Re: [PATCH RFC v2 3/3] io_uring: batch get(ctx->ref) across submits

Pavel Begunkov
2019-12-21T16:21:05Z

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On 21/12/2019 19:15, Pavel Begunkov wrote:
> Double account ctx->refs keeping number of taken refs in ctx. As
> io_uring gets per-request ctx->refs during submission, while holding
> ctx->uring_lock, this allows in most of the time to bypass
> percpu_ref_get*() and its overhead.

Jens, could you please benchmark with this one? Especially for offloaded QD1
case. I haven't got any difference for nops test and don't have a decent SSD
at hands to test it myself. We could drop it, if there is no benefit.

This rewrites that @extra_refs from the second one, so I left it for now.

>
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
> fs/io_uring.c | 32 +++++++++++++++++++++++++-------
> 1 file changed, 25 insertions(+), 7 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 5392134f042f..eef09de94609 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -84,6 +84,9 @@
> #define IORING_MAX_ENTRIES 32768
> #define IORING_MAX_CQ_ENTRIES (2 * IORING_MAX_ENTRIES)
>
> +/* Not less than IORING_MAX_ENTRIES, so can grab once per submission loop */
> +#define IORING_REFS_THRESHOLD IORING_MAX_ENTRIES
> +
> /*
> * Shift of 9 is 512 entries, or exactly one page on 64-bit archs
> */
> @@ -197,6 +200,7 @@ struct fixed_file_data {
> struct io_ring_ctx {
> struct {
> struct percpu_ref refs;
> + unsigned long taken_refs; /* used under @uring_lock */
> } ____cacheline_aligned_in_smp;
>
> struct {
> @@ -690,6 +694,13 @@ static void io_ring_ctx_ref_free(struct percpu_ref *ref)
> complete(&ctx->completions[0]);
> }
>
> +static void io_free_taken_refs(struct io_ring_ctx *ctx)
> +{
> + if (ctx->taken_refs)
> + percpu_ref_put_many(&ctx->refs, ctx->taken_refs);
> + ctx->taken_refs = 0;
> +}
> +
> static struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
> {
> struct io_ring_ctx *ctx;
> @@ -4388,7 +4399,6 @@ 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;
> int i, submitted = 0;
> - unsigned int extra_refs;
> bool mm_fault = false;
>
> /* if we have a backlog and couldn't flush it all, return BUSY */
> @@ -4398,9 +4408,15 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
> return -EBUSY;
> }
>
> - if (!percpu_ref_tryget_many(&ctx->refs, nr))
> - return -EAGAIN;
> - extra_refs = nr;
> + if (ctx->taken_refs < IORING_REFS_THRESHOLD) {
> + if (unlikely(percpu_ref_is_dying(&ctx->refs))) {
> + io_free_taken_refs(ctx);
> + return -ENXIO;
> + }
> + if (!percpu_ref_tryget_many(&ctx->refs, IORING_REFS_THRESHOLD))
> + return -EAGAIN;
> + ctx->taken_refs += IORING_REFS_THRESHOLD;
> + }
>
> if (nr > IO_PLUG_THRESHOLD) {
> io_submit_state_start(&state, nr);
> @@ -4417,8 +4433,9 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
> submitted = -EAGAIN;
> break;
> }
> - --extra_refs;
> if (!io_get_sqring(ctx, req, &sqe)) {
> + /* not submitted, but a ref is freed */
> + ctx->taken_refs--;
> __io_free_req(req);
> break;
> }
> @@ -4454,8 +4471,7 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
> io_queue_link_head(link);
> if (statep)
> io_submit_state_end(&state);
> - if (extra_refs)
> - percpu_ref_put_many(&ctx->refs, extra_refs);
> + ctx->taken_refs -= submitted;
>
> /* Commit SQ ring head once we've consumed and submitted all SQEs */
> io_commit_sqring(ctx);
> @@ -5731,6 +5747,7 @@ static int io_uring_fasync(int fd, struct file *file, int on)
> static void io_ring_ctx_wait_and_kill(struct io_ring_ctx *ctx)
> {
> mutex_lock(&ctx->uring_lock);
> + io_free_taken_refs(ctx);
> percpu_ref_kill(&ctx->refs);
> mutex_unlock(&ctx->uring_lock);
>
> @@ -6196,6 +6213,7 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
>
> if (opcode != IORING_UNREGISTER_FILES &&
> opcode != IORING_REGISTER_FILES_UPDATE) {
> + io_free_taken_refs(ctx);
> percpu_ref_kill(&ctx->refs);
>
> /*
>

--
Pavel Begunkov

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