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Message-ID: <9908bdfb-1030-4a9f-8405-3696c5d03981@linux.dev>
Date: Tue, 6 Feb 2024 17:22:45 -0800
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Marco Elver <elver@...gle.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>,
John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>,
Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>,
Shuah Khan <shuah@...nel.org>, bpf@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH] bpf: Separate bpf_local_storage_lookup() fast and slow
paths
On 2/6/24 9:04 AM, Marco Elver wrote:
> 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).
I am interested to how the tracepoint generates worse code. Can you share some
details ?
>
>> 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.
I don't see a solution either if only the cache insertion code path is in a
traceable function.
The prog->active counter has already been covered in another test. This test is
mostly only covering the lookup => delete recur case and the code path is
contained within the bpf storage logic. The future code review should be able to
cover. I would make an exception here and remove this test case considering
anything (e.g. tracepoint) we do here is likely to make it worse. (more on the
test removal below).
>
> Thanks,
> -- Marco
>
> ------ >8 ------
>
> From: Marco Elver <elver@...gle.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).
Can you share more why only putting the cache insertion code to a function
improves the larger number of maps case. In the benchmark, cacheit_lockit should
always be true and __bpf_local_storage_insert_cache() should always be called.
>
> 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@...gle.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)
Remove this BPF_PROG.
> {
> 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/??????????????????????????")
Same here. The checks related to on_lookup in prog_tests/task_local_storage.c
need to be removed also.
> int BPF_PROG(on_lookup)
> {
> struct task_struct *task = bpf_get_current_task_btf();
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