcount_dec_and_test(refcount_t *r)
return refcount_sub_and_test(1, r);
}

+static inline __must_check bool __refcount_dec_and_test(refcount_t *r)
+{
+ int old = --r->refs.counter;
+
+ return old == 0;
+}
+
/**
* refcount_dec - decrement a refcount
* @r: the refcount

--
Jens Axboe