2021-06-30 05:18:10

by Yajun Deng

Subject: [PATCH] net: core: Modify alloc_size in alloc_netdev_mqs()

Use ALIGN for 'struct net_device', and remove the unneeded
'NETDEV_ALIGN - 1'. This can save a few bytes. and modify
the pr_err content when txqs < 1.

Signed-off-by: Yajun Deng <[email protected]>
---
net/core/dev.c | 10 ++++------
1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index c253c2aafe97..c42a682a624d 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10789,7 +10789,7 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
BUG_ON(strlen(name) >= sizeof(dev->name));

if (txqs < 1) {
- pr_err("alloc_netdev: Unable to allocate device with zero queues\n");
+ pr_err("alloc_netdev: Unable to allocate device with zero TX queues\n");
return NULL;
}

@@ -10798,14 +10798,12 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
return NULL;
}

- alloc_size = sizeof(struct net_device);
+ /* ensure 32-byte alignment of struct net_device*/
+ alloc_size = ALIGN(sizeof(struct net_device), NETDEV_ALIGN);
if (sizeof_priv) {
/* ensure 32-byte alignment of private area */
- alloc_size = ALIGN(alloc_size, NETDEV_ALIGN);
- alloc_size += sizeof_priv;
+ alloc_size += ALIGN(sizeof_priv, NETDEV_ALIGN);
}
- /* ensure 32-byte alignment of whole construct */
- alloc_size += NETDEV_ALIGN - 1;

p = kvzalloc(alloc_size, GFP_KERNEL | __GFP_RETRY_MAYFAIL);
if (!p)
--
2.32.0

2021-06-30 11:07:16

by Eric Dumazet

Subject: Re: [PATCH] net: core: Modify alloc_size in alloc_netdev_mqs()

On Wed, Jun 30, 2021 at 7:11 AM Yajun Deng <[email protected]> wrote:
>
> Use ALIGN for 'struct net_device', and remove the unneeded
> 'NETDEV_ALIGN - 1'. This can save a few bytes. and modify
> the pr_err content when txqs < 1.

I think that in old times (maybe still today), SLAB debugging could
lead to not unaligned allocated zones.
The forced alignment for netdev structures came in
commit f346af6a27c0cea99522213cb813fd30489136e2 ("net_device and
netdev private struct allocation improvements.")
in linux-2.6.3 (back in 2004)

This supposedly was a win in itself, otherwise Al Viro would not have
spent time on this.

>
> Signed-off-by: Yajun Deng <[email protected]>
> ---
> net/core/dev.c | 10 ++++------
> 1 file changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index c253c2aafe97..c42a682a624d 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -10789,7 +10789,7 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
> BUG_ON(strlen(name) >= sizeof(dev->name));
>
> if (txqs < 1) {
> - pr_err("alloc_netdev: Unable to allocate device with zero queues\n");
> + pr_err("alloc_netdev: Unable to allocate device with zero TX queues\n");
> return NULL;
> }
>
> @@ -10798,14 +10798,12 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
> return NULL;
> }
>
> - alloc_size = sizeof(struct net_device);
> + /* ensure 32-byte alignment of struct net_device*/
> + alloc_size = ALIGN(sizeof(struct net_device), NETDEV_ALIGN);

This is not really needed, because struct net_device is cache line
aligned already on SMP builds.

> if (sizeof_priv) {
> /* ensure 32-byte alignment of private area */
> - alloc_size = ALIGN(alloc_size, NETDEV_ALIGN);
> - alloc_size += sizeof_priv;
> + alloc_size += ALIGN(sizeof_priv, NETDEV_ALIGN);

No longer needed, the private area starts at the end of struct net_device, whose
size is a multiple of cache line.

Really I doubt this makes sense anymore these days, we have hundreds
of structures in the kernel
that would need a similar handling if SLAB/SLUB was doing silly things.

I would simply do :

alloc_size += sizeof_priv;


> }
> - /* ensure 32-byte alignment of whole construct */
> - alloc_size += NETDEV_ALIGN - 1;
>
> p = kvzalloc(alloc_size, GFP_KERNEL | __GFP_RETRY_MAYFAIL);
> if (!p)
> --
> 2.32.0
>