Re: [PATCH v3 rcu-dev] rcuperf: Measure memory footprint during kfree_rcu() test

From: Joel Fernandes <joel@joelfernandes.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: linux-kernel@vger.kernel.org, bristot@redhat.com,
	frextrite@gmail.com, madhuparnabhowmik04@gmail.com,
	urezki@gmail.com, Davidlohr Bueso <dave@stgolabs.net>,
	Josh Triplett <josh@joshtriplett.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	rcu@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH v3 rcu-dev] rcuperf: Measure memory footprint during kfree_rcu() test
Date: Wed, 15 Jan 2020 17:45:42 -0500	[thread overview]
Message-ID: <20200115224542.GB94036@google.com> (raw)
In-Reply-To: <20200115224251.GK2935@paulmck-ThinkPad-P72>

On Wed, Jan 15, 2020 at 02:42:51PM -0800, Paul E. McKenney wrote:
> > [snip]
> > > > 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.  ;-)
> > 
> > Since the weights were concerning, I was thinking of just using a weight of
> > (1 / N) where N is the number of samples. Essentially taking the average.
> > That could be simple enough and does not cause your concerns with weight
> > tuning. I tested it and looks good, I'll post it shortly.
> 
> YES!!!  ;-)
> 
> Snapshot mem_begin before entering the loop.  For the mean value to
> be solid, you need at least 20-30 samples, which might mean upping the
> default for kfree_loops.  Have an "unsigned long long" to accumulate the
> sum, which should avoid any possibility of overflow for current systems
> and for all systems for kfree_loops less than PAGE_SIZE.  At which point,
> forget the "%" stuff and just sum up the si_mem_available() on each pass
> through the loop.
> 
> Do the division on exit from the loop, preferably checking for divide
> by zero.
> 
> Straightforward, fast, reasonably reliable, and easy to defend.

I mostly did it along these lines. Hopefully the latest posting is reasonable
enough ;-) I sent it twice because I messed up the authorship (sorry).

thanks,

 - Joel