Re: The value of FB_MTU eats two pages

Jon Maloy
2021-06-03T13:08:08Z

On 6/2/21 10:26 PM, Menglong Dong wrote:
> Hello Maloy,
>
> On Thu, Jun 3, 2021 at 3:50 AM Jon Maloy <jmaloy@redhat.com> 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
>