[PATCH 2/2] io_uring: batch getting pcpu references

Pavel Begunkov
2019-12-17T22:29:35Z

percpu_ref_tryget() has its own overhead. Instead getting a reference
for each request, grab a bunch once per io_submit_sqes().

basic benchmark with submit and wait 128 non-linked nops showed ~5%
performance gain. (7044 KIOPS vs 7423)

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---

For notice: it could be done without @extra_refs variable,
but looked too tangled because of gotos.

fs/io_uring.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index cf4138f0e504..6c85dfc62224 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -845,9 +845,6 @@ static struct io_kiocb *io_get_req(struct io_ring_ctx *ctx,
gfp_t gfp = GFP_KERNEL | __GFP_NOWARN;
struct io_kiocb *req;

- if (!percpu_ref_tryget(&ctx->refs))
- return NULL;
-
if (!state) {
req = kmem_cache_alloc(req_cachep, gfp);
if (unlikely(!req))
@@ -3929,6 +3926,7 @@ 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;
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 */
@@ -3941,6 +3939,10 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
statep = &state;
}

+ if (!percpu_ref_tryget_many(&ctx->refs, nr))
+ return -EAGAIN;
+ extra_refs = nr;
+
for (i = 0; i < nr; i++) {
struct io_kiocb *req = io_get_req(ctx, statep);

@@ -3949,6 +3951,7 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
submitted = -EAGAIN;
break;
}
+ --extra_refs;
if (!io_get_sqring(ctx, req)) {
__io_free_req(req);
break;
@@ -3976,6 +3979,8 @@ 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);

/* Commit SQ ring head once we've consumed and submitted all SQEs */
io_commit_sqring(ctx);
--
2.24.0