Re: [resend PATCH 2/2] dim: pass dim_sample to net_dim() by reference

On Wed, Oct 30, 2024 at 06:23:26PM -0600, Caleb Sander Mateos wrote:

net_dim() is currently passed a struct dim_sample argument by value.
struct dim_sample is 24 bytes. Since this is greater 16 bytes, x86-64
passes it on the stack. All callers have already initialized dim_sample
on the stack, so passing it by value requires pushing a duplicated copy
to the stack. Either witing to the stack and immediately reading it, or
perhaps dereferencing addresses relative to the stack pointer in a chain
of push instructions, seems to perform quite poorly.

In a heavy TCP workload, mlx5e_handle_rx_dim() consumes 3% of CPU time,
94% of which is attributed to the first push instruction to copy
dim_sample on the stack for the call to net_dim():
// Call ktime_get()
  0.26 |4ead2:   call   4ead7 <mlx5e_handle_rx_dim+0x47>
// Pass the address of struct dim in %rdi
       |4ead7:   lea    0x3d0(%rbx),%rdi
// Set dim_sample.pkt_ctr
       |4eade:   mov    %r13d,0x8(%rsp)
// Set dim_sample.byte_ctr
       |4eae3:   mov    %r12d,0xc(%rsp)
// Set dim_sample.event_ctr
  0.15 |4eae8:   mov    %bp,0x10(%rsp)
// Duplicate dim_sample on the stack
 94.16 |4eaed:   push   0x10(%rsp)
  2.79 |4eaf1:   push   0x10(%rsp)
  0.07 |4eaf5:   push   %rax
// Call net_dim()
  0.21 |4eaf6:   call   4eafb <mlx5e_handle_rx_dim+0x6b>

To allow the caller to reuse the struct dim_sample already on the stack,
pass the struct dim_sample by reference to net_dim().

Signed-off-by: Caleb Sander Mateos <redacted>
---

Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>