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[libc++] Do not yield from __sp_mut::lock() 

Instead of trying to be clever and design our own locking primitive,
simply rely on the OS-provided implementation to do the right thing.

Indeed, manually yielding to the OS does not provide the necessary
information for it to make good prioritization decisions. For example,
if a thread with higher priority yields while waiting for a lock held
by a thread with lower priority but the system is contended, it is
possible for the thread with lower priority to not run until the higher
priority thread has yielded 16 times and goes for __libcpp_mutex_lock().
Once that happens, the OS can bump the priority of the thread that
currently holds the lock to unblock everyone. So instead, we might as
well give the system all the information from the start so it can make
appropriate decisions.

As a fly-by change, also increase the number of locks in the table.
The size increase is modest, but has the potential to half the amount
of contention on those locks.

rdar://93598606

Differential Revision: https://reviews.llvm.org/D126882
This commit is contained in:
Louis Dionne 2022-05-26 17:42:06 -04:00
parent 4a8305ce85
commit 5b386ac912
1 changed files with 11 additions and 12 deletions
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23
libcxx/src/memory.cpp
View File

@ -138,9 +138,13 @@ __shared_weak_count::__get_deleter(const type_info&) const noexcept
#if !defined(_LIBCPP_HAS_NO_THREADS)
static constexpr std::size_t __sp_mut_count = 16;
static constexpr std::size_t __sp_mut_count = 32;
static constinit __libcpp_mutex_t mut_back[__sp_mut_count] =
{
_LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
_LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
_LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
_LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
_LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
_LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
_LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
@ -156,16 +160,7 @@ void
__sp_mut::lock() noexcept
{
auto m = static_cast<__libcpp_mutex_t*>(__lx);
unsigned count = 0;
while (!__libcpp_mutex_trylock(m))
{
if (++count > 16)
{
__libcpp_mutex_lock(m);
break;
}
this_thread::yield();
}
__libcpp_mutex_lock(m);
}
void
@ -181,7 +176,11 @@ __get_sp_mut(const void* p)
&mut_back[ 0], &mut_back[ 1], &mut_back[ 2], &mut_back[ 3],
&mut_back[ 4], &mut_back[ 5], &mut_back[ 6], &mut_back[ 7],
&mut_back[ 8], &mut_back[ 9], &mut_back[10], &mut_back[11],
&mut_back[12], &mut_back[13], &mut_back[14], &mut_back[15]
&mut_back[12], &mut_back[13], &mut_back[14], &mut_back[15],
&mut_back[16], &mut_back[17], &mut_back[18], &mut_back[19],
&mut_back[20], &mut_back[21], &mut_back[22], &mut_back[23],
&mut_back[24], &mut_back[25], &mut_back[26], &mut_back[27],
&mut_back[28], &mut_back[29], &mut_back[30], &mut_back[31]
};
return muts[hash<const void*>()(p) & (__sp_mut_count-1)];
}