|author||Luis R. Rodriguez <firstname.lastname@example.org>||2014-08-06 16:08:56 -0700|
|committer||Linus Torvalds <email@example.com>||2014-08-06 18:01:23 -0700|
printk: allow increasing the ring buffer depending on the number of CPUs
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_MAX_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. [firstname.lastname@example.org: fix build] Signed-off-by: Luis R. Rodriguez <email@example.com> Signed-off-by: Petr Mladek <firstname.lastname@example.org> Tested-by: Davidlohr Bueso <email@example.com> Tested-by: Petr Mladek <firstname.lastname@example.org> Reviewed-by: Davidlohr Bueso <email@example.com> Cc: Andrew Lunn <firstname.lastname@example.org> Cc: Stephen Warren <email@example.com> Cc: Michal Hocko <firstname.lastname@example.org> Cc: Petr Mladek <email@example.com> Cc: Joe Perches <firstname.lastname@example.org> Cc: Arun KS <email@example.com> Cc: Kees Cook <firstname.lastname@example.org> Cc: Davidlohr Bueso <email@example.com> Cc: Chris Metcalf <firstname.lastname@example.org> Cc: Jan Kara <email@example.com> Signed-off-by: Andrew Morton <firstname.lastname@example.org> Signed-off-by: Linus Torvalds <email@example.com>
Diffstat (limited to 'init/Kconfig')
1 files changed, 42 insertions, 4 deletions
diff --git a/init/Kconfig b/init/Kconfig
index 41066e49e880..a291b7ef4738 100644
@@ -807,15 +807,53 @@ config LOG_BUF_SHIFT
range 12 21
- 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_MAX_BUF_SHIFT config
+ parameter, see below. Any higher size also might be forced
+ by "log_buf_len" boot parameter.
- 17 => 128 KB
+ 17 => 128 KB
16 => 64 KB
- 15 => 32 KB
- 14 => 16 KB
+ 15 => 32 KB
+ 14 => 16 KB
13 => 8 KB
12 => 4 KB
+ int "CPU kernel log buffer size contribution (13 => 8 KB, 17 => 128KB)"
+ range 0 21
+ default 12 if !BASE_SMALL
+ default 0 if BASE_SMALL
+ This option allows to increase the default ring buffer size
+ according to the number of CPUs. The value defines the contribution
+ of each CPU as a power of 2. The used space is typically only few
+ lines however it might be much more when problems are reported,
+ e.g. backtraces.
+ The increased size means that a new buffer has to be allocated and
+ the original static one is unused. It makes sense only on systems
+ with more CPUs. Therefore this value is used only when the sum of
+ contributions is greater than the half of the default kernel ring
+ buffer as defined by LOG_BUF_SHIFT. The default values are set
+ so that more than 64 CPUs are needed to trigger the allocation.
+ Also this option is ignored when "log_buf_len" kernel parameter is
+ used as it forces an exact (power of two) size of the ring buffer.
+ 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: