Re: [PATCH RFC} io_uring: io_kiocb alloc cache

Jens Axboe
2020-05-14T14:33:42Z

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.

Can you try this with your microbenchmark? Just curious what it looks
like for that test case if we completely take slab alloc+free out of it.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index d2e37215d05a..4ecd6bd38f02 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
@@ -5784,12 +5794,10 @@ 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 +5847,7 @@ 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 +5874,31 @@ 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 */

--
Jens Axboe