netdev - Re: [PATCH net-next] net: introduce a new tracepoint for tcp_rcv_space

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Date:   Wed, 18 Apr 2018 12:42:02 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        David Miller <davem@...emloft.net>,
        Song Liu <songliubraving@...com>, netdev@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] net: introduce a new tracepoint for tcp_rcv_space_adjust

On Wed, Apr 18, 2018 at 7:44 AM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> On Mon, Apr 16, 2018 at 08:43:31AM -0700, Eric Dumazet wrote:
>>
>>
>> On 04/16/2018 08:33 AM, Yafang Shao wrote:
>> > tcp_rcv_space_adjust is called every time data is copied to user space,
>> > introducing a tcp tracepoint for which could show us when the packet is
>> > copied to user.
>> > This could help us figure out whether there's latency in user process.
>> >
>> > When a tcp packet arrives, tcp_rcv_established() will be called and with
>> > the existed tracepoint tcp_probe we could get the time when this packet
>> > arrives.
>> > Then this packet will be copied to user, and tcp_rcv_space_adjust will
>> > be called and with this new introduced tracepoint we could get the time
>> > when this packet is copied to user.
>> >
>> >     arrives time : user process time    => latency caused by user
>> >     tcp_probe      tcp_rcv_space_adjust
>> >
>> > Hence in the prink message, sk is printed as a key to connect these two
>> > tracepoints.
>> >
>>
>> socket pointer is not a key.
>>
>> TCP sockets can be reused pretty fast after free.
>>
>> I suggest you go for cookie instead, this is an unique 64bit identifier.
>> ( sock_gen_cookie() for details )
>
> I think would be even better if the stack would do this sock_gen_cookie()
> on its own in some way that user cannnot infere the order.
> In many cases we wanted to use socket cookie, but since it's not inited
> by default it's kinda useless.
> Turning this tracepoint on just to get cookie would be an ugly workaround.
>

Could we init it in sk_alloc() ?
Then in other code paths, for example sock_getsockopt or tracepoints,
we only read the value through a new inline function named
sock_read_cookie().


Thanks
Yafang