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Date: Tue, 6 Feb 2024 18:04:34 +0100
From: Marco Elver <elver@google.com>
To: Martin KaFai Lau <martin.lau@linux.dev>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>, Song Liu <song@kernel.org>,
	Yonghong Song <yonghong.song@linux.dev>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>, Stanislav Fomichev <sdf@google.com>,
	Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
	Mykola Lysenko <mykolal@fb.com>, Shuah Khan <shuah@kernel.org>,
	bpf@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-kselftest@vger.kernel.org
Subject: Re: [PATCH] bpf: Separate bpf_local_storage_lookup() fast and slow
 paths
Message-ID: <ZcJmok64Xqv6l4ZS@elver.google.com>
References: <20240131141858.1149719-1-elver@google.com>
 <b500bb70-aa3f-41d3-b058-2b634471ffef@linux.dev>
 <CANpmjNPKACDwXMnZRw9=CAgWNaMWAyFZ2W7KY2s4ck0s_ue1ag@mail.gmail.com>
 <5a08032b-ed4d-4429-b0a9-2736689d8c33@linux.dev>
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User-Agent: Mutt/2.2.12 (2023-09-09)

On Mon, Feb 05, 2024 at 03:24PM -0800, Martin KaFai Lau wrote:
[...]
> > Or can you suggest different functions to hook to for the recursion test?
> 
> I don't prefer to add another tracepoint for the selftest.

Ok - I also checked, even though it should be a no-op, it wasn't
(compiler generated worse code).

> The test in "SEC("fentry/bpf_local_storage_lookup")" is testing that the
> initial bpf_local_storage_lookup() should work and the immediate recurred
> bpf_task_storage_delete() will fail.
> 
> Depends on how the new slow path function will look like in v2. The test can
> probably be made to go through the slow path, e.g. by creating a lot of task
> storage maps before triggering the lookup.

Below is tentative v2, but I'm struggling with fixing up the test. In
particular, bpf_task_storage_delete() now only calls out to
migrate_disable/enable() and bpf_selem_unlink(), because the compiler
just ends up inlining everything it can:

<bpf_task_storage_delete>:
   endbr64
   nopl   0x0(%rax,%rax,1)
   push   %r14
   push   %rbx
   test   %rsi,%rsi
   je     ffffffff81280015 <bpf_task_storage_delete+0x75>
   mov    %rsi,%rbx
   mov    %rdi,%r14
   call   ffffffff810f2e40 <migrate_disable>
   incl   %gs:0x7eda9ba5(%rip)        # 29b68 <bpf_task_storage_busy>
   mov    0xb38(%rbx),%rax
   mov    $0xfffffffffffffffe,%rbx
   test   %rax,%rax
   je     ffffffff8128002f <bpf_task_storage_delete+0x8f>
   movzwl 0x10e(%r14),%ecx

   mov    (%rax,%rcx,8),%rdi
   test   %rdi,%rdi
   je     ffffffff8127ffef <bpf_task_storage_delete+0x4f>
   mov    (%rdi),%rcx
   cmp    %r14,%rcx
   je     ffffffff81280022 <bpf_task_storage_delete+0x82>
   mov    0x88(%rax),%rdi
   test   %rdi,%rdi
   je     ffffffff8128002f <bpf_task_storage_delete+0x8f>
   add    $0xfffffffffffffff0,%rdi
   je     ffffffff8128002f <bpf_task_storage_delete+0x8f>
   mov    0x40(%rdi),%rax
   cmp    %r14,%rax
   je     ffffffff8128001e <bpf_task_storage_delete+0x7e>
   mov    0x10(%rdi),%rdi
   test   %rdi,%rdi
   jne    ffffffff8127fffb <bpf_task_storage_delete+0x5b>
   jmp    ffffffff8128002f <bpf_task_storage_delete+0x8f>
   mov    $0xffffffffffffffea,%rbx
   jmp    ffffffff8128003b <bpf_task_storage_delete+0x9b>
   add    $0x40,%rdi
   add    $0xffffffffffffffc0,%rdi
   xor    %ebx,%ebx
   xor    %esi,%esi
   call   ffffffff8127e820 <bpf_selem_unlink>
   decl   %gs:0x7eda9b32(%rip)        # 29b68 <bpf_task_storage_busy>
   call   ffffffff810f2ed0 <migrate_enable>
   mov    %rbx,%rax
   pop    %rbx
   pop    %r14
   cs jmp ffffffff82324ea0 <__x86_return_thunk>


Could you suggest how we can fix up the tests? I'm a little stuck
because there's not much we can hook to left.

