[LTP] [PATCH V2 1/2] ltp: Add the ability to specify the latency constraint

[Cyril Hrubis]

Hi!
> > This seems to be a bit too specific to me, I would be more happy if we
> > named it "measures_sleep_time" or something that describes what the
> > testcase does rather than calling it after specific workaround.
> 
> I'm sorry, I don't get the point. The latency is a system parameter for
> any kind of run. Here it is in the common tst lib code as the constraint
> may be needed for any other new test. I don't understand the
> 'measures_sleep_time', there is no connection with the latency word.

My point of view is that this is very specific tuning parameter and in
ideal world the testcases would not need to touch kernel tuning knobs
unless the knob is what is being tested, but we do not live in an ideal
world either so some kind of solution is desirable. On the other hand
there is plenty of different tuning knobs there and I would like to
avoid putting each of them into the test library as another flag to be
set in the test structure.

> Perhaps the cpu_dma_latency needs a better explanation:
> 
> When the CPU has no task in its runqueue, the kernel will run a special
> task (the idle task). When entering in the idle routine, it will use an
> idle state process selection based on how long we predict the CPU will
> be sleeping (based on a mix of wake up statistics and timer events) as
> well as an exit latency constraint. The deeper the idle state is, the
> longer the latency to wakeup is and the more energy we save.
> 
> The number of idle states is hardware specific, as well as the idle
> state characteristics. For example, for an x86,i7 the deepest idle
> state's exit latency is 86us, on other platforms (eg. mobile), the
> deepest idle state's exit latency could more than 1500us (or less).
> 
> Under certain circumstances, you don't want your CPU to go to a too deep
> idle state because you need fast response: eg. playing a game, reading a
> mp3 or a HD video (otherwise you miss frames). The application will then
> set a cpu_dma_latency constraint and as the kernel checks this latency
> when selecting an idle state, deep idle state will be skipped.
> 
> For the specific pselect test, we measure the drift between the expected
> wakeup and the observed one. With the energy framework, we added an
> unacceptable (for the test) delay. This test does not check the latency,
> it checks if the wakeup period from the *pselect* syscall is acceptable,
> in absolute, so we need to put apart the idle routine during this
> specific test.
> 
> Technically speaking, the cpu_dma_latency is set by opening the file
> /dev/cpu_dma_latency and writing the value but as soon as you close the
> file descriptor, the constraint is removed from the kernel. So in this
> case, any test setting the cpu_dma_latency will have the file descriptor
> close at exit time, the constraint is bound to the test itself.

So if I'm getting this correctly the kernel tries to save power
agressively in your case since it knows when next timer expires and that
waking up from deep idle state would make the timer slack, right? From
that point of view this more or less looks like an workaround :-).

But what I do not understand here is why pselect requires special
handling while rest of the timer testcases that do exactly the same and
end up using the very same hrtimer framework in kernel work fine.

We do have at least select() and poll() tests that should end up using
the same timer with the same slack formula in kernel and hence should
fail in the very same way. And possibly rest of the timer testcases
should likely fail as well since these are not that different either.

Are you, by any chance, using latest stable release? Since we had
rewritten all the timer precision tests recently in:

https://github.com/linux-test-project/ltp/commit/c459654db64cd29177a382ab178fdd5ad59664e4

> > Also shouldn't this be set automatically for all timer related testcases
> > (these that make use of the sampling function) rather than just for the
> > pselect one? If so we should set the flag in the tst_timer_test.c
> > library instead. Or we can also stick the latency setup/cleanup
> > into the tst_timer_test.c as well and don't bother with adding more
> > flags into the tst_test structure (see timer_setup() and timer_cleanup()
> > in the tst_timer_test.c).
> 
> I'm not sure. We may want to measure everything together (timer + drift
> + energy framework latencies) in other testcases. I think it is fine if
> we let the test program to decide what latency he wants to set.

Are you using latest git with the timer library? I would suspect that
all of the test would need the very same treatement  since the test
paramters and maximal error margins are all defined in a single place
now.

-- 
Cyril Hrubis
chrubis@suse.cz