* Dave Jones <[email protected]> wrote:
> > could test it by e.g. trying to reproduce the same VM latency as in the
> > -rt tree. [the two zlib patches are needed if you are using 4K stacks,
> > mcount increases stack footprint.]
>
> kernel/latency.c: In function 'add_preempt_count_ti':
> kernel/latency.c:1703: warning: implicit declaration of function 'preempt_count_ti'
> kernel/latency.c:1703: error: invalid lvalue in assignment
> kernel/latency.c: In function 'sub_preempt_count_ti':
> kernel/latency.c:1764: error: invalid lvalue in assignment
indeed - i have fixed this and have uploaded a new version to:
http://redhat.com/~mingo/latency-tracing-patches/
> interesting config options ...
>
> # CONFIG_PREEMPT_NONE is not set
> CONFIG_PREEMPT_VOLUNTARY=y
> # CONFIG_PREEMPT is not set
> CONFIG_PREEMPT_BKL=y
>
> CONFIG_WAKEUP_TIMING=y
> CONFIG_WAKEUP_LATENCY_HIST=y
> CONFIG_CRITICAL_IRQSOFF_TIMING=y
> CONFIG_INTERRUPT_OFF_HIST=y
> CONFIG_LATENCY_TRACE=y
> CONFIG_USE_FRAME_POINTER=y
> CONFIG_FRAME_POINTER=y
these are various things one might be interested in gathering on a live
system. Enabling more of them means higher runtime overhead.
WAKEUP_TIMING only measures the worst-case wakeup cost, it's the
lowest-overhead option. It's activated via resetting the worst-case
cost:
echo 0 > /proc/sys/kernel/preempt_max_latency
The histogram ones are gathering a histogram (but no traces) into
/proc/latency_hist/<cpu_nr>. These have some overhead as they hook into
every preempt-disable/enable call. Output is:
$ head -15 /proc/latency_hist/wakeup_latency/CPU0
#Minimum latency: 2 microseconds.
#Average latency: 7 microseconds.
#Maximum latency: 511 microseconds.
#Total samples: 5241
#There are 0 samples greater or equal than 10240 microseconds
#usecs samples
0 0
1 0
2 7
3 2041
4 921
5 194
6 62
7 502
8 119
...
The LATENCY_TRACING option does full tracing of critical sections
[driven by e.g. WAKEUP_TIMING - but it can also be activated by
interrupts, or be completely user-driven via userspace calls], which
trace is then put into /proc/latency_trace - mcount done for every
function call in the kernel. This can add up to 30% of runtime overhead
[or worse], but is obviously very useful for debugging latencies.
Ingo
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