linux-kernel - Re: [PATCH 3/3] kernel/sysctl.c: threads-max observe limits

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Date:	Tue, 24 Feb 2015 14:20:04 -0800 (PST)
From:	David Rientjes <rientjes@...gle.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 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>,
	Oleg Nesterov <oleg@...hat.com>,
	"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 3/3] kernel/sysctl.c: threads-max observe limits

On Tue, 24 Feb 2015, Heinrich Schuchardt wrote:

> >> Users can change the maximum number of threads by writing to
> >> /proc/sys/kernel/threads-max.
> >>
> >> With the patch the value entered is checked against the same
> >> limits that apply when fork_init is called.
> >>
> > 
> > Correct me if I'm wrong, but this is a change in functionality (without 
> > update to Documentation/sysctl/kernel.txt which describes threads-max) 
> > since it does not allow the value to be lowered as before from the 
> > calculation involving totalram_pages.  The value passed to 
> > set_max_threads() only caps the value.
> 
> threads_max is set to the value the user inputs if it is inside the
> interval [20, FUTEX_TID_MASK] and it is capped by the value calculated
> from totalram_pages.
> 
> So lowering and raising is still possible (inside the limits).
> 

I believe this information should be added to the documentation cited 
above which mentions threads-max since users will otherwise be unfamiliar 
with the limits imposed.
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