Thanks,
-- Marco

------ >8 ------

From: Marco Elver <elver@google.com>
Date: Tue, 30 Jan 2024 17:57:45 +0100
Subject: [PATCH v2] bpf: Allow compiler to inline most of
 bpf_local_storage_lookup()

In various performance profiles of kernels with BPF programs attached,
bpf_local_storage_lookup() appears as a significant portion of CPU
cycles spent. To enable the compiler generate more optimal code, turn
bpf_local_storage_lookup() into a static inline function, where only the
cache insertion code path is outlined (call instruction can be elided
entirely if cacheit_lockit is a constant expression).

Based on results from './benchs/run_bench_local_storage.sh' (21 trials;
reboot between each trial) this produces improvements in throughput and
latency in the majority of cases, with an average (geomean) improvement
of 8%:

+---- Hashmap Control --------------------
|
| + num keys: 10
| :                                         <before>             | <after>
| +-+ hashmap (control) sequential get    +----------------------+----------------------
|   +- hits throughput                    | 14.789 M ops/s       | 14.745 M ops/s (  ~  )
|   +- hits latency                       | 67.679 ns/op         | 67.879 ns/op   (  ~  )
|   +- important_hits throughput          | 14.789 M ops/s       | 14.745 M ops/s (  ~  )
|
| + num keys: 1000
| :                                         <before>             | <after>
| +-+ hashmap (control) sequential get    +----------------------+----------------------
|   +- hits throughput                    | 12.233 M ops/s       | 12.170 M ops/s (  ~  )
|   +- hits latency                       | 81.754 ns/op         | 82.185 ns/op   (  ~  )
|   +- important_hits throughput          | 12.233 M ops/s       | 12.170 M ops/s (  ~  )
|
| + num keys: 10000
| :                                         <before>             | <after>
| +-+ hashmap (control) sequential get    +----------------------+----------------------
|   +- hits throughput                    | 7.220 M ops/s        | 7.204 M ops/s  (  ~  )
|   +- hits latency                       | 138.522 ns/op        | 138.842 ns/op  (  ~  )
|   +- important_hits throughput          | 7.220 M ops/s        | 7.204 M ops/s  (  ~  )
|
| + num keys: 100000
| :                                         <before>             | <after>
| +-+ hashmap (control) sequential get    +----------------------+----------------------
|   +- hits throughput                    | 5.061 M ops/s        | 5.165 M ops/s  (+2.1%)
|   +- hits latency                       | 198.483 ns/op        | 194.270 ns/op  (-2.1%)
|   +- important_hits throughput          | 5.061 M ops/s        | 5.165 M ops/s  (+2.1%)
|
| + num keys: 4194304
| :                                         <before>             | <after>
| +-+ hashmap (control) sequential get    +----------------------+----------------------
|   +- hits throughput                    | 2.864 M ops/s        | 2.882 M ops/s  (  ~  )
|   +- hits latency                       | 365.220 ns/op        | 361.418 ns/op  (-1.0%)
|   +- important_hits throughput          | 2.864 M ops/s        | 2.882 M ops/s  (  ~  )
|
+---- Local Storage ----------------------
|
| + num_maps: 1
| :                                         <before>             | <after>
| +-+ local_storage cache sequential get  +----------------------+----------------------
|   +- hits throughput                    | 33.005 M ops/s       | 39.068 M ops/s (+18.4%)
|   +- hits latency                       | 30.300 ns/op         | 25.598 ns/op   (-15.5%)
|   +- important_hits throughput          | 33.005 M ops/s       | 39.068 M ops/s (+18.4%)
| :
| :                                         <before>             | <after>
| +-+ local_storage cache interleaved get +----------------------+----------------------
|   +- hits throughput                    | 37.151 M ops/s       | 44.926 M ops/s (+20.9%)
|   +- hits latency                       | 26.919 ns/op         | 22.259 ns/op   (-17.3%)
|   +- important_hits throughput          | 37.151 M ops/s       | 44.926 M ops/s (+20.9%)
|
| + num_maps: 10
| :                                         <before>             | <after>
| +-+ local_storage cache sequential get  +----------------------+----------------------
|   +- hits throughput                    | 32.288 M ops/s       | 38.099 M ops/s (+18.0%)
|   +- hits latency                       | 30.972 ns/op         | 26.248 ns/op   (-15.3%)
|   +- important_hits throughput          | 3.229 M ops/s        | 3.810 M ops/s  (+18.0%)
| :
| :                                         <before>             | <after>
| +-+ local_storage cache interleaved get +----------------------+----------------------
|   +- hits throughput                    | 34.473 M ops/s       | 41.145 M ops/s (+19.4%)
|   +- hits latency                       | 29.010 ns/op         | 24.307 ns/op   (-16.2%)
|   +- important_hits throughput          | 12.312 M ops/s       | 14.695 M ops/s (+19.4%)
|
| + num_maps: 16
| :                                         <before>             | <after>
| +-+ local_storage cache sequential get  +----------------------+----------------------
