Commit 0b85ffc2 authored by Steven Rostedt (Red Hat)'s avatar Steven Rostedt (Red Hat) Committed by Steven Rostedt
Browse files

tracing: Add config option to allow snapshot to swap per cpu 



When the preempt or irq latency tracers are enabled, they require
the ring buffer to be able to swap the per cpu sub buffers between
two main buffers. This adds a slight overhead to tracing as the
trace recording needs to perform some checks to synchronize
between recording and swaps that might be happening on other CPUs.

The config RING_BUFFER_ALLOW_SWAP is set when a user of the ring
buffer needs the "swap cpu" feature, otherwise the extra checks
are not implemented and removed from the tracing overhead.

The snapshot feature will swap per CPU if the RING_BUFFER_ALLOW_SWAP
config is set. But that only gets set by things like OPROFILE
and the irqs and preempt latency tracers.

This config is added to let the user decide to include this feature
with the snapshot agnostic from whether or not another user of
the ring buffer sets this config.

Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
parent f1affcaa

kernel/trace/Kconfig

+23 −0
Original line number Diff line number Diff line
@@ -192,6 +192,7 @@ config IRQSOFF_TRACER 
	select TRACER_MAX_TRACE

	select RING_BUFFER_ALLOW_SWAP

	select TRACER_SNAPSHOT

	select TRACER_SNAPSHOT_PER_CPU_SWAP

	help

	  This option measures the time spent in irqs-off critical

	  sections, with microsecond accuracy.
@@ -215,6 +216,7 @@ config PREEMPT_TRACER 
	select TRACER_MAX_TRACE

	select RING_BUFFER_ALLOW_SWAP

	select TRACER_SNAPSHOT

	select TRACER_SNAPSHOT_PER_CPU_SWAP

	help

	  This option measures the time spent in preemption-off critical

	  sections, with microsecond accuracy.
@@ -266,6 +268,27 @@ config TRACER_SNAPSHOT 
	      echo 1 > /sys/kernel/debug/tracing/snapshot

	      cat snapshot



config TRACER_SNAPSHOT_PER_CPU_SWAP

        bool "Allow snapshot to swap per CPU"

	depends on TRACER_SNAPSHOT

	select RING_BUFFER_ALLOW_SWAP

	help

	  Allow doing a snapshot of a single CPU buffer instead of a

	  full swap (all buffers). If this is set, then the following is

	  allowed:



	      echo 1 > /sys/kernel/debug/tracing/per_cpu/cpu2/snapshot



	  After which, only the tracing buffer for CPU 2 was swapped with

	  the main tracing buffer, and the other CPU buffers remain the same.



	  When this is enabled, this adds a little more overhead to the

	  trace recording, as it needs to add some checks to synchronize

	  recording with swaps. But this does not affect the performance

	  of the overall system. This is enabled by default when the preempt

	  or irq latency tracers are enabled, as those need to swap as well

	  and already adds the overhead (plus a lot more).



config TRACE_BRANCH_PROFILING

	bool

	select GENERIC_TRACER

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