Message-ID: <20190819193327.GF117548@google.com>
Date:   Mon, 19 Aug 2019 15:33:27 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, byungchul.park@....com,
        Davidlohr Bueso <dave@...olabs.net>,
        Josh Triplett <josh@...htriplett.org>, kernel-team@...roid.com,
        kernel-team@....com, Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        max.byungchul.park@...il.com, Rao Shoaib <rao.shoaib@...cle.com>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v4 2/2] rcuperf: Add kfree_rcu() performance Tests

On Wed, Aug 14, 2019 at 03:58:50PM -0700, Paul E. McKenney wrote:
> On Wed, Aug 14, 2019 at 12:04:11PM -0400, Joel Fernandes (Google) wrote:
> > This test runs kfree_rcu in a loop to measure performance of the new
> > kfree_rcu batching functionality.
> 
> kfree_rcu().

Fixed.

> > The following table shows results when booting with arguments:
> > rcuperf.kfree_loops=200000 rcuperf.kfree_alloc_num=1000 rcuperf.kfree_rcu_test=1
> > 
> > In addition, rcuperf.kfree_no_batch is used to toggle the batching of
> > kfree_rcu()s for a test run.
> > 
> > rcuperf.kfree_no_batch	GPs	time (seconds)
> >  0 (default)		1732	15.9
> >  1			9133 	14.5
> > 
> > Note that the results are the same for the case:
> > 1. Patch is not applied and rcuperf.kfree_no_batch=0
> > 2. Patch is applied     and rcuperf.kfree_no_batch=1
> > 
> > On a 16 CPU system with the above boot parameters, we see that the total
> > number of grace periods that elapse during the test drops from 9133 when
> > not batching to 1732 when batching (a 5X improvement). The kfree_rcu()
> > flood itself slows down a bit when batching, though, as shown. This is
> > likely due to rcuperf threads contending with the additional worker
> > threads that are now running both before (the monitor) and after (the
> > work done to kfree()) the grace period.
> 
> Another possibility is that the batching approach is resulting in a
> greater number of objects waiting to be freed (noted below), and it
> takes the extra 1.4 seconds to catch up.  How would you decide which
> effect is the most important?  (Your path of least resistance is to
> remove the speculation.)

I will remove the speculation since the slightly extra time is understandable
and not concerning. I hope we agree on that.

> > Note that the active memory consumption during the kfree_rcu() flood
> > does increase to around 300-400MB due to the batching (from around 50MB
> > without batching). However, this memory consumption is relatively
> > constant and is just an effect of the buffering. In other words, the
> > system is able to keep up with the kfree_rcu() load. The memory
> > consumption comes down to 200-300MB if KFREE_DRAIN_JIFFIES is
> > increased from HZ/50 to HZ/80.
> > 
> > Also, when running the test, please disable CONFIG_DEBUG_PREEMPT and
> > CONFIG_PROVE_RCU for realistic comparisons with/without batching.
> > 
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> 
> Looks pretty close, just a very few issues needing fixing below.

Thanks!

> > ---
> >  .../admin-guide/kernel-parameters.txt         |  17 ++
> >  kernel/rcu/rcuperf.c                          | 189 +++++++++++++++++-
> >  2 files changed, 198 insertions(+), 8 deletions(-)
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 7ccd158b3894..a9156ca5de24 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -3895,6 +3895,23 @@
> >  			test until boot completes in order to avoid
> >  			interference.
> >  
> > +	rcuperf.kfree_rcu_test= [KNL]
> > +			Set to measure performance of kfree_rcu() flooding.
> > +
> > +	rcuperf.kfree_nthreads= [KNL]
> > +			The number of threads running loops of kfree_rcu().
> > +
> > +	rcuperf.kfree_alloc_num= [KNL]
> > +			Number of allocations and frees done in an iteration.
> > +
> > +	rcuperf.kfree_loops= [KNL]
> > +			Number of loops doing rcuperf.kfree_alloc_num number
> > +			of allocations and frees.
> > +
> > +	rcuperf.kfree_no_batch= [KNL]
> > +			Use the non-batching (slower) version of kfree_rcu.
> > +			This is useful for comparing with the batched version.
> 
> I suggest s/slower/more efficient/ given that the batching takes more
> wall-clock time than does the no-batching.