|   +- hits throughput                    | 32.524 M ops/s       | 38.341 M ops/s (+17.9%)
|   +- hits latency                       | 30.748 ns/op         | 26.083 ns/op   (-15.2%)
|   +- important_hits throughput          | 2.033 M ops/s        | 2.396 M ops/s  (+17.9%)
| :
| :                                         <before>             | <after>
| +-+ local_storage cache interleaved get +----------------------+----------------------
|   +- hits throughput                    | 34.575 M ops/s       | 41.338 M ops/s (+19.6%)
|   +- hits latency                       | 28.925 ns/op         | 24.193 ns/op   (-16.4%)
|   +- important_hits throughput          | 11.001 M ops/s       | 13.153 M ops/s (+19.6%)
|
| + num_maps: 17
| :                                         <before>             | <after>
| +-+ local_storage cache sequential get  +----------------------+----------------------
|   +- hits throughput                    | 28.861 M ops/s       | 32.756 M ops/s (+13.5%)
|   +- hits latency                       | 34.649 ns/op         | 30.530 ns/op   (-11.9%)
|   +- important_hits throughput          | 1.700 M ops/s        | 1.929 M ops/s  (+13.5%)
| :
| :                                         <before>             | <after>
| +-+ local_storage cache interleaved get +----------------------+----------------------
|   +- hits throughput                    | 31.529 M ops/s       | 36.110 M ops/s (+14.5%)
|   +- hits latency                       | 31.719 ns/op         | 27.697 ns/op   (-12.7%)
|   +- important_hits throughput          | 9.598 M ops/s        | 10.993 M ops/s (+14.5%)
|
| + num_maps: 24
| :                                         <before>             | <after>
| +-+ local_storage cache sequential get  +----------------------+----------------------
|   +- hits throughput                    | 18.602 M ops/s       | 19.937 M ops/s (+7.2%)
|   +- hits latency                       | 53.767 ns/op         | 50.166 ns/op   (-6.7%)
|   +- important_hits throughput          | 0.776 M ops/s        | 0.831 M ops/s  (+7.2%)
| :
| :                                         <before>             | <after>
| +-+ local_storage cache interleaved get +----------------------+----------------------
|   +- hits throughput                    | 21.718 M ops/s       | 23.332 M ops/s (+7.4%)
|   +- hits latency                       | 46.047 ns/op         | 42.865 ns/op   (-6.9%)
|   +- important_hits throughput          | 6.110 M ops/s        | 6.564 M ops/s  (+7.4%)
|
| + num_maps: 32
| :                                         <before>             | <after>
| +-+ local_storage cache sequential get  +----------------------+----------------------
|   +- hits throughput                    | 14.118 M ops/s       | 14.626 M ops/s (+3.6%)
|   +- hits latency                       | 70.856 ns/op         | 68.381 ns/op   (-3.5%)
|   +- important_hits throughput          | 0.442 M ops/s        | 0.458 M ops/s  (+3.6%)
| :
| :                                         <before>             | <after>
| +-+ local_storage cache interleaved get +----------------------+----------------------
|   +- hits throughput                    | 17.111 M ops/s       | 17.906 M ops/s (+4.6%)
|   +- hits latency                       | 58.451 ns/op         | 55.865 ns/op   (-4.4%)
|   +- important_hits throughput          | 4.776 M ops/s        | 4.998 M ops/s  (+4.6%)
|
| + num_maps: 100
| :                                         <before>             | <after>
| +-+ local_storage cache sequential get  +----------------------+----------------------
|   +- hits throughput                    | 5.281 M ops/s        | 5.528 M ops/s  (+4.7%)
|   +- hits latency                       | 192.398 ns/op        | 183.059 ns/op  (-4.9%)
|   +- important_hits throughput          | 0.053 M ops/s        | 0.055 M ops/s  (+4.9%)
| :
| :                                         <before>             | <after>
| +-+ local_storage cache interleaved get +----------------------+----------------------
|   +- hits throughput                    | 6.265 M ops/s        | 6.498 M ops/s  (+3.7%)
|   +- hits latency                       | 161.436 ns/op        | 152.877 ns/op  (-5.3%)
|   +- important_hits throughput          | 1.636 M ops/s        | 1.697 M ops/s  (+3.7%)
|
| + num_maps: 1000
| :                                         <before>             | <after>
| +-+ local_storage cache sequential get  +----------------------+----------------------
|   +- hits throughput                    | 0.355 M ops/s        | 0.354 M ops/s  (  ~  )
|   +- hits latency                       | 2826.538 ns/op       | 2827.139 ns/op (  ~  )
|   +- important_hits throughput          | 0.000 M ops/s        | 0.000 M ops/s  (  ~  )
| :
| :                                         <before>             | <after>
| +-+ local_storage cache interleaved get +----------------------+----------------------
|   +- hits throughput                    | 0.404 M ops/s        | 0.403 M ops/s  (  ~  )
|   +- hits latency                       | 2481.190 ns/op       | 2487.555 ns/op (  ~  )
|   +- important_hits throughput          | 0.102 M ops/s        | 0.101 M ops/s  (  ~  )

