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Date: Thu, 19 Dec 2019 11:34:49 -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 v2 rcu-dev] rcuperf: Measure memory footprint during
kfree_rcu() test
On Thu, Dec 19, 2019 at 12:58:53PM -0500, Joel Fernandes wrote:
> On Thu, Dec 19, 2019 at 09:14:02AM -0800, Paul E. McKenney wrote:
> > On Thu, Dec 19, 2019 at 11:22:42AM -0500, Joel Fernandes (Google) wrote:
> > > During changes to kfree_rcu() code, we often check how much is free
> > > memory. Instead of doing so manually, add a measurement in the test
> > > itself. We measure 4 times during the test for available memory and
> > > compare with the beginning.
> > >
> > > 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>
> >
> > Does the following make sense for the commit log?
> >
> > ------------------------------------------------------------------------
> >
> > 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.
> >
> > Something like the following is printed at the end of the run:
>
> Yes! I'll incorporate this!
>
> >
> > Total time taken by all kfree'ers: 6369738407 ns, loops: 10000, batches: 764, memory footprint: 216MB
Ah, and I presume that the 216MB reflects other memory being pushed out
by the rcuperf run. Either way, some guidance to the user should be
added somewhere, perhaps as a comment preceding the print statement.
> > ------------------------------------------------------------------------
> >
> > And some questions below. I have queued this for testing and further
> > review with the commit log above in the meantime.
> >
> > Thanx, Paul
> >
> > > ---
> > > v1->v2 : Minor corrections
> > >
> > > Cc: bristot@...hat.com
> > > Cc: frextrite@...il.com
> > > Cc: madhuparnabhowmik04@...il.com
> > > Cc: urezki@...il.com
> > >
> > > kernel/rcu/rcuperf.c | 14 ++++++++++++--
> > > 1 file changed, 12 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
> > > index da94b89cd531..91f0650914cc 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>
> > > @@ -611,6 +612,7 @@ kfree_perf_thread(void *arg)
> > > long me = (long)arg;
> > > struct kfree_obj *alloc_ptr;
> > > u64 start_time, end_time;
> > > + long mem_begin, mem_during = 0;
> > >
> > > VERBOSE_PERFOUT_STRING("kfree_perf_thread task started");
> > > set_cpus_allowed_ptr(current, cpumask_of(me % nr_cpu_ids));
> > > @@ -626,6 +628,12 @@ kfree_perf_thread(void *arg)
> > > }
> > >
> > > do {
> > > + if (!mem_during) {
> > > + mem_during = mem_begin = si_mem_available();
> >
> > So we sample at the beginning of the test before we have either allocated
> > or freed. Or did I miss a beginning-of-test allocation somehow?
>
> Yes, this is the assignment at the beginning of test. I did mess something up
> though, if all threads don't start at around the same time, and then if the
> thread that started late also ends later than everyone else and becomes the
> chosen one to do the reporting, then there is a chance that the mem_begin
> it prints will be larger considering the threads that started earlier than
> the chosen one may have allocated some memory in the meanwhile. I did not
> really see this happen in my testing but I'll fix that in the v3!
>
> >
> > > + } else if (loop % (kfree_loops / 4) == 0) {
> > > + mem_during = (mem_during + si_mem_available()) / 2;
> >
> > This is the digital-filter step. The truncating nature of integer
> > division could actually get us four samples counting the first one if
> > kfree_loops evenly divides by four, or five otherwise, correct? In the
> > latter case, we would have a measurement near the end of the test,
> > but not exactly at the end of the test, right?
>
> Yes there is an off-by-one possibility depending on kfree_loops value. I did
> not mind that too much because I was looking for an approximate measurement
> which would be better than my manual calculation of memory footprint anyway.
> And I did not see how the off-by-one could affect the results.
>
> > And I have to ask, having studied control systems back in the day...
> >
> > Why digitally filter by 0.5 as opposed to any other choice? For
> > example, you could weight recent history more heavily:
> >
> > mem_during = (mem_during + 3 * si_mem_available()) / 4;
> >
> > Or vice versa:
> >
> > mem_during = (3 * mem_during + si_mem_available()) / 4;
> >
> > So why the specific choice of 0.5?
>
> There wasn't a particular reason and I agree your weighted approach is better
> and absorbs any quick fluctuations in the signal better.
Actually, I wasn't advocating in any particular direction, just asking.
But either way, it would be good to add a comment describing the
rationale.
