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tcg: introduce dynamic TLB sizing

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Disabled in all TCG backends for now.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <20190116170114.26802-3-cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Emilio G. Cota 2019-01-16 12:01:13 -05:00 • committed by Richard Henderson
parent 3cea94bbc9
commit 86e1eff8bc
12 changed files with 282 additions and 7 deletions
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202
accel/tcg/cputlb.c
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@ -74,6 +74,187 @@ QEMU_BUILD_BUG_ON(sizeof(target_ulong) > sizeof(run_on_cpu_data));
QEMU_BUILD_BUG_ON(NB_MMU_MODES > 16);
#define ALL_MMUIDX_BITS ((1 << NB_MMU_MODES) - 1)
#if TCG_TARGET_IMPLEMENTS_DYN_TLB
static inline size_t sizeof_tlb(CPUArchState *env, uintptr_t mmu_idx)
{
return env->tlb_mask[mmu_idx] + (1 << CPU_TLB_ENTRY_BITS);
}
static void tlb_window_reset(CPUTLBWindow *window, int64_t ns,
size_t max_entries)
{
window->begin_ns = ns;
window->max_entries = max_entries;
}
static void tlb_dyn_init(CPUArchState *env)
{
int i;
for (i = 0; i < NB_MMU_MODES; i++) {
CPUTLBDesc *desc = &env->tlb_d[i];
size_t n_entries = 1 << CPU_TLB_DYN_DEFAULT_BITS;
tlb_window_reset(&desc->window, get_clock_realtime(), 0);
desc->n_used_entries = 0;
env->tlb_mask[i] = (n_entries - 1) << CPU_TLB_ENTRY_BITS;
env->tlb_table[i] = g_new(CPUTLBEntry, n_entries);
env->iotlb[i] = g_new(CPUIOTLBEntry, n_entries);
}
}
/**
* tlb_mmu_resize_locked() - perform TLB resize bookkeeping; resize if necessary
* @env: CPU that owns the TLB
* @mmu_idx: MMU index of the TLB
*
* Called with tlb_lock_held.
*
* We have two main constraints when resizing a TLB: (1) we only resize it
* on a TLB flush (otherwise we'd have to take a perf hit by either rehashing
* the array or unnecessarily flushing it), which means we do not control how
* frequently the resizing can occur; (2) we don't have access to the guest's
* future scheduling decisions, and therefore have to decide the magnitude of
* the resize based on past observations.
*
* In general, a memory-hungry process can benefit greatly from an appropriately
* sized TLB, since a guest TLB miss is very expensive. This doesn't mean that
* we just have to make the TLB as large as possible; while an oversized TLB
* results in minimal TLB miss rates, it also takes longer to be flushed
* (flushes can be _very_ frequent), and the reduced locality can also hurt
* performance.
*
* To achieve near-optimal performance for all kinds of workloads, we:
*
* 1. Aggressively increase the size of the TLB when the use rate of the
* TLB being flushed is high, since it is likely that in the near future this
* memory-hungry process will execute again, and its memory hungriness will
* probably be similar.
*
* 2. Slowly reduce the size of the TLB as the use rate declines over a
* reasonably large time window. The rationale is that if in such a time window
* we have not observed a high TLB use rate, it is likely that we won't observe
* it in the near future. In that case, once a time window expires we downsize
* the TLB to match the maximum use rate observed in the window.
*
* 3. Try to keep the maximum use rate in a time window in the 30-70% range,
* since in that range performance is likely near-optimal. Recall that the TLB
* is direct mapped, so we want the use rate to be low (or at least not too
* high), since otherwise we are likely to have a significant amount of
* conflict misses.
