Re: [PATCH] net/mlx5e: fix high stack usage

[Arnd Bergmann]

On 11/2/18, Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> On Fri, Nov 02, 2018 at 04:33:03PM +0100, Arnd Bergmann wrote:
>> A patch that looks harmless causes the stack usage of the
>> mlx5e_grp_sw_update_stats()
>> function to drastically increase with x86 gcc-4.9 and higher (tested up to
>> 8.1):
>>
>> drivers/net/ethernet/mellanox/mlx5/core/en_stats.c: In function
>> âmlx5e_grp_sw_update_statsâ:
>> drivers/net/ethernet/mellanox/mlx5/core/en_stats.c:216:1: warning: the
>> frame size of 1276 bytes is larger than 500 bytes [-Wframe-larger-than=]
>
> Why is the stack size so big here? The mlx5e_sw_stats is < 500 bytes
> and all the other on-stack stuff looks pretty small?

I am not entirely sure, but my analysis indicates that gcc tries loop unrolling
or some other optimization that leads to two copies on the stack.

>> By splitting out the loop body into a non-inlined function, the stack size
>> goes
>> back down to under 500 bytes.
>
> Does this actually reduce the stack consumed or does this just suppress
> the warning?

It definitely reduces the total stack usage, the separate functions just
had the expected stack usage that was a few hundred bytes combined.

       Arnd