I think you mean, slower -> less efficient (due to taking up more grace
period cycles per second in the no batching case). I will update it
accordingly.

[snip]
> > @@ -592,6 +593,175 @@ rcu_perf_shutdown(void *arg)
> >  	return -EINVAL;
> >  }
> >  
> > +/*
> > + * kfree_rcu performance tests: Start a kfree_rcu loop on all CPUs for number
> > + * of iterations and measure total time and number of GP for all iterations to complete.
> > + */
> > +
> > +torture_param(int, kfree_nthreads, -1, "Number of threads running loops of kfree_rcu().");
> > +torture_param(int, kfree_alloc_num, 8000, "Number of allocations and frees done in an iteration.");
> > +torture_param(int, kfree_loops, 10, "Number of loops doing kfree_alloc_num allocations and frees.");
> > +torture_param(int, kfree_no_batch, 0, "Use the non-batching (slower) version of kfree_rcu.");
> > +
> > +static struct task_struct **kfree_reader_tasks;
> > +static int kfree_nrealthreads;
> > +static atomic_t n_kfree_perf_thread_started;
> > +static atomic_t n_kfree_perf_thread_ended;
> > +
> > +struct kfree_obj {
> > +	char kfree_obj[8];
> > +	struct rcu_head rh;
> > +};
> 
> (Aside from above, no need to change this part of the patch, at least not
> that I know of at the moment.)
> 
> 24 bytes on a 64-bit system, 16 on a 32-bit system.  So there might
> have been 10 million extra objects awaiting free in the batching case
> given the 400M-50M=350M excess for the batching approach.  If freeing
> each object took about 100ns, that could account for the additional
> wall-clock time for the batching approach.

Makes sense, and this comes down to 200-220MB range with the additional list.

> > +	set_user_nice(current, MAX_NICE);
> > +
> > +	alloc_ptrs = (struct kfree_obj **)kmalloc(sizeof(struct kfree_obj *) * kfree_alloc_num,
> > +						  GFP_KERNEL);
> > +	if (!alloc_ptrs)
> > +		return -ENOMEM;
> > +
> > +	start_time = ktime_get_mono_fast_ns();
> > +
> > +	if (atomic_inc_return(&n_kfree_perf_thread_started) >= kfree_nrealthreads) {
> > +		if (gp_exp)
> > +			b_rcu_gp_test_started = cur_ops->exp_completed() / 2;
> > +		else
> > +			b_rcu_gp_test_started = cur_ops->get_gp_seq();
> > +	}
> > +
> > +	do {
> > +		for (i = 0; i < kfree_alloc_num; i++) {
> > +			alloc_ptrs[i] = kmalloc(sizeof(struct kfree_obj), GFP_KERNEL);
> > +			if (!alloc_ptrs[i])
> > +				return -ENOMEM;
> > +		}
> > +
> > +		for (i = 0; i < kfree_alloc_num; i++) {
> > +			if (!kfree_no_batch) {
> > +				kfree_rcu(alloc_ptrs[i], rh);
> > +			} else {
> > +				rcu_callback_t cb;
> > +
> > +				cb = (rcu_callback_t)(unsigned long)offsetof(struct kfree_obj, rh);
> > +				kfree_call_rcu_nobatch(&(alloc_ptrs[i]->rh), cb);
> > +			}
> > +		}
> 
> The point of allocating a large batch and then kfree_rcu()ing them in a
> loop is to defeat the per-CPU pool optimization?  Either way, a comment
> would be very good!

It was a reasoning like this, added it as a comment:

	/* While measuring kfree_rcu() time, we also end up measuring kmalloc()
	 * time. So the strategy here is to do a few (kfree_alloc_num) number
	 * of kmalloc() and kfree_rcu() every loop so that the current loop's
	 * deferred kfree()ing overlaps with the next loop's kmalloc().
	 */

Will post it soon with other patches on top of -rcu dev.

thanks,

 - Joel

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