Signed-off-by: Marco Elver <elver@google.com>
---
v2:
* Inline most of bpf_local_storage_lookup(), which produces greater
  speedup and avoids regressing the cases with large map arrays.
* Drop "unlikely()" hint, it didn't produce much benefit.
* Re-run benchmark and collect 21 trials of results.
---
 include/linux/bpf_local_storage.h             | 30 ++++++++++-
 kernel/bpf/bpf_local_storage.c                | 52 +++++--------------
 .../selftests/bpf/progs/cgrp_ls_recursion.c   |  2 +-
 .../selftests/bpf/progs/task_ls_recursion.c   |  2 +-
 4 files changed, 43 insertions(+), 43 deletions(-)

diff --git a/include/linux/bpf_local_storage.h b/include/linux/bpf_local_storage.h
index 173ec7f43ed1..dcddb0aef7d8 100644
--- a/include/linux/bpf_local_storage.h
+++ b/include/linux/bpf_local_storage.h
@@ -129,10 +129,36 @@ bpf_local_storage_map_alloc(union bpf_attr *attr,
 			    struct bpf_local_storage_cache *cache,
 			    bool bpf_ma);
 
-struct bpf_local_storage_data *
+void __bpf_local_storage_insert_cache(struct bpf_local_storage *local_storage,
+				      struct bpf_local_storage_map *smap,
+				      struct bpf_local_storage_elem *selem);
+/* If cacheit_lockit is false, this lookup function is lockless */
+static inline struct bpf_local_storage_data *
 bpf_local_storage_lookup(struct bpf_local_storage *local_storage,
 			 struct bpf_local_storage_map *smap,
-			 bool cacheit_lockit);
+			 bool cacheit_lockit)
+{
+	struct bpf_local_storage_data *sdata;
+	struct bpf_local_storage_elem *selem;
+
+	/* Fast path (cache hit) */
+	sdata = rcu_dereference_check(local_storage->cache[smap->cache_idx],
+				      bpf_rcu_lock_held());
+	if (sdata && rcu_access_pointer(sdata->smap) == smap)
+		return sdata;
+
+	/* Slow path (cache miss) */
+	hlist_for_each_entry_rcu(selem, &local_storage->list, snode,
+				  rcu_read_lock_trace_held())
+		if (rcu_access_pointer(SDATA(selem)->smap) == smap)
+			break;
+
+	if (!selem)
+		return NULL;
+	if (cacheit_lockit)
+		__bpf_local_storage_insert_cache(local_storage, smap, selem);
+	return SDATA(selem);
+}
 
 void bpf_local_storage_destroy(struct bpf_local_storage *local_storage);
 
diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c
index 146824cc9689..bdea1a459153 100644
--- a/kernel/bpf/bpf_local_storage.c
+++ b/kernel/bpf/bpf_local_storage.c
@@ -414,47 +414,21 @@ void bpf_selem_unlink(struct bpf_local_storage_elem *selem, bool reuse_now)
 	bpf_selem_unlink_storage(selem, reuse_now);
 }
 
