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Kernel: Optimize futexes 

Eeach futex object now has its own mutex to prevent unnecessary locking
of the process/global futex lock. This basically removes sys_futex from
profiles when running software with llvmpipe
This commit is contained in:
Bananymous 2026-01-13 19:18:52 +02:00
parent c30fc9d60f
commit a83fa6f4c6
2 changed files with 36 additions and 19 deletions
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1
kernel/include/kernel/Process.h
View File

@ -348,6 +348,7 @@ namespace Kernel
struct futex_t
{
Mutex mutex;
ThreadBlocker blocker;
uint32_t waiters { 0 };
uint32_t to_wakeup { 0 };

46
kernel/kernel/Process.cpp
View File

@ -3142,11 +3142,32 @@ namespace Kernel
return SystemTimer::get().ns_since_boot() + (abs_ns - real_ns);
}());
if (op == FUTEX_WAIT && BAN::atomic_load(*addr) != val)
return BAN::Error::from_errno(EAGAIN);
futex_t* futex;
{
auto& futex_lock = is_private ? m_futex_lock : s_futex_lock;
auto& futexes = is_private ? m_futexes : s_futexes;
LockGuard _(futex_lock);
auto it = futexes.find(paddr);
if (it != futexes.end())
futex = it->value.ptr();
else switch (op)
{
case FUTEX_WAIT:
futex = TRY(futexes.emplace(paddr, TRY(BAN::UniqPtr<futex_t>::create())))->value.ptr();
break;
case FUTEX_WAKE:
return 0;
}
}
LockGuard _(futex->mutex);
switch (op)
{
case FUTEX_WAIT:
@ -3154,20 +3175,17 @@ namespace Kernel
if (BAN::atomic_load(*addr) != val)
return BAN::Error::from_errno(EAGAIN);
auto it = futexes.find(paddr);
if (it == futexes.end())
it = TRY(futexes.emplace(paddr, TRY(BAN::UniqPtr<futex_t>::create())));
futex_t* const futex = it->value.ptr();
futex->waiters++;
BAN::ScopeGuard _([futex] {
// TODO: Lazily deallocate unused futex objects at some point (?)
futex->waiters--;
});
// TODO: Deallocate unused futex objects at some point (?)
// We don't want to do it on every operation as allocation
// and deletion slows this down a lot and the same addresses
// will be probably used again
BAN::ScopeGuard cleanup([futex] { futex->waiters--; });
for (;;)
{
TRY(Thread::current().block_or_eintr_or_waketime_ns(futex->blocker, wake_time_ns, true, &futex_lock));
TRY(Thread::current().block_or_eintr_or_waketime_ns(futex->blocker, wake_time_ns, true, &futex->mutex));
if (BAN::atomic_load(*addr) == val || futex->to_wakeup == 0)
continue;
futex->to_wakeup--;
@ -3176,18 +3194,16 @@ namespace Kernel
}
case FUTEX_WAKE:
{
auto it = futexes.find(paddr);
if (it == futexes.end())
return 0;
futex_t* const futex = it->value.ptr();
if (BAN::Math::will_addition_overflow(futex->to_wakeup, val))
futex->to_wakeup = BAN::numeric_limits<uint32_t>::max();
else
futex->to_wakeup += val;
futex->to_wakeup = BAN::Math::min(futex->to_wakeup, futex->waiters);
if (futex->to_wakeup > 0)
futex->blocker.unblock();
return 0;
}
}