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

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Date:   Fri, 2 Nov 2018 11:00:20 -0600
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Arnd Bergmann <arnd@...db.de>
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 Fri, Nov 02, 2018 at 05:23:26PM +0100, Arnd Bergmann wrote:
> 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.

Wow how strange

Did you consier adding a
  __attribute__((optimize("no-unroll-loops"))) 

type macro?

Jason