linux-kernel - Re: [RFC] EPOLL_KILLME: New flag to epoll_wait() that subscribes process to death row (new syscall)

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Message-Id: <1509549397.2561228.1158168688.4CFA4326@webmail.messagingengine.com>
Date:   Wed, 01 Nov 2017 11:16:37 -0400
From:   Colin Walters <walters@...bum.org>
To:     Shawn Landden <slandden@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [RFC] EPOLL_KILLME: New flag to epoll_wait() that subscribes process
 to death row (new syscall)

On Wed, Nov 1, 2017, at 01:32 AM, Shawn Landden wrote:
> It is common for services to be stateless around their main event loop.
> If a process passes the EPOLL_KILLME flag to epoll_wait5() then it
> signals to the kernel that epoll_wait5() may not complete, and the kernel
> may send SIGKILL if resources get tight.
> 

I've thought about something like this in the past too and would love
to see it land.  Bigger picture, this also comes up in (server) container
environments, see e.g.:

https://docs.openshift.com/container-platform/3.3/admin_guide/idling_applications.html

There's going to be a long slog getting apps to actually make use
of this, but I suspect if it gets wrapped up nicely in some "framework"
libraries for C/C++, and be bound in the language ecosystems like golang
we could see a fair amount of adoption on the order of a year or two.

However, while I understand why it feels natural to tie this to epoll,
as the maintainer of glib2 which is used by a *lot* of things; I'm not
sure we're going to port to epoll anytime soon.

Why not just make this a prctl()?  It's not like it's really any less racy to do:

prctl(PR_SET_IDLE)
epoll()

and this also allows:

prctl(PR_SET_IDLE)
poll()

And as this is most often just going to be an optional hint it's easier to e.g. just ignore EINVAL
from the prctl().