> > Oh, and integer overflow is a problem on 32-bit platforms with more
> > than 2GB of memory, for example x86 or ARM physical-address-extension
> > (PAE) systems. I therefore changed the declarations to "long long"
> > (and adjusted the format accordingly), but please let me know if I am
> > missing some other effect that prevents overflow.
>
> Yes you're right. Thanks for fixing that. I'll fold that into my v3.
Sounds good -- I will replace the currently queued commit with the
v3 version when it arrives.
Thanx, Paul
> > This does not address the possible problem of 64-bit systems that really
> > have 64 bits worth of physical memory, but I am happy to leave that one
> > for the time being, to be fixed if and when. ;-)
> > Adjusted patch shown below. Please let me know if I have messed anything
> > up, and if there is nothing obviously wrong, please give it a good testing.
>
> Will do!
>
> thanks,
>
> - Joel
>
> >
> > > + }
> > > +
> > > for (i = 0; i < kfree_alloc_num; i++) {
> > > alloc_ptr = kmalloc(sizeof(struct kfree_obj), GFP_KERNEL);
> > > if (!alloc_ptr)
> > > @@ -645,9 +653,11 @@ kfree_perf_thread(void *arg)
> > > else
> > > b_rcu_gp_test_finished = cur_ops->get_gp_seq();
> > >
> > > - pr_alert("Total time taken by all kfree'ers: %llu ns, loops: %d, batches: %ld\n",
> > > + pr_alert("Total time taken by all kfree'ers: %llu ns, loops: %d, batches: %ld, memory footprint: %ldMB\n",
> > > (unsigned long long)(end_time - start_time), kfree_loops,
> > > - rcuperf_seq_diff(b_rcu_gp_test_finished, b_rcu_gp_test_started));
> > > + rcuperf_seq_diff(b_rcu_gp_test_finished, b_rcu_gp_test_started),
> > > + (mem_begin - mem_during) >> (20 - PAGE_SHIFT));
> > > +
> > > if (shutdown) {
> > > smp_mb(); /* Assign before wake. */
> > > wake_up(&shutdown_wq);
> > > --
> > > 2.24.1.735.g03f4e72817-goog
> >
> > ------------------------------------------------------------------------
> >
> > commit 8bf389d441538030d07a9a0f9e38ec0843f7a83e
> > Author: Joel Fernandes (Google) <joel@...lfernandes.org>
> > Date: Thu Dec 19 11:22:42 2019 -0500
> >
> > rcuperf: Measure memory footprint during kfree_rcu() test
> >
> > 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.
> >
> > Something like the following is printed at the end of the run:
> >
> > 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>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> >
> > diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
> > index da94b89..a4a8d09 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>
> > @@ -611,6 +612,7 @@ kfree_perf_thread(void *arg)
> > long me = (long)arg;
> > struct kfree_obj *alloc_ptr;
> > u64 start_time, end_time;
> > + long long mem_begin, mem_during = 0;
> >
> > VERBOSE_PERFOUT_STRING("kfree_perf_thread task started");
> > set_cpus_allowed_ptr(current, cpumask_of(me % nr_cpu_ids));
> > @@ -626,6 +628,12 @@ kfree_perf_thread(void *arg)
> > }
> >
> > do {
> > + if (!mem_during) {
> > + mem_during = mem_begin = si_mem_available();
> > + } else if (loop % (kfree_loops / 4) == 0) {
> > + mem_during = (mem_during + si_mem_available()) / 2;
> > + }
> > +
> > for (i = 0; i < kfree_alloc_num; i++) {
> > alloc_ptr = kmalloc(sizeof(struct kfree_obj), GFP_KERNEL);
> > if (!alloc_ptr)
> > @@ -645,9 +653,11 @@ kfree_perf_thread(void *arg)
> > else
> > b_rcu_gp_test_finished = cur_ops->get_gp_seq();
> >
> > - pr_alert("Total time taken by all kfree'ers: %llu ns, loops: %d, batches: %ld\n",
> > + pr_alert("Total time taken by all kfree'ers: %llu ns, loops: %d, batches: %ld, memory footprint: %lldMB\n",
> > (unsigned long long)(end_time - start_time), kfree_loops,
> > - rcuperf_seq_diff(b_rcu_gp_test_finished, b_rcu_gp_test_started));
> > + rcuperf_seq_diff(b_rcu_gp_test_finished, b_rcu_gp_test_started),
> > + (mem_begin - mem_during) >> (20 - PAGE_SHIFT));
> > +
> > if (shutdown) {
> > smp_mb(); /* Assign before wake. */
> > wake_up(&shutdown_wq);
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