All atomic 64 bit ints are changed to atomicbitops.(Ui|I)nt64. A nogo checker
enforces that sync/atomic 64 bit functions are not called.
For reviewers: the interesting changes are in the atomicbitops and checkaligned
packages.
Why do this?
- It is very easy to accidentally use atomic values without sync/atomic funcs.
- We have checkatomics, but this is optional and is forgotten in several places.
- Using a type+checker to enforce this seems less error prone and simpler.
- We get NoCopy protection.
- Use of 64 bit atomics can break 32 bit builds. We have types to handle this
without any runtime cost, so we might as well use them.
PiperOrigin-RevId: 440473398
Some synchronization patterns require the ability to simultaneously wake and
sleep a goroutine. For the sleep package, this is the case when a waker must be
asserted when a subsequent fetch is imminent.
Currently, this operation results in significant P churn in the runtime, which
ping-pongs execution between multiple system threads and cores and consumes a
significant amount of host CPU (and because of the context switches, this can
be significant worse with mitigations for side channel vulnerabilities).
The solution is to introduce a dedicated mechanism for a synchronous switch
which does not wake another runtime P (see golang/go#32113). This can be used
by the `AssertAndFetch` API in the sleep package.
The benchmark results for this package are very similiar to raw channel
operations for all cases, with the exception of operations that do not wait.
The primary advantage is more precise control over scheduling. This will be
used in a subsequent change.
```
BenchmarkGoAssertNonWaiting
BenchmarkGoAssertNonWaiting-8 261364384 4.976 ns/op
BenchmarkGoSingleSelect
BenchmarkGoSingleSelect-8 20946358 57.77 ns/op
BenchmarkGoMultiSelect
BenchmarkGoMultiSelect-8 6071697 197.0 ns/op
BenchmarkGoWaitOnSingleSelect
BenchmarkGoWaitOnSingleSelect-8 4978051 235.4 ns/op
BenchmarkGoWaitOnMultiSelect
BenchmarkGoWaitOnMultiSelect-8 2309224 520.2 ns/op
BenchmarkSleeperAssertNonWaiting
BenchmarkSleeperAssertNonWaiting-8 447325033 2.657 ns/op
BenchmarkSleeperSingleSelect
BenchmarkSleeperSingleSelect-8 21488844 55.19 ns/op
BenchmarkSleeperMultiSelect
BenchmarkSleeperMultiSelect-8 21851674 54.89 ns/op
BenchmarkSleeperWaitOnSingleSelect
BenchmarkSleeperWaitOnSingleSelect-8 2860327 416.4 ns/op
BenchmarkSleeperWaitOnSingleSelectSync
BenchmarkSleeperWaitOnSingleSelectSync-8 2741733 427.1 ns/op
BenchmarkSleeperWaitOnMultiSelect
BenchmarkSleeperWaitOnMultiSelect-8 2867484 418.1 ns/op
BenchmarkSleeperWaitOnMultiSelectSync
BenchmarkSleeperWaitOnMultiSelectSync-8 2789158 427.9 ns/op
```
PiperOrigin-RevId: 415581417
Instead of passing the event mask at registratrion time, pass the mask as part
of the waiter. This makes the mask immutable and simplifies the architecture of
waiters. This is also necessary for a future fix that will allow the fdnotifier
to keep persistent entries, as opposed to requiring constant updates.
This change is intended to be a no-op in terms of function. The only exception
is signalfd, where this mask was abused. To handle this case, the operation of
signalfd changed to allow one layer of indirection.
PiperOrigin-RevId: 409702998
Consider the following benchmark, which is equivalent to
BenchmarkSleeperWaitOnSingleSelect with names changed to more closely reflect
the behavior of BenchmarkGoWaitOnSingleSelect:
var (
empty Sleeper
emptyCond Waker
full Sleeper
fullCond Waker
)
empty.AddWaker(&emptyCond)
full.AddWaker(&fullCond)
go func() {
for i := 0; i < b.N; i++ {
empty.Fetch(true)
fullCond.Assert()
}
}()
for i := 0; i < b.N; i++ {
emptyCond.Assert()
full.Fetch(true)
}
The unfairness arises because runtime.chansend and runtime.chanrecv don't
actually work this way. If runtime.chansend blocks, it has already enqueued the
element to be sent on runtime.hchan.sendq, which runtime.chanrecv dequeues
before calling goready(); in sleep-like terms, by the time empty.Fetch()
returns, fullCond.Assert() has already happened and been fetched by the other
goroutine. The same property applies to runtime.chanrecv/runtime.hchan.recvq.
