Re: The value of FB_MTU eats two pages

[Menglong Dong]

Hello Maloy,

On Thu, Jun 3, 2021 at 3:50 AM Jon Maloy <jmaloy@xxxxxxxxxx> wrote:

[...]
> Hi Dong,
> The value is based on empiric knowledge.
> When I determined it I made a small loop in a kernel driver where I
> allocated skbs (using tipc_buf_acquire) with an increasing size
> (incremented with 1 each iteration), and then printed out the
> corresponding truesize.
>
> That gave the value we are using now.
>
> Now, when re-running the test I get a different value, so something has
> obviously changed since then.
>
> [ 1622.158586] skb(513) =>> truesize 2304, prev skb(512) => prev
> truesize 1280
> [ 1622.162074] skb(1537) =>> truesize 4352, prev skb(1536) => prev
> truesize 2304
> [ 1622.165984] skb(3585) =>> truesize 8448, prev skb(3584) => prev
> truesize 4352
>
> As you can see, the optimal value now, for an x86_64 machine compiled
> with gcc, is 3584 bytes, not 3744.

I'm not sure if this is a perfect way to determine the value of FB_MTU.
If 'struct skb_shared_info' changes, this value seems should change,
too.

How about we make it this:

#define FB_MTU (PAGE_SIZE - \
         SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) - \
         SKB_DATA_ALIGN(BUF_HEADROOM + BUF_TAILROOM + 3 + \
                 MAX_H_SIZ))

The value 'BUF_HEADROOM + BUF_TAILROOM + 3' come from 'tipc_buf_acquire()':

#ifdef CONFIG_TIPC_CRYPTO
    unsigned int buf_size = (BUF_HEADROOM + size + BUF_TAILROOM + 3) & ~3u;
#else
    unsigned int buf_size = (BUF_HEADROOM + size + 3) & ~3u;
#endif

Is it a good idea?

Thanks
Menglong Dong