linux-kernel - Re: [RFT 2/2] printk: allow increasing the ring buffer depending on the number of CPUs

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Date:	Wed, 18 Jun 2014 21:21:34 +0200
From:	"Luis R. Rodriguez" <mcgrof@...e.com>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>, hpa@...ux.intel.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Andrew Lunn <andrew@...n.ch>,
	Stephen Warren <swarren@...dotorg.org>,
	Michal Hocko <mhocko@...e.cz>, Petr Mladek <pmladek@...e.cz>,
	Joe Perches <joe@...ches.com>,
	Arun KS <arunks.linux@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Chris Metcalf <cmetcalf@...era.com>
Subject: Re: [RFT 2/2] printk: allow increasing the ring buffer depending
	on the number of CPUs

On Wed, Jun 18, 2014 at 11:11:12AM -0700, Davidlohr Bueso wrote:
> On Wed, 2014-06-18 at 04:14 -0700, Luis R. Rodriguez wrote:
> > From: "Luis R. Rodriguez" <mcgrof@...e.com>
> > Signed-off-by: Luis R. Rodriguez <mcgrof@...e.com>
> 
> Looks good Luis, thanks a lot for doing this -- it will definitely help
> my everyday debugging issues on huge machines.
> 
> I ran this on my 160-core Westmere. Some nits below, otherwise:
> 
> Reviewed-and-tested-by: Davidlohr Bueso <davidlohr@...com>

Great thanks for testing and your review!

> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index af164a7..7c7b599 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -848,12 +849,43 @@ static int __init log_buf_len_setup(char *str)
> >  }
> >  early_param("log_buf_len", log_buf_len_setup);
> >  
> > +static void __init log_buf_add_cpu(void)
> > +{
> > +	int cpu_extra;
> 
> unsigned int

Amended.

> > +
> > +	/*
> > +	 * archs should set up cpu_possible_bits properly with
> > +	 * set_cpu_possible() after setup_arch() but just in
> > +	 * case lets ensure this is valid. During an early
> > +	 * call before setup_arch()() this will be 1.
> > +	 */
> > +	if (num_possible_cpus() <= 1)
> 
> This can never return 0, so how about making it == 1?

I was originally concerned over the early boot code which had
not yet called setup_arch() but since we now have a check for
early on setup_log_buf() before calling log_buf_add_cpu() I
think its safe to check for 1 then, will change! I'll also
remove the note about this always returning 1 on early init
before setup_arch() as I only confirmed that for x86 -- unless
of course there is code that ensures this for early boot for
all archs, I just can't find it.

> > +		return;
> > +
> > +	cpu_extra = (num_possible_cpus() - 1) * __LOG_CPU_MIN_BUF_LEN;
> > +
> > +	/* by default this will only continue through for large > 64 CPUs */
> > +	if (cpu_extra <= __LOG_BUF_LEN / 2)
> > +		return;
> > +
> > +	pr_info("log_buf_len cpu_extra contribution: %d\n", cpu_extra);
> 
> We should add 'bytes' for units. 

That should mean also amending the orignal setup_log_buf() for the final size,
will do that too.

> Also, while at it, how about making it easier for users and also print
> the individual contribution of each CPU

Sure, done. While at it I renamed LOG_CPU_MIN_BUF_SHIFT to MAX to annotate folks
want to to consider the worst case scenario to help with debugging on production.

  Luis
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