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

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Date:   Fri, 2 Nov 2018 17:23:26 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Saeed Mahameed <saeedm@...lanox.com>,
        Leon Romanovsky <leon@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Tariq Toukan <tariqt@...lanox.com>,
        Eran Ben Elisha <eranbe@...lanox.com>,
        Boris Pismenny <borisp@...lanox.com>,
        Ilya Lesokhin <ilyal@...lanox.com>,
        Moshe Shemesh <moshe@...lanox.com>,
        Kamal Heib <kamalh@...lanox.com>, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net/mlx5e: fix high stack usage

On 11/2/18, Jason Gunthorpe <jgg@...pe.ca> 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
>> ‘mlx5e_grp_sw_update_stats’:
>> 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