Message-ID: <20200106195200.GS13449@paulmck-ThinkPad-P72>
Date:   Mon, 6 Jan 2020 11:52:00 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, bristot@...hat.com,
        frextrite@...il.com, madhuparnabhowmik04@...il.com,
        urezki@...il.com, Davidlohr Bueso <dave@...olabs.net>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v3 rcu-dev] rcuperf: Measure memory footprint during
 kfree_rcu() test

On Fri, Dec 20, 2019 at 10:37:14PM -0500, Joel Fernandes wrote:
> On Fri, Dec 20, 2019 at 04:07:29PM -0800, Paul E. McKenney wrote:
> > On Thu, Dec 19, 2019 at 04:13:49PM -0500, Joel Fernandes (Google) wrote:
> > > During changes to kfree_rcu() code, we often check the amount of free
> > > memory.  As an alternative to checking this manually, this commit adds a
> > > measurement in the test itself.  It measures four times during the test
> > > for available memory, digitally filters these measurements to produce a
> > > running average with a weight of 0.5, and compares this digitally
> > > filtered value with the amount of available memory at the beginning of
> > > the test.
> > > 
> > > We apply the digital filter only once we are more than 25% into the
> > > test. At the 25% mark, we just read available memory and don't apply any
> > > filtering. This prevents the first sample from skewing the results
> > > as we would not consider memory readings that were before memory was
> > > allocated.
> > > 
> > > A sample run shows something like:
> > > 
> > > Total time taken by all kfree'ers: 6369738407 ns, loops: 10000, batches: 764, memory footprint: 216MB
> > > 
> > > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > 
> > Much better!  A few comments below.
> > 
> > 							Thanx, Paul
> > 
> > > ---
> > > v1->v2: Minor corrections
> > > v1->v3: Use long long to prevent 32-bit system's overflow
> > > 	Handle case where some threads start later than others.
> > > 	Start measuring only once 25% into the test. Slightly more accurate.
> > > 
> > > Cc: bristot@...hat.com
> > > Cc: frextrite@...il.com
> > > Cc: madhuparnabhowmik04@...il.com
> > > Cc: urezki@...il.com
> > > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > > 
> > >  kernel/rcu/rcuperf.c | 23 +++++++++++++++++++++--
> > >  1 file changed, 21 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
> > > index da94b89cd531..67e0f804ea97 100644
> > > --- a/kernel/rcu/rcuperf.c
> > > +++ b/kernel/rcu/rcuperf.c
> > > @@ -12,6 +12,7 @@
> > >  #include <linux/types.h>
> > >  #include <linux/kernel.h>
> > >  #include <linux/init.h>
> > > +#include <linux/mm.h>
> > >  #include <linux/module.h>
> > >  #include <linux/kthread.h>
> > >  #include <linux/err.h>
> > > @@ -604,6 +605,8 @@ struct kfree_obj {
> > >  	struct rcu_head rh;
> > >  };
> > >  
> > > +long long mem_begin;
> > > +
> > >  static int
> > >  kfree_perf_thread(void *arg)
> > >  {
> > > @@ -611,6 +614,7 @@ kfree_perf_thread(void *arg)
> > >  	long me = (long)arg;
> > >  	struct kfree_obj *alloc_ptr;
> > >  	u64 start_time, end_time;
> > > +	long long mem_during = si_mem_available();
> > 
> > You initialize here, which makes quite a bit of sense...
> > 
> > >  	VERBOSE_PERFOUT_STRING("kfree_perf_thread task started");
> > >  	set_cpus_allowed_ptr(current, cpumask_of(me % nr_cpu_ids));
> > > @@ -626,6 +630,15 @@ kfree_perf_thread(void *arg)
> > >  	}
> > >  
> > >  	do {
> > > +		// Moving average of memory availability measurements.
> > > +		// Start measuring only from when we are at least 25% into the test.
> > > +		if (loop && kfree_loops > 3 && (loop % (kfree_loops / 4) == 0)) {
> > > +			if (loop == (kfree_loops / 4))
> > > +				mem_during = si_mem_available();
> > 
> > But then you reinitialize here.  Perhaps to avoid the compiler being
> > confused into complaining about uninitialized variables?  (But if so,
> > please comment it.)
> 
> It is reinitialized here like that, because if kfree_loops is < 4, then
> mem_during needs to hold some value to avoid the (mem_begin - mem_during) to
> falsely appear quite large. That's why I initialized it in the beginning. If
> kfree_loops is >= 4, then yes it will be initialized twice but that's Ok.
> 
> I can add a comment to the earlier initialization if you like.

Could we just force kfree_loops >= 4?  Complain if not, set it to 4?

> If that is the only comment on the patch left, I appreciate if you can
> change:
>  + long long mem_during = si_mem_available();
>  to 
>  + long long mem_during = si_mem_available(); // for kfree_loops < 4 case

Let's see what we end up with.

> > The thing is that by the fourth measurement, the initial influence has
> > been diluted by a factor of something like 16 or 32, correct?  I don't
> > believe that your measurements are any more accurate than that, given the
> > bursty nature of the RCU reclamation process.  So why not just initialize
> > it and average it at each sample point?
> 
> Yes but diluting 200MB of delta by 16 is still high and causes a skew.

You get similar errors by only sampling four times, though.  Assuming a
reasonably long test run compared to the typical grace-period duration,
each of the four samples has a 50% chance of being above the median,
thus a 1/16 chance of all four samples being above the median.

> > If you want more accuracy, you could increase the number of sample
> > points, while changing the filter constants to more heavily weight
> > past measurements.
> 
> At the moment the results are showing me consistent numbers in my tests and
> I'm quite happy with them. And it is better to have some measurement, than
> not having anything.

No argument in general, but consistent numbers do not rule out the
possibility of a deterministic bias.

> We can certainly refine it further but at this time I am thinking of spending
> my time reviewing Lai's patches and learning some other RCU things I need to
> catch up on. If you hate this patch too much, we can also defer this patch
> review for a bit and I can carry it in my tree for now as it is only a patch
> to test code. But honestly, in its current form I am sort of happy with it.

OK, I will keep it as is for now and let's look again later on.  It is not
in the bucket for the upcoming merge window in any case, so we do have
quite a bit of time.

It is not that I hate it, but rather that I want to be able to give
good answers to questions that might come up.  And given that I have
occasionally given certain people a hard time about their statistics,
it is only reasonable to expect them to return the favor.  I wouldn't
want you to be caught in the crossfire.  ;-)

> > Actually, I strongly suggest recording frequent raw data, and applying
> > various filtering techniques offline to see what works best.  I might
> > be mistaken, but it feels like you are shooting in the dark here.
> 
> The number I am aiming for is close to a constant. For example in a run, I
> see the available memory goes down from 460000 pages to ~ 400000 pages. That
> is around ~240MB of memory. This deviates by +- 20MB. I would rather take
> many samples than just 1 sample and apply some form of reduction to combine
> them into a single sample.  My current filtering works well in my testing and
> I could do more complex things, but then again at the moment I am happy with
> the test. I am open to any other filtering related suggestions though!
> 
> Sorry if I missed something!

My thoughts are to use an explicit flag to catch the first-time case,
take something like 30 samples as the default, and restructure the loop
a bit to reduce at least some of the indentation.

But let's keep it on the back burner for a bit while we get other things
sorted out.

							Thanx, Paul

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