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Kernel: Make TSC based timer monotonic

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Prior to this I was just blindly rebasing the current TSC stats every
once in a while. Now I check for drift and scale the multiplier
accordingly to keep the timer from drifting
This commit is contained in:
Bananymous
2026-06-26 01:32:04 +03:00
parent ac0ef53e87
commit 59ec05c898
8 changed files with 172 additions and 75 deletions
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117
kernel/kernel/Timer/Timer.cpp
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@@ -59,62 +59,65 @@ namespace Kernel
void SystemTimer::initialize_tsc()
{
if (!CPUID::has_invariant_tsc())
{
dwarnln("CPU does not have an invariant TSC");
return;
}
if (CPUID::has_invariant_tsc())
return initialize_invariant_tsc();
dwarnln("No supported TSC based timers available");
}
const uint64_t tsc_freq = get_tsc_frequency();
void SystemTimer::initialize_invariant_tsc()
{
const uint64_t tsc_freq = [this]() -> uint64_t {
if (const auto cpuid_freq = CPUID::get_tsc_frequency())
return cpuid_freq;
// take 5x 50 ms samples and use the median value
constexpr size_t tsc_sample_count = 5;
constexpr size_t tsc_sample_ns = 50'000'000;
uint64_t tsc_freq_samples[tsc_sample_count];
for (size_t i = 0; i < tsc_sample_count; i++)
{
const auto start_ns = m_timer->ns_since_boot();
const auto start_tsc = __builtin_ia32_rdtsc();
while (m_timer->ns_since_boot() < start_ns + tsc_sample_ns)
Processor::pause();
const auto stop_tsc = __builtin_ia32_rdtsc();
const auto stop_ns = m_timer->ns_since_boot();
const auto duration_ns = stop_ns - start_ns;
const auto count_tsc = stop_tsc - start_tsc;
tsc_freq_samples[i] = count_tsc * 1'000'000'000 / duration_ns;
}
BAN::sort::sort(tsc_freq_samples, tsc_freq_samples + tsc_sample_count);
return tsc_freq_samples[tsc_sample_count / 2];
}();
m_tsc_info = { .invariant = {
.shift = 0,
.mult = static_cast<uint32_t>((1'000'000'000ull << 32) / tsc_freq),
}};
m_tsc_type = TSCType::Invariant;
Processor::initialize_tsc(m_boot_time);
dprintln("Initialized invariant TSC ({} Hz)", tsc_freq);
const uint8_t tsc_shift = 22;
const uint64_t tsc_mult = (static_cast<uint64_t>(1'000'000'000) << tsc_shift) / tsc_freq;
Processor::initialize_tsc(tsc_shift, tsc_mult, m_boot_time);
m_has_invariant_tsc = true;
}
uint64_t SystemTimer::get_tsc_frequency() const
void SystemTimer::update_tsc()
{
// take 5x 50 ms samples and use the median value
constexpr size_t tsc_sample_count = 5;
constexpr size_t tsc_sample_ns = 50'000'000;
uint64_t tsc_freq_samples[tsc_sample_count];
for (size_t i = 0; i < tsc_sample_count; i++)
{
const auto start_ns = m_timer->ns_since_boot();
const auto start_tsc = __builtin_ia32_rdtsc();
while (m_timer->ns_since_boot() < start_ns + tsc_sample_ns)
Processor::pause();
const auto stop_tsc = __builtin_ia32_rdtsc();
const auto stop_ns = m_timer->ns_since_boot();
const auto duration_ns = stop_ns - start_ns;
const auto count_tsc = stop_tsc - start_tsc;
tsc_freq_samples[i] = count_tsc * 1'000'000'000 / duration_ns;
}
BAN::sort::sort(tsc_freq_samples, tsc_freq_samples + tsc_sample_count);
return tsc_freq_samples[tsc_sample_count / 2];
}
void SystemTimer::update_tsc() const
{
if (!m_has_invariant_tsc)
if (m_tsc_type == TSCType::None)
return;
// only update every 100 ms
if (++m_timer_ticks < 100)
// only update once per second
const uint64_t current_ns = Processor::ns_since_boot_tsc();
if (current_ns < m_tsc_update_ns)
return;
m_timer_ticks = 0;
m_tsc_update_ns = current_ns + 1'000'000'000;
Processor::update_tsc();
Processor::broadcast_smp_message({
@@ -123,28 +126,38 @@ namespace Kernel
});
}
uint64_t SystemTimer::ns_since_boot_no_tsc() const
SystemTimer::TSCInfo SystemTimer::tsc_info() const
{
return m_timer->ns_since_boot();
switch (m_tsc_type)
{
case TSCType::None:
ASSERT_NOT_REACHED();
case TSCType::Invariant:
return {
.shift = m_tsc_info.invariant.shift,
.mult = m_tsc_info.invariant.mult,
};
}
ASSERT_NOT_REACHED();
}
uint64_t SystemTimer::ms_since_boot() const
{
if (!m_has_invariant_tsc)
if (m_tsc_type == TSCType::None)
return m_timer->ms_since_boot();
return Processor::ns_since_boot_tsc() / 1'000'000;
}
uint64_t SystemTimer::ns_since_boot() const
{
if (!m_has_invariant_tsc)
if (m_tsc_type == TSCType::None)
return m_timer->ns_since_boot();
return Processor::ns_since_boot_tsc();
}
timespec SystemTimer::time_since_boot() const
{
if (!m_has_invariant_tsc)
if (m_tsc_type == TSCType::None)
return m_timer->time_since_boot();
const auto ns_since_boot = Processor::ns_since_boot_tsc();
return {