netdev - Re: [resend PATCH 2/2] dim: pass dim_sample to net

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Message-ID: <5ab60ad8-5625-41d3-b20a-4137a8f4c19a@broadcom.com>
Date: Thu, 31 Oct 2024 10:17:17 -0700
From: Florian Fainelli <florian.fainelli@...adcom.com>
To: Caleb Sander Mateos <csander@...estorage.com>,
 Andrew Lunn <andrew+netdev@...n.ch>,
 AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
 Arthur Kiyanovski <akiyano@...zon.com>, Brett Creeley
 <brett.creeley@....com>,
 Broadcom internal kernel review list
 <bcm-kernel-feedback-list@...adcom.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
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 <darinzon@...zon.com>, "David S. Miller" <davem@...emloft.net>,
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Cc: intel-wired-lan@...ts.osuosl.org, linux-arm-kernel@...ts.infradead.org,
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Subject: Re: [resend PATCH 2/2] dim: pass dim_sample to net_dim() by reference

On 10/30/24 17:23, 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 <csander@...estorage.com>

Reviewed-by: Florian Fainelli <florian.fainelli@...adcom.com> 
#bcm{sysport,genet}.c

Thanks!
-- 
Florian