linux-kernel - Re: [PATCH 4/4 v4] kernel/fork.c: memory hotplug updates max

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Date:	Mon, 23 Feb 2015 22:46:35 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Heinrich Schuchardt <xypron.glpk@....de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Aaron Tomlin <atomlin@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Davidlohr Bueso <dave@...olabs.net>,
	David Rientjes <rientjes@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Fabian Frederick <fabf@...net.be>,
	Guenter Roeck <linux@...ck-us.net>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Jens Axboe <axboe@...com>, Joe Perches <joe@...ches.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Kees Cook <keescook@...omium.org>,
	Michael Marineau <mike@...ineau.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vladimir Davydov <vdavydov@...allels.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4 v4] kernel/fork.c: memory hotplug updates
	max_threads

On 02/23, Heinrich Schuchardt wrote:
>
> On 23.02.2015 21:54, Oleg Nesterov wrote:
> >
> >> And it changes the swapper/0's rlimits. This is pointless after we fork
> >> /sbin/init.
>
> So should writing to /proc/sys/max_threads update the limits of all
> processes?

Why?

No, I think it should not touch rlimits at all.

> >> It seems to me these patches need some cleanups. Plus I am not sure the
> >> kernel should update max_threads automatically, we have the "threads-max"
> >> sysctl.
>
> The idea in the original version of fork_init is that max_threads should
> be chosen such that the memory needed to store the meta-information of
> max_threads threads should only be 1/8th of the total memory.
>
> Somebody adding or removing memory will not necessarily update
> /proc/sys/kernel/threads-max.
>
> This means that if I remove 90 % of the memory I get to a situation
> where max_threads allows so many threads to be created that the
> meta-information occupies all memory.
>
> With patch 4/4 max_threads is automatically reduced in this case.

I understand. But I think that if you remove 90 % of the memory you can
also update /proc/sys/kernel/threads-max.

And, suppose that admin specially limited max_threads, then you add more
memory. Should the kernel bump the limit silently?

And if hotplug should update max_threads, why it doesn't update, say,
files_stat.max_files?

IOW, I do not think that kernel should control max_threads after boot.
But I won't really argue. Just this looks a bit strange to me.

Oleg.

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