While looking at io_fill_cqe_req()'s asm I stumbled on our trace points
turning into the chunk below:
trace_io_uring_complete(req->ctx, req, req->cqe.user_data,
req->cqe.res, req->cqe.flags,
req->extra1, req->extra2);
io_uring/io_uring.c:898: trace_io_uring_complete(req->ctx, req, req->cqe.user_data,
movq 232(%rbx), %rdi # req_44(D)->big_cqe.extra2, _5
movq 224(%rbx), %rdx # req_44(D)->big_cqe.extra1, _6
movl 84(%rbx), %r9d # req_44(D)->cqe.D.81184.flags, _7
movl 80(%rbx), %r8d # req_44(D)->cqe.res, _8
movq 72(%rbx), %rcx # req_44(D)->cqe.user_data, _9
movq 88(%rbx), %rsi # req_44(D)->ctx, _10
./arch/x86/include/asm/jump_label.h:27: asm_volatile_goto("1:"
1:jmp .L1772 # objtool NOPs this #
...
It does a jump_label for actual tracing, but those 6 moves will stay
there in the hottest io_uring path. As an optimisation, add a
trace_io_uring_complete_enabled() check, which is also uses jump_labels,
it tricks the compiler into behaving. It removes the junk without
changing anything else int the hot path.
Note: apparently, it's not only me noticing it, and people are also
working it around. We should remove the check when it's solved
generically or rework tracing.
Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
---
io_uring/io_uring.h | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)
diff --git a/io_uring/io_uring.h b/io_uring/io_uring.h
index 3e6ff3cd9a24..465598223386 100644
--- a/io_uring/io_uring.h
+++ b/io_uring/io_uring.h
@@ -145,10 +145,11 @@ static inline bool io_fill_cqe_req(struct io_ring_ctx *ctx, struct io_kiocb *req
if (unlikely(!cqe))
return false;
- trace_io_uring_complete(req->ctx, req, req->cqe.user_data,
- req->cqe.res, req->cqe.flags,
- (req->flags & REQ_F_CQE32_INIT) ? req->extra1 : 0,
- (req->flags & REQ_F_CQE32_INIT) ? req->extra2 : 0);
+ if (trace_io_uring_complete_enabled())
+ trace_io_uring_complete(req->ctx, req, req->cqe.user_data,
+ req->cqe.res, req->cqe.flags,
+ (req->flags & REQ_F_CQE32_INIT) ? req->extra1 : 0,
+ (req->flags & REQ_F_CQE32_INIT) ? req->extra2 : 0);
memcpy(cqe, &req->cqe, sizeof(*cqe));
--
2.41.0
