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

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Message-ID: <54EE18DF.2040201@gmx.de>
Date:	Wed, 25 Feb 2015 19:47:59 +0100
From:	Heinrich Schuchardt <xypron.glpk@....de>
To:	David Rientjes <rientjes@...gle.com>
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, Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH 3/3] kernel/sysctl.c: threads-max observe limits

On 24.02.2015 23:20, David Rientjes wrote:
> 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.

Hello David,

I guess the documentation fix should be put into a separate patch (of
the same patch series) as Jonathan Corbet uses a separate git tree to
consolidate documentation changes. Is that ok with you?

Best regards

Heinrich
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