-/* If cacheit_lockit is false, this lookup function is lockless */
-struct bpf_local_storage_data *
-bpf_local_storage_lookup(struct bpf_local_storage *local_storage,
-			 struct bpf_local_storage_map *smap,
-			 bool cacheit_lockit)
+void __bpf_local_storage_insert_cache(struct bpf_local_storage *local_storage,
+				      struct bpf_local_storage_map *smap,
+				      struct bpf_local_storage_elem *selem)
 {
-	struct bpf_local_storage_data *sdata;
-	struct bpf_local_storage_elem *selem;
-
-	/* Fast path (cache hit) */
-	sdata = rcu_dereference_check(local_storage->cache[smap->cache_idx],
-				      bpf_rcu_lock_held());
-	if (sdata && rcu_access_pointer(sdata->smap) == smap)
-		return sdata;
-
-	/* Slow path (cache miss) */
-	hlist_for_each_entry_rcu(selem, &local_storage->list, snode,
-				  rcu_read_lock_trace_held())
-		if (rcu_access_pointer(SDATA(selem)->smap) == smap)
-			break;
-
-	if (!selem)
-		return NULL;
-
-	sdata = SDATA(selem);
-	if (cacheit_lockit) {
-		unsigned long flags;
-
-		/* spinlock is needed to avoid racing with the
-		 * parallel delete.  Otherwise, publishing an already
-		 * deleted sdata to the cache will become a use-after-free
-		 * problem in the next bpf_local_storage_lookup().
-		 */
-		raw_spin_lock_irqsave(&local_storage->lock, flags);
-		if (selem_linked_to_storage(selem))
-			rcu_assign_pointer(local_storage->cache[smap->cache_idx],
-					   sdata);
-		raw_spin_unlock_irqrestore(&local_storage->lock, flags);
-	}
+	unsigned long flags;
 
-	return sdata;
+	/* spinlock is needed to avoid racing with the
+	 * parallel delete.  Otherwise, publishing an already
+	 * deleted sdata to the cache will become a use-after-free
+	 * problem in the next bpf_local_storage_lookup().
+	 */
+	raw_spin_lock_irqsave(&local_storage->lock, flags);
+	if (selem_linked_to_storage(selem))
+		rcu_assign_pointer(local_storage->cache[smap->cache_idx], SDATA(selem));
+	raw_spin_unlock_irqrestore(&local_storage->lock, flags);
 }
 
 static int check_flags(const struct bpf_local_storage_data *old_sdata,
diff --git a/tools/testing/selftests/bpf/progs/cgrp_ls_recursion.c b/tools/testing/selftests/bpf/progs/cgrp_ls_recursion.c
index 610c2427fd93..6e93f3c8b318 100644
--- a/tools/testing/selftests/bpf/progs/cgrp_ls_recursion.c
+++ b/tools/testing/selftests/bpf/progs/cgrp_ls_recursion.c
@@ -33,7 +33,7 @@ static void __on_lookup(struct cgroup *cgrp)
 	bpf_cgrp_storage_delete(&map_b, cgrp);
 }
 
-SEC("fentry/bpf_local_storage_lookup")
+SEC("fentry/??????????????????????????")
 int BPF_PROG(on_lookup)
 {
 	struct task_struct *task = bpf_get_current_task_btf();
diff --git a/tools/testing/selftests/bpf/progs/task_ls_recursion.c b/tools/testing/selftests/bpf/progs/task_ls_recursion.c
index 4542dc683b44..d73b33a4c153 100644
--- a/tools/testing/selftests/bpf/progs/task_ls_recursion.c
+++ b/tools/testing/selftests/bpf/progs/task_ls_recursion.c
@@ -27,7 +27,7 @@ struct {
 	__type(value, long);
 } map_b SEC(".maps");
 
-SEC("fentry/bpf_local_storage_lookup")
+SEC("fentry/??????????????????????????")
 int BPF_PROG(on_lookup)
 {
 	struct task_struct *task = bpf_get_current_task_btf();