This property has no correspondence to the actual usage of the sleep package,
so change the channel benchmarks to explicitly exchange control using buffered
channels instead. Also remove some stale comments and align the syncevent
benchmarks with the sleep ones.
BenchmarkSleeperWaitOnSingleSelect
BenchmarkSleeperWaitOnSingleSelect-12 2118603 472.5 ns/op
BenchmarkGoWaitOnSingleSelect
BenchmarkGoWaitOnSingleSelect-12 2224262 517.7 ns/op
BenchmarkSleeperWaitOnMultiSelect
BenchmarkSleeperWaitOnMultiSelect-12 2630569 459.8 ns/op
BenchmarkGoWaitOnMultiSelect
BenchmarkGoWaitOnMultiSelect-12 807918 1312 ns/op
BenchmarkWaiterPingPong
BenchmarkWaiterPingPong-12 2955579 385.8 ns/op
BenchmarkSleeperPingPong
BenchmarkSleeperPingPong-12 2454367 474.3 ns/op
BenchmarkChannelPingPong
BenchmarkChannelPingPong-12 2302662 513.5 ns/op
BenchmarkWaiterPingPongMulti
BenchmarkWaiterPingPongMulti-12 3023676 388.8 ns/op
BenchmarkSleeperPingPongMulti
BenchmarkSleeperPingPongMulti-12 2574064 471.5 ns/op
BenchmarkChannelPingPongMulti
BenchmarkChannelPingPongMulti-12 1000000 1088 ns/op
PiperOrigin-RevId: 407760956
In a subsequent change, the Sleeper API will be plumbed through and used for
arbitrary task wakeups. This requires a non-static association of Wakers and
Sleepers, which means that a fixed ID no longer works. This is a relatively
simple change that removes the ID from the Waker association, and simply uses
the Waker pointer itself.
That change also makes minor improvements to the tests to ensure that the
benchmarks are more representative by removing goroutine start from the hot
path (and uses Wakers for required synchronization), adds assertion checks to
AddWaker, and clears relevant fields during Done (to allow assertions to pass).
PiperOrigin-RevId: 407719630
Our "Preconditions:" blocks are very useful to determine the input invariants,
but they are bit inconsistent throughout the codebase, which makes them harder
to read (particularly cases with 5+ conditions in a single paragraph).
I've reformatted all of the cases to fit in simple rules:
1. Cases with a single condition are placed on a single line.
2. Cases with multiple conditions are placed in a bulleted list.
This format has been added to the style guide.
I've also mentioned "Postconditions:", though those are much less frequently
used, and all uses already match this style.
PiperOrigin-RevId: 327687465
The variable name is g which is collision with the reserved name
for R28. This leads to bazel build failure on ARM with following
information:
(register+register) not supported on this architecture
rename it from g to ptr (referenced from golang source
code)
Signed-off-by: Howard Zhang <howard.zhang@arm.com>
None of the dependencies have changed in 1.15. It may be possible to simplify
some of the wrappers in rawfile following 1.13, but that can come in a later
change.
PiperOrigin-RevId: 313863264
Package syncevent is intended to subsume ~all uses of channels in the sentry
(including //pkg/waiter), as well as //pkg/sleep.
Compared to channels:
- Delivery of events to a syncevent.Receiver allows *synchronous* execution of
an arbitrary callback, whereas delivery of events to a channel requires a
goroutine to receive from that channel, resulting in substantial scheduling
overhead. (This is also part of the motivation for the waiter package.)
- syncevent.Waiter can wait on multiple event sources without the high O(N)
overhead of select. (This is the same motivation as for the sleep package.)
Compared to the waiter package:
- syncevent.Waiters are intended to be persistent (i.e. per-kernel.Task), and
syncevent.Broadcaster (analogous to waiter.Queue) is a hash table rather than
a linked list, such that blocking is (usually) allocation-free.
- syncevent.Source (analogous to waiter.Waitable) does not include an equivalent
to waiter.Waitable.Readiness(), since this is inappropriate for transient
events (see e.g. //pkg/sentry/kernel/time.ClockEventSource).
Compared to the sleep package:
- syncevent events are represented by bits in a bitmask rather than discrete
sleep.Waker objects, reducing overhead and making it feasible to broadcast
events to multiple syncevent.Receivers.
