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SubjectRe: [PATCH v4 2/2] rcuperf: Add kfree_rcu() performance Tests
On Tue, Aug 20, 2019 at 08:31:32PM -0400, Joel Fernandes wrote:
> On Tue, Aug 20, 2019 at 08:27:05PM -0400, Joel Fernandes wrote:
> [snip]
> > > > > Or is the idea to time the kfree_rcu() loop separately? (I don't see
> > > > > any such separate timing, though.)
> > > >
> > > > The kmalloc() times are included within the kfree loop. The timing of
> > > > kfree_rcu() is not separate in my patch.
> > >
> > > You lost me on this one. What happens when you just interleave the
> > > kmalloc() and kfree_rcu(), without looping, compared to the looping
> > > above? Does this get more expensive? Cheaper? More vulnerable to OOM?
> > > Something else?
> >
> > You mean pairing a single kmalloc() with a single kfree_rcu() and doing this
> > several times? The results are very similar to doing kfree_alloc_num
> > kmalloc()s, then do kfree_alloc_num kfree_rcu()s; and repeat the whole thing
> > kfree_loops times (as done by this rcuperf patch we are reviewing).
> >
> > Following are some numbers. One change is the case where we are not at all
> > batching does seem to complete even faster when we fully interleave kmalloc()
> > with kfree() while the case of batching in the same scenario completes at the
> > same time as did the "not fully interleaved" scenario. However, the grace
> > period reduction improvements and the chances of OOM'ing are pretty much the
> > same in either case.
> [snip]
> > Not fully interleaved: do kfree_alloc_num kmallocs, then do kfree_alloc_num kfree_rcu()s. And repeat this kfree_loops times.
> > =======================
> > (1) Batching
> > rcuperf.kfree_loops=20000 rcuperf.kfree_alloc_num=8000 rcuperf.kfree_no_batch=0 rcuperf.kfree_rcu_test=1
> >
> > root@(none):/# free -m
> > total used free shared buff/cache available
> > Mem: 977 251 686 0 39 684
> > Swap: 0 0 0
> >
> > [ 15.574402] Total time taken by all kfree'ers: 14185970787 ns, loops: 20000, batches: 1548
> >
> > (2) No Batching
> > rcuperf.kfree_loops=20000 rcuperf.kfree_alloc_num=8000 rcuperf.kfree_no_batch=1 rcuperf.kfree_rcu_test=1
> >
> > root@(none):/# free -m
> > total used free shared buff/cache available
> > Mem: 977 82 855 0 39 853
> > Swap: 0 0 0
> >
> > [ 13.724554] Total time taken by all kfree'ers: 12246217291 ns, loops: 20000, batches: 7262
>
> And the diff for changing the test to do this case is as follows (I don't
> plan to fold this diff in, since I feel the existing test suffices and
> results are similar):

But why not? It does look to be a nice simplification, after all.

Thanx, Paul

> diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
> index 46f9c4449348..e4e4be4aaf51 100644
> --- a/kernel/rcu/rcuperf.c
> +++ b/kernel/rcu/rcuperf.c
> @@ -618,18 +618,13 @@ kfree_perf_thread(void *arg)
> {
> int i, loop = 0;
> long me = (long)arg;
> - struct kfree_obj **alloc_ptrs;
> + struct kfree_obj *alloc_ptr;
> u64 start_time, end_time;
>
> VERBOSE_PERFOUT_STRING("kfree_perf_thread task started");
> set_cpus_allowed_ptr(current, cpumask_of(me % nr_cpu_ids));
> 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) {
> @@ -646,19 +641,17 @@ kfree_perf_thread(void *arg)
> */
> do {
> for (i = 0; i < kfree_alloc_num; i++) {
> - alloc_ptrs[i] = kmalloc(sizeof(struct kfree_obj), GFP_KERNEL);
> - if (!alloc_ptrs[i])
> + alloc_ptr = kmalloc(sizeof(struct kfree_obj), GFP_KERNEL);
> + if (!alloc_ptr)
> return -ENOMEM;
> - }
>
> - for (i = 0; i < kfree_alloc_num; i++) {
> if (!kfree_no_batch) {
> - kfree_rcu(alloc_ptrs[i], rh);
> + kfree_rcu(alloc_ptr, 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);
> + kfree_call_rcu_nobatch(&(alloc_ptr->rh), cb);
> }
> }
>
> @@ -682,7 +675,6 @@ kfree_perf_thread(void *arg)
> }
> }
>
> - kfree(alloc_ptrs);
> torture_kthread_stopping("kfree_perf_thread");
> return 0;
> }

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