Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933273AbaFLNFt (ORCPT ); Thu, 12 Jun 2014 09:05:49 -0400 Received: from cantor2.suse.de ([195.135.220.15]:53281 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754234AbaFLNFo (ORCPT ); Thu, 12 Jun 2014 09:05:44 -0400 Date: Thu, 12 Jun 2014 15:05:32 +0200 From: Petr =?iso-8859-1?Q?Ml=E1dek?= To: "Luis R. Rodriguez" Cc: "Luis R. Rodriguez" , linux-kernel@vger.kernel.org, Michal Hocko , Andrew Morton , Joe Perches , Arun KS , Kees Cook , Mel Gorman Subject: Re: [RFC] printk: allow increasing the ring buffer depending on the number of CPUs Message-ID: <20140612130532.GN7772@pathway.suse.cz> References: <1402448685-30634-1-git-send-email-mcgrof@do-not-panic.com> <20140611093447.GL7772@pathway.suse.cz> <20140611214741.GH6042@wotan.suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20140611214741.GH6042@wotan.suse.de> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 2014-06-11 23:47:41, Luis R. Rodriguez wrote: > On Wed, Jun 11, 2014 at 11:34:47AM +0200, Petr Ml?dek wrote: > > On Tue 2014-06-10 18:04:45, Luis R. Rodriguez wrote: > > > From: "Luis R. Rodriguez" > > > 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. > > Not sure if I follow what you mean by CPU_NUM-specific, can you > elaborate? I wanted to say that the space requested by LOG_CPU_BUF_SHIFT depends on the number of CPUs. If LOG_CPU_BUF_SHIFT is not zero, your patch always allocates new ringbuffer and leave the static "__log_buf" unused. I think that this is not necessary for machines with small amount of CPUs and probably also with small amount of memory. I would rename the variable to LOG_CPU_BUF_MIN_SHIFT or so. It would represent minimal size that is needed to print CPU-specific messages. If they take only "small" part of the default ring buffer size, we could still use the default rind buffer. For example, if we left 50% of the default buffer for CPU-specific messages, the code might look like: #define __LOG_CPU_MIN_BUF_LEN (1 << CONFIG_LOG_CPU_MIN_BUF_SHIFT) int cpu_extra = (num_possible_cpus() - 1) * __LOG_CPU_MIN_BUF_LEN; if (!new_log_buf_len && (cpu_extra > __LOG_BUF_LEN / 2)) new_log_buf_len = __LOG_BUF_LEN + cpu_extra; if (!new_log_buf_len) return; allocate the new ring buffer... > The default in this patch is to ignore this, do you mean that upstream > should probably default to a non-zero value here and then let distributions > select 0 for some kernel builds ? If the change has effect only for huge systems, the default value might be non-zero everywhere. > If so then perhaps adding a sysctl override value might be good to > allow only small systems to override this to 0? I think that it won't help to lover the value using sysctl because the huge buffer would be already allocated during boot. If I did not miss anything. [...] > > > > 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 (!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; > > The check for cpu_extra > 1 does that -- the default in the patch was 0 > and 1 << 0 is 1, so if in the case that the default is used we'd bail > just like before. Or did I perhaps miss what you were saying here? The problem is that we do not bail out because you removed the "return". If "new_log_buf_len=0" and "cpu_extra=1" then we keep "new_log_buf_len" as is. Then we continue, try to allocate zero memory and print error: "log_buf_len: 0 bytes not available". Do I get it right? > > Also I would feel more comfortable if we somehow limit the maximum > > size of cpu_extra. > > Michal had similar concerns and I thought up to limit it to 1024 max > CPUs, but after my second implementation I did some math on the values > that would be used if say LOG_CPU_BUF_SHIFT was 12, it turns out to not > be *that* bad for even huge num_possible_cpus(). For example for 4096 > num_possible_cpus() this comes out to with LOG_BUF_SHIFT of 18: > > > ((1 << 18) + ((4096 - 1) * (1 << 12))) / 1024 = 16636 KB > > ~16 MB doesn't seem that bad for such a monster box which I'd presume > would have an insane amount of memory. If this logic however does > seems unreasonable and we should cap it -- then by all means lets > pick a sensible number, its just not clear to me what that number > should be. Another reason why I stayed away from capping this was > that we'd then likely end up capping this in the future, and I was > trying to find a solution that would not require mucking as > technology evolves. The reasoning above is also why I had opted to > make the default to 0, only distributions would have a good sense > of what might be reasonable, which I guess begs more for a sysctl > value here. I am not sure but I think that the huge buffer would be allocated before any sysctl value could be modified. So, I think that sysctl would not really help here. I think that the 10% or 20% of the total memory size is a good limit. Nobody would want to use more than 20% of memory for logging. So, it needs not be higher. The main purpose of the limit is that the system does not die immediately after allocating the ring buffer. The 80% reserve for the rest of the system sounds fine as well. Note that the limit won't be needed on 99,9% of systems but it would help with debugging the last 0.1% :-) > > 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. > > When I tested disabling smp I saw the log was still amended to include > information about the disabled CPUs, I however hadn't tested on a machine > with hot pluggable CPUs and with tons of CPUs disabled, so not sure if > that adds more info as well. This also though points more to this being > more a system specific thing, which is another reason to perhaps keep this > disabled and leave this instead as a system config? > > > The question is how to do it. I am still not much familiar with the > > memory subsystem. I wonder if 10% of memory defined by the > > "total_rampages" variable would be a reasonable limit. > > Not sure either, curious if Mel might have a suggestion? > > > > > > if (early) { > > > new_log_buf = > > > -- > > > 2.0.0.rc3.18.g00a5b79 > > > > > > > > LocalWords: buf len cpu boottime > > What's this? :) Heh, emacs added this when doing spell check. It is strage, it does it only from time to time :-) Best Regards, Petr -- 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/