linux-kernel - Re: [RFC v2] prctl: prctl(PR_SET_IDLE, PR_IDLE_MODE

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Date:   Fri, 3 Nov 2017 10:09:15 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Shawn Landden <slandden@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-api@...r.kernel.org
Subject: Re: [RFC v2] prctl: prctl(PR_SET_IDLE, PR_IDLE_MODE_KILLME), for
 stateless idle loops

On Thu 02-11-17 23:35:44, Shawn Landden wrote:
> It is common for services to be stateless around their main event loop.
> If a process sets PR_SET_IDLE to PR_IDLE_MODE_KILLME then it
> signals to the kernel that epoll_wait() and friends may not complete,
> and the kernel may send SIGKILL if resources get tight.
> 
> See my systemd patch: https://github.com/shawnl/systemd/tree/prctl
> 
> Android uses this memory model for all programs, and having it in the
> kernel will enable integration with the page cache (not in this
> series).
> 
> 16 bytes per process is kinda spendy, but I want to keep
> lru behavior, which mem_score_adj does not allow. When a supervisor,
> like Android's user input is keeping track this can be done in user-space.
> It could be pulled out of task_struct if an cross-indexing additional
> red-black tree is added to support pid-based lookup.

This is still an abuse and the patch is wrong. We really do have an API
to use I fail to see why you do not use it.

[...]
> @@ -1018,6 +1060,24 @@ bool out_of_memory(struct oom_control *oc)
>  			return true;
>  	}
>  
> +	/*
> +	 * Check death row for current memcg or global.
> +	 */
> +	l = oom_target_get_queue(current);
> +	if (!list_empty(l)) {
> +		struct task_struct *ts = list_first_entry(l,
> +				struct task_struct, se.oom_target_queue);
> +
> +		pr_debug("Killing pid %u from EPOLL_KILLME death row.",
> +			 ts->pid);
> +
> +		/* We use SIGKILL instead of the oom killer
> +		 * so as to cleanly interrupt ep_poll()
> +		 */
> +		send_sig(SIGKILL, ts, 1);
> +		return true;
> +	}

Still not NUMA aware and completely backwards. If this is a memcg OOM
then it is _memcg_ to evaluate not the current. The oom might happen up
the hierarchy due to hard limit.

But still, you should be very clear _why_ the existing oom tuning is not
appropropriate and we can think of a way to hanle it better but cramming
the oom selection this way is simply not acceptable.
-- 
Michal Hocko
SUSE Labs