- syncevent.Receiver invokes an arbitrary callback, which is required by the
sentry's epoll implementation. (syncevent.Waiter, which is analogous to
sleep.Sleeper, pairs a syncevent.Receiver with a callback that wakes a
waiting goroutine; the implementation of this aspect is nearly identical to
that of sleep.Sleeper, except that it represents *runtime.g as unsafe.Pointer
rather than uintptr.)
- syncevent.Waiter.Wait (analogous to sleep.Sleeper.Fetch(block=true)) does not
automatically un-assert returned events. This is useful in cases where the
path for handling an event is not the same as the path that observes it, such
as for application signals (a la Linux's TIF_SIGPENDING).
- Unlike sleep.Sleeper, which Fetches Wakers in the order that they were
Asserted, the event bitmasks used by syncevent.Receiver have no way of
preserving event arrival order. (This is similar to select, which goes out of
its way to randomize event ordering.)
The disadvantage of the syncevent package is that, since events are represented
by bits in a uint64 bitmask, each syncevent.Receiver can "only" multiplex
between 64 distinct events; this does not affect any known use case.
Benchmarks:
BenchmarkBroadcasterSubscribeUnsubscribe
BenchmarkBroadcasterSubscribeUnsubscribe-12 45133884 26.3 ns/op
BenchmarkMapSubscribeUnsubscribe
BenchmarkMapSubscribeUnsubscribe-12 28504662 41.8 ns/op
BenchmarkQueueSubscribeUnsubscribe
BenchmarkQueueSubscribeUnsubscribe-12 22747668 45.6 ns/op
BenchmarkBroadcasterSubscribeUnsubscribeBatch
BenchmarkBroadcasterSubscribeUnsubscribeBatch-12 31609177 37.8 ns/op
BenchmarkMapSubscribeUnsubscribeBatch
BenchmarkMapSubscribeUnsubscribeBatch-12 17563906 62.1 ns/op
BenchmarkQueueSubscribeUnsubscribeBatch
BenchmarkQueueSubscribeUnsubscribeBatch-12 26248838 46.6 ns/op
BenchmarkBroadcasterBroadcastRedundant
BenchmarkBroadcasterBroadcastRedundant/0
BenchmarkBroadcasterBroadcastRedundant/0-12 100907563 11.8 ns/op
BenchmarkBroadcasterBroadcastRedundant/1
BenchmarkBroadcasterBroadcastRedundant/1-12 85103068 13.3 ns/op
BenchmarkBroadcasterBroadcastRedundant/4
BenchmarkBroadcasterBroadcastRedundant/4-12 52716502 22.3 ns/op
BenchmarkBroadcasterBroadcastRedundant/16
BenchmarkBroadcasterBroadcastRedundant/16-12 20278165 58.7 ns/op
BenchmarkBroadcasterBroadcastRedundant/64
BenchmarkBroadcasterBroadcastRedundant/64-12 5905428 205 ns/op
BenchmarkMapBroadcastRedundant
BenchmarkMapBroadcastRedundant/0
BenchmarkMapBroadcastRedundant/0-12 87532734 13.5 ns/op
BenchmarkMapBroadcastRedundant/1
BenchmarkMapBroadcastRedundant/1-12 28488411 36.3 ns/op
BenchmarkMapBroadcastRedundant/4
BenchmarkMapBroadcastRedundant/4-12 19628920 60.9 ns/op
BenchmarkMapBroadcastRedundant/16
BenchmarkMapBroadcastRedundant/16-12 6026980 192 ns/op
BenchmarkMapBroadcastRedundant/64
BenchmarkMapBroadcastRedundant/64-12 1640858 754 ns/op
BenchmarkQueueBroadcastRedundant
BenchmarkQueueBroadcastRedundant/0
BenchmarkQueueBroadcastRedundant/0-12 96904807 12.0 ns/op
BenchmarkQueueBroadcastRedundant/1
BenchmarkQueueBroadcastRedundant/1-12 73521873 16.3 ns/op
BenchmarkQueueBroadcastRedundant/4
BenchmarkQueueBroadcastRedundant/4-12 39209468 31.2 ns/op
