Re: The value of FB_MTU eats two pages

[Jon Maloy]

On 6/2/21 10:26 PM, Menglong Dong wrote:
> 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?
Yes, I think that makes sense. I was always aware of the "fragility" of 
my approach, -this one looks more future safe.

///jon

> Thanks
> Menglong Dong