netdev - Re: [PATCH net-next 2/2] tcp: schedule EPOLLOUT after a partial sendmsg

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Date:   Mon, 14 Sep 2020 16:58:40 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     soheil.kdev@...il.com
Cc:     netdev@...r.kernel.org, edumazet@...gle.com, soheil@...gle.com
Subject: Re: [PATCH net-next 2/2] tcp: schedule EPOLLOUT after a partial
 sendmsg

From: Soheil Hassas Yeganeh <soheil.kdev@...il.com>
Date: Mon, 14 Sep 2020 17:52:10 -0400

> From: Soheil Hassas Yeganeh <soheil@...gle.com>
> 
> For EPOLLET, applications must call sendmsg until they get EAGAIN.
> Otherwise, there is no guarantee that EPOLLOUT is sent if there was
> a failure upon memory allocation.
> 
> As a result on high-speed NICs, userspace observes multiple small
> sendmsgs after a partial sendmsg until EAGAIN, since TCP can send
> 1-2 TSOs in between two sendmsg syscalls:
> 
> // One large partial send due to memory allocation failure.
> sendmsg(20MB)   = 2MB
> // Many small sends until EAGAIN.
> sendmsg(18MB)   = 64KB
> sendmsg(17.9MB) = 128KB
> sendmsg(17.8MB) = 64KB
> ...
> sendmsg(...)    = EAGAIN
> // At this point, userspace can assume an EPOLLOUT.
> 
> To fix this, set the SOCK_NOSPACE on all partial sendmsg scenarios
> to guarantee that we send EPOLLOUT after partial sendmsg.
> 
> After this commit userspace can assume that it will receive an EPOLLOUT
> after the first partial sendmsg. This EPOLLOUT will benefit from
> sk_stream_write_space() logic delaying the EPOLLOUT until significant
> space is available in write queue.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Soheil Hassas Yeganeh <soheil@...gle.com>

Applied.