BenchmarkQueueBroadcastRedundant/16
BenchmarkQueueBroadcastRedundant/16-12 10810058 105 ns/op
BenchmarkQueueBroadcastRedundant/64
BenchmarkQueueBroadcastRedundant/64-12 2998046 376 ns/op
BenchmarkBroadcasterBroadcastAck
BenchmarkBroadcasterBroadcastAck/1
BenchmarkBroadcasterBroadcastAck/1-12 44472397 26.4 ns/op
BenchmarkBroadcasterBroadcastAck/4
BenchmarkBroadcasterBroadcastAck/4-12 17653509 69.7 ns/op
BenchmarkBroadcasterBroadcastAck/16
BenchmarkBroadcasterBroadcastAck/16-12 4082617 260 ns/op
BenchmarkBroadcasterBroadcastAck/64
BenchmarkBroadcasterBroadcastAck/64-12 1220534 1027 ns/op
BenchmarkMapBroadcastAck
BenchmarkMapBroadcastAck/1
BenchmarkMapBroadcastAck/1-12 26760705 44.2 ns/op
BenchmarkMapBroadcastAck/4
BenchmarkMapBroadcastAck/4-12 11495636 100 ns/op
BenchmarkMapBroadcastAck/16
BenchmarkMapBroadcastAck/16-12 2937590 343 ns/op
BenchmarkMapBroadcastAck/64
BenchmarkMapBroadcastAck/64-12 861037 1344 ns/op
BenchmarkQueueBroadcastAck
BenchmarkQueueBroadcastAck/1
BenchmarkQueueBroadcastAck/1-12 19832679 55.0 ns/op
BenchmarkQueueBroadcastAck/4
BenchmarkQueueBroadcastAck/4-12 5618214 189 ns/op
BenchmarkQueueBroadcastAck/16
BenchmarkQueueBroadcastAck/16-12 1569980 713 ns/op
BenchmarkQueueBroadcastAck/64
BenchmarkQueueBroadcastAck/64-12 437672 2814 ns/op
BenchmarkWaiterNotifyRedundant
BenchmarkWaiterNotifyRedundant-12 650823090 1.96 ns/op
BenchmarkSleeperNotifyRedundant
BenchmarkSleeperNotifyRedundant-12 619871544 1.61 ns/op
BenchmarkChannelNotifyRedundant
BenchmarkChannelNotifyRedundant-12 298903778 3.67 ns/op
BenchmarkWaiterNotifyWaitAck
BenchmarkWaiterNotifyWaitAck-12 68358360 17.8 ns/op
BenchmarkSleeperNotifyWaitAck
BenchmarkSleeperNotifyWaitAck-12 25044883 41.2 ns/op
BenchmarkChannelNotifyWaitAck
BenchmarkChannelNotifyWaitAck-12 29572404 40.2 ns/op
BenchmarkSleeperMultiNotifyWaitAck
BenchmarkSleeperMultiNotifyWaitAck-12 16122969 73.8 ns/op
BenchmarkWaiterTempNotifyWaitAck
BenchmarkWaiterTempNotifyWaitAck-12 46111489 25.8 ns/op
BenchmarkSleeperTempNotifyWaitAck
BenchmarkSleeperTempNotifyWaitAck-12 15541882 73.6 ns/op
BenchmarkWaiterNotifyWaitMultiAck
BenchmarkWaiterNotifyWaitMultiAck-12 65878500 18.2 ns/op
BenchmarkSleeperNotifyWaitMultiAck
BenchmarkSleeperNotifyWaitMultiAck-12 28798623 41.5 ns/op
BenchmarkChannelNotifyWaitMultiAck
BenchmarkChannelNotifyWaitMultiAck-12 11308468 101 ns/op
BenchmarkWaiterNotifyAsyncWaitAck
BenchmarkWaiterNotifyAsyncWaitAck-12 2475387 492 ns/op
BenchmarkSleeperNotifyAsyncWaitAck
BenchmarkSleeperNotifyAsyncWaitAck-12 2184507 518 ns/op
BenchmarkChannelNotifyAsyncWaitAck
BenchmarkChannelNotifyAsyncWaitAck-12 2120365 562 ns/op
BenchmarkWaiterNotifyAsyncWaitMultiAck
BenchmarkWaiterNotifyAsyncWaitMultiAck-12 2351247 494 ns/op
BenchmarkSleeperNotifyAsyncWaitMultiAck
BenchmarkSleeperNotifyAsyncWaitMultiAck-12 2205799 522 ns/op
BenchmarkChannelNotifyAsyncWaitMultiAck
BenchmarkChannelNotifyAsyncWaitMultiAck-12 1238079 928 ns/op
Updates #1074
PiperOrigin-RevId: 295834087
Jamie LiuandgVisor bot
Adin ScannellandgVisor bot
Kevin KrakauerandgVisor bot
Ayush RanjanandgVisor bot
Michael PrattandgVisor bot
Howard Zhang