*/
static void tlb_mmu_resize_locked(CPUArchState *env, int mmu_idx)
{
CPUTLBDesc *desc = &env->tlb_d[mmu_idx];
size_t old_size = tlb_n_entries(env, mmu_idx);
size_t rate;
size_t new_size = old_size;
int64_t now = get_clock_realtime();
int64_t window_len_ms = 100;
int64_t window_len_ns = window_len_ms * 1000 * 1000;
bool window_expired = now > desc->window.begin_ns + window_len_ns;
if (desc->n_used_entries > desc->window.max_entries) {
desc->window.max_entries = desc->n_used_entries;
}
rate = desc->window.max_entries * 100 / old_size;
if (rate > 70) {
new_size = MIN(old_size << 1, 1 << CPU_TLB_DYN_MAX_BITS);
} else if (rate < 30 && window_expired) {
size_t ceil = pow2ceil(desc->window.max_entries);
size_t expected_rate = desc->window.max_entries * 100 / ceil;
/*
* Avoid undersizing when the max number of entries seen is just below
* a pow2. For instance, if max_entries == 1025, the expected use rate
* would be 1025/2048==50%. However, if max_entries == 1023, we'd get
* 1023/1024==99.9% use rate, so we'd likely end up doubling the size
* later. Thus, make sure that the expected use rate remains below 70%.
* (and since we double the size, that means the lowest rate we'd
* expect to get is 35%, which is still in the 30-70% range where
* we consider that the size is appropriate.)
*/
if (expected_rate > 70) {
ceil *= 2;
}
new_size = MAX(ceil, 1 << CPU_TLB_DYN_MIN_BITS);
}
if (new_size == old_size) {
if (window_expired) {
tlb_window_reset(&desc->window, now, desc->n_used_entries);
}
return;
}
g_free(env->tlb_table[mmu_idx]);
g_free(env->iotlb[mmu_idx]);
tlb_window_reset(&desc->window, now, 0);
/* desc->n_used_entries is cleared by the caller */
env->tlb_mask[mmu_idx] = (new_size - 1) << CPU_TLB_ENTRY_BITS;
env->tlb_table[mmu_idx] = g_try_new(CPUTLBEntry, new_size);
env->iotlb[mmu_idx] = g_try_new(CPUIOTLBEntry, new_size);
/*
* If the allocations fail, try smaller sizes. We just freed some
* memory, so going back to half of new_size has a good chance of working.
* Increased memory pressure elsewhere in the system might cause the
* allocations to fail though, so we progressively reduce the allocation
* size, aborting if we cannot even allocate the smallest TLB we support.
*/
while (env->tlb_table[mmu_idx] == NULL || env->iotlb[mmu_idx] == NULL) {
if (new_size == (1 << CPU_TLB_DYN_MIN_BITS)) {
error_report("%s: %s", __func__, strerror(errno));
abort();
}
new_size = MAX(new_size >> 1, 1 << CPU_TLB_DYN_MIN_BITS);
env->tlb_mask[mmu_idx] = (new_size - 1) << CPU_TLB_ENTRY_BITS;
g_free(env->tlb_table[mmu_idx]);
g_free(env->iotlb[mmu_idx]);
env->tlb_table[mmu_idx] = g_try_new(CPUTLBEntry, new_size);
env->iotlb[mmu_idx] = g_try_new(CPUIOTLBEntry, new_size);
}
}
static inline void tlb_table_flush_by_mmuidx(CPUArchState *env, int mmu_idx)
{
tlb_mmu_resize_locked(env, mmu_idx);
memset(env->tlb_table[mmu_idx], -1, sizeof_tlb(env, mmu_idx));
env->tlb_d[mmu_idx].n_used_entries = 0;
}
static inline void tlb_n_used_entries_inc(CPUArchState *env, uintptr_t mmu_idx)
{
env->tlb_d[mmu_idx].n_used_entries++;
}
static inline void tlb_n_used_entries_dec(CPUArchState *env, uintptr_t mmu_idx)
{
env->tlb_d[mmu_idx].n_used_entries--;
