linux-kernel - Re: NUMA emulation x86_64: numa=fake parameter for custom nodes distance

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Message-ID: <20111119003114.GA10179@stainedmachine.redhat.com>
Date:	Sat, 19 Nov 2011 01:31:15 +0100
From:	Petr Holasek <pholasek@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, Anton Arapov <anton@...hat.com>
Subject: Re: NUMA emulation x86_64: numa=fake parameter for custom nodes
 distance

On Fri, 18 Nov 2011, Andrew Morton wrote:

> Date: Fri, 18 Nov 2011 11:53:36 -0800
> From: Andrew Morton <akpm@...ux-foundation.org>
> To: Petr Holasek <pholasek@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
>  "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
>  x86@...nel.org, Anton Arapov <anton@...hat.com>
> Subject: Re: [PATCH 1/2] NUMA emulation x86_64: numa=fake parameter for
>  custom nodes distance
> 
> On Fri, 18 Nov 2011 12:55:07 +0100
> Petr Holasek <pholasek@...hat.com> wrote:
> 
> > As default, when numa emulation is turned on, node distance table
> > uses physical distance, so for 4 nodes emulated on 1 physical table is
> > 
> > node   0   1   2   3
> > 0:  10  10  10  10
> > 1:  10  10  10  10
> > 2:  10  10  10  10
> > 3:  10  10  10  10
> > 
> > This patch adds new [distance] argument to
> > 
> > numa=fake=<number/size of nodes>[,distance]
> > 
> > When distance argument is used, it sets linear distance between nodes
> > like that:
> > 
> >     __distance__
> > ___|___     ____|___     ________     ________
> > |       |   |        |   |        |   |        |
> > | node1 |---| node 2 |---| node 3 |---| node 4 |
> > |_______|   |________|   |________|   |________|
> > |                        |             |
> > |                        |             |
> > |____distance * 2________|             |
> > |                                      |
> > |____________distance * 3______________|
> > 
> > This feature might be useful for testing some numa awareness features in
> > both user and kernel spaces.
> > 
> 
> "might" is a red flag.  We don't merge things which might be useful!
> 
> *Is* it useful?  If so then please tell us why and explain how it might
> be useful to others.

A lot of developers still have no access to large NUMA machines and 
possibility of NUMA emulation could involve more of them to thinking
about NUMA awareness of their apps/kernel code.

> 
> > @@ -404,6 +406,17 @@ void __init numa_emulation(struct numa_meminfo *numa_meminfo, int numa_dist_cnt)
> >  		if (emu_nid_to_phys[i] == NUMA_NO_NODE)
> >  			emu_nid_to_phys[i] = dfl_phys_nid;
> >  
> > +	/* load distance level parameter */
> > +	dist_level = -1;
> > +	c = strchr(emu_cmdline, ',');
> > +	if (c) {
> > +		c++;
> > +		ret = kstrtoul(c, 10, &dist_level);
> > +		if (ret < 0 || dist_level < LOCAL_DISTANCE ||
> > +				dist_level * max_emu_nid > ULONG_MAX)
> > +			dist_level = -1;
> 
> If this happens, the user goofed and we should tell them, with a printk.
> 
> 
> [patch 2/2] adds the documentation for the feature and should be
> included in the same patch as the implementation.

Apologize, I'll send v2 of patch with printk() and documentation all-in-one
if it is necessary.

thanks,
Petr H

> 
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