Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751834AbaFLSBN (ORCPT ); Thu, 12 Jun 2014 14:01:13 -0400 Received: from g4t3427.houston.hp.com ([15.201.208.55]:42476 "EHLO g4t3427.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751592AbaFLSBL (ORCPT ); Thu, 12 Jun 2014 14:01:11 -0400 Message-ID: <1402596066.2627.1.camel@buesod1.americas.hpqcorp.net> Subject: Re: [RFC] printk: allow increasing the ring buffer depending on the number of CPUs From: Davidlohr Bueso To: Petr =?ISO-8859-1?Q?Ml=E1dek?= Cc: "Luis R. Rodriguez" , linux-kernel@vger.kernel.org, "Luis R. Rodriguez" , Michal Hocko , Andrew Morton , Joe Perches , Arun KS , Kees Cook Date: Thu, 12 Jun 2014 11:01:06 -0700 In-Reply-To: <20140611093447.GL7772@pathway.suse.cz> References: <1402448685-30634-1-git-send-email-mcgrof@do-not-panic.com> <20140611093447.GL7772@pathway.suse.cz> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.6.4 (3.6.4-3.fc18) Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2014-06-11 at 11:34 +0200, Petr Mládek wrote: > On Tue 2014-06-10 18:04:45, Luis R. Rodriguez wrote: > > From: "Luis R. Rodriguez" > > > > 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. > > Thanks for looking at this. It is a pity to lose stracktrace when a huge > machine Oopses just because the default ring buffer is too small. Agreed, I would very much welcome something like this. > > 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 scenerio. > > > > 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 option is diabled by default, and if used > > the kernel ring buffer size then can be computed as follows: > > > > size = __LOG_BUF_LEN + (num_possible_cpus() - 1 ) * __LOG_CPU_BUF_LEN > > > > Cc: Michal Hocko > > Cc: Petr Mladek > > Cc: Andrew Morton > > Cc: Joe Perches > > Cc: Arun KS > > Cc: Kees Cook > > Cc: linux-kernel@vger.kernel.org > > Signed-off-by: Luis R. Rodriguez > > --- > > init/Kconfig | 28 ++++++++++++++++++++++++++++ > > kernel/printk/printk.c | 6 ++++-- > > 2 files changed, 32 insertions(+), 2 deletions(-) > > > > diff --git a/init/Kconfig b/init/Kconfig > > index 9d3585b..1814436 100644 > > --- a/init/Kconfig > > +++ b/init/Kconfig > > @@ -806,6 +806,34 @@ config LOG_BUF_SHIFT > > 13 => 8 KB > > 12 => 4 KB > > > > +config LOG_CPU_BUF_SHIFT > > + int "CPU kernel log buffer size contribution (13 => 8 KB, 17 => 128KB)" > > + range 0 21 > > + default 0 > > + help > > + The kernel ring buffer will get additional data logged onto it > > + when multiple CPUs are supported. Typically the contributions is a > > + few lines when idle however under under load this can vary and in the > > + worst case it can mean loosing logging information. You can use this > > + to set the maximum expected mount of amount of logging contribution > > + under load by each CPU in the worst case scenerio. Select a size as > > + a power of 2. For example if LOG_BUF_SHIFT is 18 and if your > > + LOG_CPU_BUF_SHIFT is 12 your kernel ring buffer size will be as > > + follows having 16 CPUs as possible. > > + > > + ((1 << 18) + ((16 - 1) * (1 << 12))) / 1024 = 316 KB > > It might be better to use the CPU_NUM-specific value as a minimum of > the needed space. Linux distributions might want to distribute kernel > with non-zero value and still use the static "__log_buf" on reasonable > small systems. It should also depend on SMP and !BASE_SMALL. I was wondering about disabling this by default as it would defeat the purpose of being a proactive feature. Similarly, I worry about distros choosing a correct default value on their own. > > + Where as typically you'd only end up with 256 KB. This is disabled > > + by default with a value of 0. > > I would add: > > This value is ignored when "log_buf_len" commandline parameter > is used. It forces the exact size of the ring buffer. ... and update Documentation/kernel-parameters.txt to be more descriptive about this new functionality. > > + Examples: > > + 17 => 128 KB > > + 16 => 64 KB > > + 15 => 32 KB > > + 14 => 16 KB > > + 13 => 8 KB > > + 12 => 4 KB > > I think that we should make it more cleat that it is per-CPU here, > for example: > > 17 => 128 KB for each CPU > 16 => 64 KB for each CPU > 15 => 32 KB for each CPU > 14 => 16 KB for each CPU > 13 => 8 KB for each CPU > 12 => 4 KB for each CPU > Agreed. > > # > > # Architectures with an unreliable sched_clock() should select this: > > # > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > > index 7228258..2023424 100644 > > --- a/kernel/printk/printk.c > > +++ b/kernel/printk/printk.c > > @@ -246,6 +246,7 @@ static u32 clear_idx; > > #define LOG_ALIGN __alignof__(struct printk_log) > > #endif > > #define __LOG_BUF_LEN (1 << CONFIG_LOG_BUF_SHIFT) > > +#define __LOG_CPU_BUF_LEN (1 << CONFIG_LOG_CPU_BUF_SHIFT) > > static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN); > > static char *log_buf = __log_buf; > > static u32 log_buf_len = __LOG_BUF_LEN; > > @@ -752,9 +753,10 @@ void __init setup_log_buf(int early) > > unsigned long flags; > > char *new_log_buf; > > int free; > > + int cpu_extra = (num_possible_cpus() - 1) * __LOG_CPU_BUF_LEN; If depending on SMP, you can remove the - 1 here. > > - if (!new_log_buf_len) > > - return; > > + if (!new_log_buf_len && cpu_extra > 1) > > + new_log_buf_len = __LOG_BUF_LEN + cpu_extra; > > We still should return when both new_log_buf_len and cpu_extra are > zero and call here: > > if (!new_log_buf_len) > return; Yep. > Also I would feel more comfortable if we somehow limit the maximum > size of cpu_extra. I wonder if there might be a crazy setup with a lot > of possible CPUs and possible memory but with some minimal amount of > CPUs and memory at the boot time. Maybe. But considering that systems with a lot of CPUs *do* have a lot of memory, I wouldn't worry much about this, just like we don't worry about it now. Considering a _large_ 1024 core system and using the max value 21 for CONFIG_LOG_BUF_SHIFT, we would only allocate just over 2Gb of extra space -- trivial for such a system. And if it does break something, then heck, go fix you box and/or just reduce the percpu value. I guess that's a good reason to keep the default to 0 and let users play with it as they wish without compromising uninterested parties. afaict only x86 would be exposed to systems not booting if we fail to allocate. Thanks, Davidlohr -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/