Re: [PATCH net-next] tcp: forbid direct reclaim if MSG_DONTWAIT is set in send path

From: Eric Dumazet
Date: Tue Oct 09 2018 - 21:44:12 EST

On 10/09/2018 06:30 PM, Yafang Shao wrote:
> On Tue, Oct 9, 2018 at 11:38 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>> On Tue, Oct 9, 2018 at 7:58 AM Eric Dumazet <edumazet@xxxxxxxxxx> 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 ?