On Thu, Jun 12, 2014 at 03:05:32PM +0200, Petr Mládek wrote: > On Wed 2014-06-11 23:47:41, Luis R. Rodriguez wrote: > > On Wed, Jun 11, 2014 at 11:34:47AM +0200, Petr Mládek wrote: > > > On Tue 2014-06-10 18:04:45, Luis R. Rodriguez wrote: > > > > From: "Luis R. Rodriguez" <mcg...@suse.com> > > > > 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.
True, which is why I disabled it by default if we want to only leave this disabled for < certain amount of num CPU systems, what is that number, I see below a recommendation and I do like it. > 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. True, and will rename this, that still leaves open the question of a number of CPUs that is sensible to keep but you resolve that below. > 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... Yeah I like these heuristics a lot, will fold them in and send a v2 now in patch form. To be clear with this CONFIG_LOG_CPU_MIN_BUF_SHIFT could actually be left now to something other than non zeo by default and only if that contribution is seen to go above 1/2 of __LOG_BUF_LEN will we allocate more for the ring buffer. With default values of LOG_BUF_SHIFT at 18 and say a default value of 12 for LOG_CPU_MIN_BUF_SHIFT this would mean we'd need if we remove the -1 we'd require 64 CPUs in order to trigger an allocation for more memory. If we keep the -1 we'd require anything over 64 number of CPUs. Do we want to keep the -1 and the > 64 CPU requirement as default? Is the LOG_CPU_MIN_BUF_SHIFT default of 12 resonable to start with (assumes 4KB in the worst case before the kernel ring buffer flips over). > > 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. Sure. > > 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. > > [...] Yeah true, a sensible default would be best, with the systctl we'd also have to handle dynamic re-allocations and while the tracing code already added code to make this easier I'd prefer we don't make this a popular path. > > > > > > 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? Yeah sorry, I meant to add the else.. and bail with a return if the default was of 0 was not used or if the kernel parameter to increase the size was not passed. > > > 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. Sure. > 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% :-) Oh, what we do for the the 0.1%. Luis
pgpcHPKkha4mp.pgp
Description: PGP signature