}
#else /* !TCG_TARGET_IMPLEMENTS_DYN_TLB */
static inline void tlb_dyn_init(CPUArchState *env)
{
}
static inline void tlb_table_flush_by_mmuidx(CPUArchState *env, int mmu_idx)
{
memset(env->tlb_table[mmu_idx], -1, sizeof(env->tlb_table[0]));
}
static inline void tlb_n_used_entries_inc(CPUArchState *env, uintptr_t mmu_idx)
{
}
static inline void tlb_n_used_entries_dec(CPUArchState *env, uintptr_t mmu_idx)
{
}
#endif /* TCG_TARGET_IMPLEMENTS_DYN_TLB */
void tlb_init(CPUState *cpu)
{
CPUArchState *env = cpu->env_ptr;
@ -82,6 +263,8 @@ void tlb_init(CPUState *cpu)
/* Ensure that cpu_reset performs a full flush. */
env->tlb_c.dirty = ALL_MMUIDX_BITS;
tlb_dyn_init(env);
}
/* flush_all_helper: run fn across all cpus
@ -122,7 +305,7 @@ void tlb_flush_counts(size_t *pfull, size_t *ppart, size_t *pelide)
static void tlb_flush_one_mmuidx_locked(CPUArchState *env, int mmu_idx)
{
memset(env->tlb_table[mmu_idx], -1, sizeof(env->tlb_table[0]));
tlb_table_flush_by_mmuidx(env, mmu_idx);
memset(env->tlb_v_table[mmu_idx], -1, sizeof(env->tlb_v_table[0]));
env->tlb_d[mmu_idx].large_page_addr = -1;
env->tlb_d[mmu_idx].large_page_mask = -1;
@ -234,12 +417,14 @@ static inline bool tlb_entry_is_empty(const CPUTLBEntry *te)
}
/* Called with tlb_c.lock held */
static inline void tlb_flush_entry_locked(CPUTLBEntry *tlb_entry,
static inline bool tlb_flush_entry_locked(CPUTLBEntry *tlb_entry,
target_ulong page)
{
if (tlb_hit_page_anyprot(tlb_entry, page)) {
memset(tlb_entry, -1, sizeof(*tlb_entry));
return true;
}
return false;
}
/* Called with tlb_c.lock held */
@ -250,7 +435,9 @@ static inline void tlb_flush_vtlb_page_locked(CPUArchState *env, int mmu_idx,
assert_cpu_is_self(ENV_GET_CPU(env));
for (k = 0; k < CPU_VTLB_SIZE; k++) {
tlb_flush_entry_locked(&env->tlb_v_table[mmu_idx][k], page);
if (tlb_flush_entry_locked(&env->tlb_v_table[mmu_idx][k], page)) {
tlb_n_used_entries_dec(env, mmu_idx);
}
}
}
@ -267,7 +454,9 @@ static void tlb_flush_page_locked(CPUArchState *env, int midx,
midx, lp_addr, lp_mask);
tlb_flush_one_mmuidx_locked(env, midx);
} else {
tlb_flush_entry_locked(tlb_entry(env, midx, page), page);
if (tlb_flush_entry_locked(tlb_entry(env, midx, page), page)) {
tlb_n_used_entries_dec(env, midx);
}
tlb_flush_vtlb_page_locked(env, midx, page);
}
}
@ -444,8 +633,9 @@ void tlb_reset_dirty(CPUState *cpu, ram_addr_t start1, ram_addr_t length)
qemu_spin_lock(&env->tlb_c.lock);
for (mmu_idx = 0; mmu_idx < NB_MMU_MODES; mmu_idx++) {
unsigned int i;
unsigned int n = tlb_n_entries(env, mmu_idx);
for (i = 0; i < CPU_TLB_SIZE; i++) {
for (i = 0; i < n; i++) {
tlb_reset_dirty_range_locked(&env->tlb_table[mmu_idx][i], start1,
length);
}
@ -607,6 +797,7 @@ void tlb_set_page_with_attrs(CPUState *cpu, target_ulong vaddr,
/* Evict the old entry into the victim tlb. */
copy_tlb_helper_locked(tv, te);
env->iotlb_v[mmu_idx][vidx] = env->iotlb[mmu_idx][index];
tlb_n_used_entries_dec(env, mmu_idx);
}
/* refill the tlb */
@ -658,6 +849,7 @@ void tlb_set_page_with_attrs(CPUState *cpu, target_ulong vaddr,
}
copy_tlb_helper_locked(te, &tn);
tlb_n_used_entries_inc(env, mmu_idx);
qemu_spin_unlock(&env->tlb_c.lock);
}