Date:	Wed, 23 Jun 2010 13:56:45 -0700
From:	Daniel Walker <dwalker@...eaurora.org>
To:	Patrick Pannuto <ppannuto@...eaurora.org>
Cc:	linux-kernel@...r.kernel.org, sboyd@...eaurora.org,
	tglx@...utronix.de, mingo@...e.hu, heiko.carstens@...ibm.com,
	eranian@...gle.com, schwidefsky@...ibm.com,
	linux-arm-msm@...r.kernel.org
Subject: Re: [RFC] [PATCH] timer: Added usleep[_range][_interruptable] timer

On Wed, 2010-06-23 at 13:21 -0700, Patrick Pannuto wrote:
> Daniel Walker wrote:
> > On Wed, 2010-06-23 at 12:22 -0700, Patrick Pannuto wrote:
> >> *** INTRO ***
> >>
> >> As discussed here ( http://lkml.org/lkml/2007/8/3/250 ), msleep(1) is not
> >> precise enough for many drivers (yes, sleep precision is an unfair notion,
> >> but consistently sleeping for ~an order of magnitude greater than requested
> >> is worth fixing). This patch adds a usleep API so that udelay does not have
> >> to be used. Obviously not every udelay can be replaced (those in atomic
> >> contexts or being used for simple bitbanging come to mind), but there are
> >> many, many examples of
> >>
> >> mydriver_write(...)
> >> /* Wait for hardware to latch */
> >> udelay(100)
> >>
> >> in various drivers where a busy-wait loop is neither beneficial nor
> >> necessary, but msleep simply does not provide enough precision and people
> >> are using a busy-wait loop instead.
> > 
> > I think one thing for you to answer would be, why do you think udelay is
> > a problem? I don't honestly see that many udelay()'s around, and
> > especially not in important code paths .. Instead of adding a new API
> > like this you might just rework the problem areas.
> > 
> > Are you approaching this from performance? or battery life? or what?
> 
> First and foremost: power. If switching from udelay to usleep lets the processor
> go to a lower C-state once in awhile, then I would consider this a win.

It's not clear if your changes would actually do that tho.. Since your
adding little tiny length timers instead of super long timers.. You want
more long length timers to get into a lower power state.

> > 
> >> *** SOME QUANTIFIABLE (?) NUMBERS ***
> >>
> > 
> >> then averaged the results to see if there was any benefit:
> >>
> >> === ORIGINAL (99 samples) ========================================= ORIGINAL ===
> >>     Avg: 188.760000 wakeups in 9.911010 secs (19.045486 wkups/sec) [18876 total]
> >> Wakeups: Min - 179, Max - 208, Mean - 190.666667, Stdev - 6.601194
> >>
> >> === USLEEP (99 samples) ============================================= USLEEP ===
> >>     Avg: 188.200000 wakeups in 9.911230 secs (18.988561 wkups/sec) [18820 total]
> >> Wakeups: Min - 181, Max - 213, Mean - 190.101010, Stdev - 6.950757
> >>
> >> While not particularly rigorous, the results seem to indicate that there may be
> >> some benefit from pursuing this.
> > 
> > This is sort of ambiguous .. I don't think you replaced enough of these
> > for it to have much of an impact. It's actually counter intuitive
> > because your changes add more timers, yet they reduced average wakeups
> > by a tiny amount .. Why do you think that is ?
> > 
> 
> Yes, this test was leftover from a different project that involved refactoring
> timers, so it was available and easy.  My guess for the reduction in number of
> wakeups is that the processor is able to do other work during the 100us it was
> previously busy-waiting, and thus had to wake up less often.

As I said in the prior email the udelay()'s don't preclude other types
of work since you can get preempted.

I think your results are just showing noise ..

> I don't know a good way to test this, if you do, please advise and I will
> happily pursue it.

You could test residency in specific power states .. Since you want to
test if your reducing power consumption .. However, I'd replace a ton
more of these udelay()'s , I don't think you'll get a decent results
with out that.

> >> *** HOW MUCH BENEFIT? ***
> >>
> >> Somewhat arbitrarily choosing 100 as a cut-off for udelay VS usleep:
> >>
> >> 	git grep 'udelay([[:digit:]]\+)' | 
> >> 		perl -F"[\(\)]" -anl -e 'print if $F[1] >= 100' | wc -l
> >>
> >> yeilds 1093 on Linus's tree. There are 313 instances of >= 1000 and still
> >> another 53 >= 10000us of busy wait! (If AVOID_POPS is configured in, the
> >> es18xx driver will udelay(100000) or *0.1 seconds of busy wait*)
> > 
> > I'd say a better question is how often do they run?
> 
> The i2c guys will get hit any time there is contention / heavy traffic on the
> i2c bus (they're in the i2c_poll_notbusy path, also the i2c_poll_writeready),
> so any time there is a lot of peripheral traffic (e.g. the phone is probably
> doing a lot of stuff), then there are long (ish) busy-wait loops that are
> unnecessary.

If the phone is "doing a lot of stuff" then pretty good chance power
saving is at a minimum anyway. Try to be more specific .. For example,
is there some specific app that is causes power problems, and maybe that
app eventually gets into those i2c calls.

> I haven't researched extensively, but I imagine there are a fair number of
> other code paths like this; udelays polling until devices aren't busy - and
> devices are generally only busy under some degree of load, not a good time
> to busy wait if you don't have to IMHO


The busy waits your replacing are small in length on average .. If you
have timers that trigger in small intervals then your not going to
increase residency in any _lower_ power states .. It's possible that you
could increase residency in the top level power state, but it seem like
it would be really marginal .. You need to show that udelay()'s have an
outside of noise impact on something ..

> > 
> > Another thing is that your usleep() can't replace udelay() in critical
> > sections. However, if your doing udelay() in non-critical areas, I don't
> > think there is anything stopping preemption during the udelay() .. So
> > udelay() doesn't really cut off the whole system when it runs unless it
> > _is_ in a critical section.
> > 
> 
> I mentioned elsewhere that this can't replace all udelays; as for those that
> can be pre-empted, it seems like only a win to give up your time slice to
> something that will do real work (or sleep at a lower c-state and use less
> power) than to sit and loop. Yes, you *could* be pre-empted from doing
> absolutely nothing, but I don't think you should *have* to be for the
> system to make a more productive use of those cycles.

I think you need to do some more research on what your actually doing to
the system. From what your showing us one could make a lot of different
arguments as to what this change will actually do. You really need some
sort of test that doesn't leave a lot of room for argument.

Daniel

-- 
Sent by a consultant of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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