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Subject: Re: The value of FB_MTU eats two pages
To:     Menglong Dong <menglong8.dong@gmail.com>
Cc:     ying.xue@windriver.com, David Miller <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        tipc-discussion@lists.sourceforge.net,
        LKML <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>
References: <CADxym3baupJJ7Q9otxtoQ-DH5e-J2isg-LZj2CsOqRPo70AL4A@mail.gmail.com>
 <e91baaba-e00a-4b16-0787-e9460dacfbb9@redhat.com>
 <CADxym3ZdyqJ7b_PqdcjbNhKWP7_nsPRQ9Q0TtFC6Qzr75ekK+g@mail.gmail.com>
From:   Jon Maloy <jmaloy@redhat.com>
Message-ID: <85310a8b-35ab-376d-ca87-7487b97232c8@redhat.com>
Date:   Thu, 3 Jun 2021 09:08:01 -0400
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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
>