On Wed, 2014-06-18 at 04:14 -0700, Luis R. Rodriguez wrote: > From: "Luis R. Rodriguez" <[email protected]> > > 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 <[email protected]> > Cc: Stephen Warren <[email protected]> > Cc: Michal Hocko <[email protected]> > Cc: Petr Mladek <[email protected]> > Cc: Andrew Morton <[email protected]> > Cc: Joe Perches <[email protected]> > Cc: Arun KS <[email protected]> > Cc: Kees Cook <[email protected]> > Cc: Davidlohr Bueso <[email protected]> > Cc: Chris Metcalf <[email protected]> > Cc: [email protected] > Signed-off-by: Luis R. Rodriguez <[email protected]>
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 <[email protected]> > --- > Documentation/kernel-parameters.txt | 8 ++++-- > init/Kconfig | 53 > ++++++++++++++++++++++++++++++++++++- > kernel/printk/printk.c | 32 ++++++++++++++++++++++ > 3 files changed, 90 insertions(+), 3 deletions(-) > > diff --git a/Documentation/kernel-parameters.txt > b/Documentation/kernel-parameters.txt > index 6eaa9cd..229d031 100644 > --- a/Documentation/kernel-parameters.txt > +++ b/Documentation/kernel-parameters.txt > @@ -1685,8 +1685,12 @@ bytes respectively. Such letter suffixes can also be > entirely omitted. > 7 (KERN_DEBUG) debug-level messages > > log_buf_len=n[KMG] Sets the size of the printk ring buffer, > - in bytes. n must be a power of two. The default > - size is set in the kernel config file. > + in bytes. n must be a power of two and greater > + than the minimal size. The minimal size is defined > + by LOG_BUF_SHIFT kernel config parameter. There is > + also CONFIG_LOG_CPU_MIN_BUF_SHIFT config parameter > + that allows to increase the default size depending on > + the number of CPUs. See init/Kconfig for more details. > > logo.nologo [FB] Disables display of the built-in Linux logo. > This may be used to provide more screen space for > diff --git a/init/Kconfig b/init/Kconfig > index 9d76b99..69bdbcf 100644 > --- a/init/Kconfig > +++ b/init/Kconfig > @@ -807,7 +807,11 @@ config LOG_BUF_SHIFT > range 12 21 > default 17 > help > - Select kernel log buffer size as a power of 2. > + Select the minimal kernel log buffer size as a power of 2. > + The final size is affected by LOG_CPU_MIN_BUF_SHIFT config > + parameter, see below. Any higher size also might be forced > + by "log_buf_len" boot parameter. > + > Examples: > 17 => 128 KB > 16 => 64 KB > @@ -816,6 +820,53 @@ config LOG_BUF_SHIFT > 13 => 8 KB > 12 => 4 KB > > +config LOG_CPU_MIN_BUF_SHIFT > + int "CPU kernel log buffer size contribution (13 => 8 KB, 17 => 128KB)" > + range 0 21 > + default 12 > + depends on SMP > + depends on !BASE_SMALL > + help > + The kernel ring buffer will get additional data logged onto it > + when multiple CPUs are supported. Typically the contributions are > + only a few lines when idle however under under load this can vary > + and in the worst case it can mean losing 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 scenario, 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 / 2 bytes)) in order to trigger an increase upon > + bootup. If an increase is required the ring buffer is increated to > + the next power of 2 that can fit both the minimum kernel ring buffer > + (LOG_BUF_SHIFT) plus the additional worst case CPU contributions. > + For example if LOG_BUF_SHIFT is 18 (256 KB) you'd require at laest > + 128 KB contributions by other CPUs in order to trigger an increase. > + 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 new minimum required kernel ring buffer size > + would be: > + > + ((1 << 18) + ((128 - 1) * (1 << 12))) / 1024 = 764 KB > + > + Since we only allow powers of two for the kernel ring buffer size the > + new kernel ring buffer size would be 1024 KB. > + > + CPU contributions are ignored when "log_buf_len" kernel parameter is > + used as it forces an exact (power of two) size of the ring buffer to > + an expected value. > + > + The number of possible CPUs is used for this computation ignoring > + hotplugging making the compuation optimal for the the worst case > + scenerio while allowing a simple algorithm to be used from bootup. > + > + Examples shift values and their meaning: > + 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 > + > # > # Architectures with an unreliable sched_clock() should select this: > # > 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 > @@ -266,6 +266,7 @@ static u32 clear_idx; > #define LOG_ALIGN __alignof__(struct printk_log) > #endif > #define __LOG_BUF_LEN (1 << CONFIG_LOG_BUF_SHIFT) > +#define __LOG_CPU_MIN_BUF_LEN (1 << CONFIG_LOG_CPU_MIN_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; > @@ -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 > + > + /* > + * 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? > + 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. Also, while at it, how about making it easier for users and also print the total size (num_possible_cpus() * cpu_extra)? > + pr_info("log_buf_len min size: %d\n", __LOG_BUF_LEN); > + > + log_buf_len_update(cpu_extra + __LOG_BUF_LEN); > +} > + > void __init setup_log_buf(int early) > { > unsigned long flags; > char *new_log_buf; > int free; > > + if (log_buf != __log_buf) > + return; > + > + if (!early && !new_log_buf_len) > + log_buf_add_cpu(); > + > if (!new_log_buf_len) > return; > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
