netdev - Re: [PATCH net-next] tcp: forbid direct reclaim if MSG

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Date:   Tue, 9 Oct 2018 18:44:05 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Yafang Shao <laoar.shao@...il.com>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] tcp: forbid direct reclaim if MSG_DONTWAIT is
 set in send path



On 10/09/2018 06:30 PM, Yafang Shao wrote:
> On Tue, Oct 9, 2018 at 11:38 PM Eric Dumazet <edumazet@...gle.com> wrote:
>>
>> On Tue, Oct 9, 2018 at 7:58 AM Eric Dumazet <edumazet@...gle.com> wrote:
>>>
>>
>>> We do not add bloat in the kernel if no application is ever going to
>>> use it, especially in the TCP fast path.
>>>
>>
>> BTW, are you willing to change all memory allocations in the kernel as well ?
>>
>> Let say an application is using a system call providing a pathname
>> (open(), stat(), ...), how this system call
>> is going to ask the kernel for no direct reclaim ?
>>
>> Even allocating a socket with socket() or accept() has no ability to
>> avoid direct reclaim.
>>
>> So tcp_sendmsg() is only the tip of the iceberg.
> 
> If we can really find a solution that is good enough to hanlde direct
> reclaim in tcp_sendmsg,
> we could also implement it in other syscalls.
> Unexpected latency is hateful.

We have thousands of other places in the kernel, I want to find a generic solution,
not patch all the places one by one.

So come back when you have something more generic, and once applications have a way
to handle gracefully (without calling sendmsg() in infinite loop ...)
to these memory allocation issues.

How is EPOLLOUT going to be generated ?