1 | /* |
2 | * Copyright 2018-present Facebook, Inc. |
3 | * |
4 | * Licensed under the Apache License, Version 2.0 (the "License"); |
5 | * you may not use this file except in compliance with the License. |
6 | * You may obtain a copy of the License at |
7 | * |
8 | * http://www.apache.org/licenses/LICENSE-2.0 |
9 | * |
10 | * Unless required by applicable law or agreed to in writing, software |
11 | * distributed under the License is distributed on an "AS IS" BASIS, |
12 | * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
13 | * See the License for the specific language governing permissions and |
14 | * limitations under the License. |
15 | */ |
16 | |
17 | #pragma once |
18 | |
19 | #include <algorithm> |
20 | #include <chrono> |
21 | #include <thread> |
22 | |
23 | #include <folly/portability/Asm.h> |
24 | #include <folly/synchronization/WaitOptions.h> |
25 | |
26 | namespace folly { |
27 | namespace detail { |
28 | |
29 | enum class spin_result { |
30 | success, // condition passed |
31 | timeout, // exceeded deadline |
32 | advance, // exceeded current wait-options component timeout |
33 | }; |
34 | |
35 | template <typename Clock, typename Duration, typename F> |
36 | spin_result spin_pause_until( |
37 | std::chrono::time_point<Clock, Duration> const& deadline, |
38 | WaitOptions const& opt, |
39 | F f) { |
40 | if (opt.spin_max() <= opt.spin_max().zero()) { |
41 | return spin_result::advance; |
42 | } |
43 | |
44 | auto tbegin = Clock::now(); |
45 | while (true) { |
46 | if (f()) { |
47 | return spin_result::success; |
48 | } |
49 | |
50 | auto const tnow = Clock::now(); |
51 | if (tnow >= deadline) { |
52 | return spin_result::timeout; |
53 | } |
54 | |
55 | // Backward time discontinuity in Clock? revise pre_block starting point |
56 | tbegin = std::min(tbegin, tnow); |
57 | if (tnow >= tbegin + opt.spin_max()) { |
58 | return spin_result::advance; |
59 | } |
60 | |
61 | // The pause instruction is the polite way to spin, but it doesn't |
62 | // actually affect correctness to omit it if we don't have it. Pausing |
63 | // donates the full capabilities of the current core to its other |
64 | // hyperthreads for a dozen cycles or so. |
65 | asm_volatile_pause(); |
66 | } |
67 | } |
68 | |
69 | template <typename Clock, typename Duration, typename F> |
70 | spin_result spin_yield_until( |
71 | std::chrono::time_point<Clock, Duration> const& deadline, |
72 | F f) { |
73 | while (true) { |
74 | if (f()) { |
75 | return spin_result::success; |
76 | } |
77 | |
78 | auto const max = std::chrono::time_point<Clock, Duration>::max(); |
79 | if (deadline != max && Clock::now() >= deadline) { |
80 | return spin_result::timeout; |
81 | } |
82 | |
83 | std::this_thread::yield(); |
84 | } |
85 | } |
86 | |
87 | } // namespace detail |
88 | } // namespace folly |
89 | |