Re: [BUG] Strange 1-second pauses during Resume-from-RAM

Ingo Molnar
2007-11-16T11:24:20Z

* Ingo Molnar <mingo@elte.hu> wrote:

> once that tracer bug was fixed, the best method to generate a trace
> was to do this:
>
> echo 1 > /proc/sys/kernel/stackframe_tracing
> echo 1 > /proc/sys/kernel/syscall_tracing
> ./trace-cmd bash -c "echo mem > /sys/power/state" > trace.txt

so here's an UP suspend+resume trace i did:

http://redhat.com/~mingo/latency-tracing-patches/misc/trace-suspend-long.txt.bz2

tons of detail - which might be interesting to other folks as well. Fact
is, our suspend-to-RAM+resume cycle is very, very slow, even on fast
hardware - and this trace shows all the reasons why.

This was a fully cached system - i.e. i've done a suspend+resume before
to warm up the caches. (not that suspend+resume does much IO normally.)

The trace shows that a suspend+resume cycle is 7.95 seconds long
(without counting the time the box spent suspended) - ouch! This was a
T60 with Core2Duo 1.83GHz.

For example here is where freezing starts:

bash-2397 0.... 31686us : remove_wait_queue (vt_waitactive)
bash-2397 0.... 31688us : freeze_processes (enter_state)
bash-2397 0.... 31689us : printk (freeze_processes)

here is where the ACPI code triggers the suspend:

bash-2397 0D... 1904138us : acpi_hw_low_level_write (acpi_hw_register_write)

but this is a whopping 1.9 seconds into the trace already!

first sign of life after i opened the laptop lid again:

bash-2397 0D... 1904138us : __restore_processor_state (restore_processor_state)
bash-2397 0D... 1904138us : enable_sep_cpu (__restore_processor_state)

(in the trace there's no delay visible - the period of time spent
suspended is not visible to the tracer.)

One good way to start looking at such traces is to filter out
rescheduling events alone:

grep ': schedule <' trace-suspend-long.txt

that gives a rough outline of what's going on:

<idle>-0 0D... 1776566us : schedule <bash-2397> (0 20)
bash-2397 0D... 1786748us : schedule <<idle>-0> (20 0)
scsi_eh_-419 0D... 1786814us : schedule <bash-2397> (0 -5)
bash-2397 0D... 1786960us : schedule <scsi_eh_-419> (-5 0)
scsi_eh_-421 0D... 1787020us : schedule <bash-2397> (0 -5)
bash-2397 0D... 1787125us : schedule <scsi_eh_-421> (-5 0)

so you can zoom in on the real area of interest by searching for the
timestamp.

Hope this helps,

Ingo