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 22:43:35 +0200
From:	"Luis R. Rodriguez" <mcgrof@...e.com>
To:	Petr Mládek <pmladek@...e.cz>
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>, Joe Perches <joe@...ches.com>,
	Arun KS <arunks.linux@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Davidlohr Bueso <davidlohr@...com>,
	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 05:42:55PM +0200, Petr Mládek wrote:
> On Wed 2014-06-18 04:14:25, Luis R. Rodriguez wrote:
> > From: "Luis R. Rodriguez" <mcgrof@...e.com>
> > 
> > The default size of the ring buffer is too small for machines
> > with a large amount of CPUs under heavy load. What ends up
> > happening when debugging is the ring buffer overlaps and chews
> > up old messages making debugging impossible unless the size is
> > passed as a kernel parameter. An idle system upon boot up will
> > on average spew out only about one or two extra lines but where
> > this really matters is on heavy load and that will vary widely
> > depending on the system and environment.
> > 
> > There are mechanisms to help increase the kernel ring buffer
> > for tracing through debugfs, and those interfaces even allow growing
> > the kernel ring buffer per CPU. We also have a static value which
> > can be passed upon boot. Relying on debugfs however is not ideal
> > for production, and relying on the value passed upon bootup is
> > can only used *after* an issue has creeped up. Instead of being
> > reactive this adds a proactive measure which lets you scale the
> > amount of contributions you'd expect to the kernel ring buffer
> > under load by each CPU in the worst case scenario.
> > 
> > We use num_possible_cpus() to avoid complexities which could be
> > introduced by dynamically changing the ring buffer size at run
> > time, num_possible_cpus() lets us use the upper limit on possible
> > number of CPUs therefore avoiding having to deal with hotplugging
> > CPUs on and off. This introduces the kernel configuration option
> > LOG_CPU_MIN_BUF_SHIFT which is used to specify the maximum amount
> > of contributions to the kernel ring buffer in the worst case before
> > the kernel ring buffer flips over, the size is specified as a power
> > of 2. The total amount of contributions made by each CPU must be
> > greater than half of the default kernel ring buffer size
> > (1 << LOG_BUF_SHIFT bytes) in order to trigger an increase upon
> > bootup. The kernel ring buffer is increased to the next power of
> > two that would fit the required minimum kernel ring buffer size
> > plus the additional CPU contribution. For example if LOG_BUF_SHIFT
> > is 18 (256 KB) you'd require at least 128 KB contributions by
> > other CPUs in order to trigger an increase of the kernel ring buffer.
> > With a LOG_CPU_BUF_SHIFT of 12 (4 KB) you'd require at least
> > anything over > 64 possible CPUs to trigger an increase. If you
> > had 128 possible CPUs the amount of minimum required kernel ring
> > buffer bumps to:
> > 
> >    ((1 << 18) + ((128 - 1) * (1 << 12))) / 1024 = 764 KB
> > 
> > Since we require the ring buffer to be a power of two the new
> > required size would be 1024 KB.
> > 
> > This CPU contributions are ignored when the "log_buf_len" kernel parameter
> > is used as it forces the exact size of the ring buffer to an expected power
> > of two value.
> > 
> > Cc: Andrew Lunn <andrew@...n.ch>
> > Cc: Stephen Warren <swarren@...dotorg.org>
> > Cc: Michal Hocko <mhocko@...e.cz>
> > Cc: Petr Mladek <pmladek@...e.cz>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: Joe Perches <joe@...ches.com>
> > Cc: Arun KS <arunks.linux@...il.com>
> > Cc: Kees Cook <keescook@...omium.org>
> > Cc: Davidlohr Bueso <davidlohr@...com>
> > Cc: Chris Metcalf <cmetcalf@...era.com>
> > Cc: linux-kernel@...r.kernel.org
> > Signed-off-by: Luis R. Rodriguez <mcgrof@...e.com>
> 
> It is where we ended after several versions. I am happy with this
> state.
> 
> Signed-off-by: Petr Mladek <pmladek@...e.cz>
> Tested-by: Petr Mladek <pmladek@...e.cz>

Thanks, I did some minor spell fixes, and also renamed min to max
for the kconfig variable. Will send as final patches now.

  Luis
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