[PATCH 3/3] arm64: tlb: skip tlbi broadcast

Andrea Arcangeli
2020-02-23T19:25:38Z

With multiple NUMA nodes and multiple sockets, the tlbi broadcast
shall be delivered through the interconnects in turn increasing the
CPU interconnect traffic and the latency of the tlbi broadcast
instruction. To avoid the synchronous delivery of the tlbi broadcast
before the tlbi instruction can be retired, the hardware would need to
implement a replicated mm_cpumask bitflag for each ASID and every CPU
would need to tell every other CPU which ASID is being loaded. Exactly
what x86 does with mm_cpumask in software.

Even within a single NUMA node the latency of the tlbi broadcast
instruction increases almost linearly with the number of CPUs trying
to send tlbi broadcasts at the same time.

If a single thread of the process is running and it's also running in
the CPU issuing the TLB flush, or if no thread of the process are
running, we can achieve full SMP scalability in the arm64 TLB flushng
by skipping the tlbi broadcasting.

After the local TLB flush this means the ASID context goes out of sync
in all CPUs except the local one. This can be tracked on the per-mm
cpumask: if the bit is set it means the ASID context is stale for that
CPU. This results in an extra local ASID TLB flush only when threads
are running in new CPUs after a TLB flush.

Skipping the tlbi instruction broadcasting is already implemented in
local_flush_tlb_all(), this patch only extends it to flush_tlb_mm(),
flush_tlb_range() and flush_tlb_page() too.

The below benchmarks are measured on a non-NUMA 32 CPUs system (ARMv8
Ampere), so it should be far from a worst case scenario: the
enterprise kernel config allows multiple NUMA nodes with NR_CPUS set
by default to 4096.

=== stock ===

# cat for-each-cpu.sh
#!/bin/bash

for i in $(seq `nproc`); do
"$@" &>/dev/null &
done
wait
# perf stat -r 10 -e dummy ./for-each-cpu.sh ./mprotect-threaded 10000
[..]
2.1696 +- 0.0122 seconds time elapsed ( +- 0.56% )

# perf stat -r 10 -e dummy ./for-each-cpu.sh ./gperftools/tcmalloc_large_heap_fragmentation_unittest
[..]
0.99018 +- 0.00360 seconds time elapsed ( +- 0.36% )

# cat sort-compute
#!/bin/bash

for x in `seq 256`; do
for i in `seq 32`; do /usr/bin/sort </usr/bin/sort >/dev/null; done &
done
wait
# perf stat -r 10 -e dummy ./sort-compute
[..]
1.8094 +- 0.0139 seconds time elapsed ( +- 0.77% )

=== patch applied ===

# perf stat -r 10 -e dummy ./for-each-cpu.sh ./mprotect-threaded 10000
[..]
0.13941 +- 0.00449 seconds time elapsed ( +- 3.22% )

# perf stat -r 10 -e dummy ./for-each-cpu.sh ./gperftools/tcmalloc_large_heap_fragmentation_unittest
[..]
0.90510 +- 0.00262 seconds time elapsed ( +- 0.29% )

# perf stat -r 10 -e dummy ./sort-compute
[..]
1.64025 +- 0.00618 seconds time elapsed ( +- 0.38% )

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
arch/arm64/include/asm/efi.h | 2 +-
arch/arm64/include/asm/mmu.h | 4 +-
arch/arm64/include/asm/mmu_context.h | 33 ++++++++--
arch/arm64/include/asm/tlbflush.h | 95 +++++++++++++++++++++++++++-
arch/arm64/mm/context.c | 54 ++++++++++++++++
5 files changed, 177 insertions(+), 11 deletions(-)

diff --git a/arch/arm64/include/asm/efi.h b/arch/arm64/include/asm/efi.h
index 44531a69d32b..5d9a1433d918 100644
--- a/arch/arm64/include/asm/efi.h
+++ b/arch/arm64/include/asm/efi.h
@@ -131,7 +131,7 @@ static inline void efifb_setup_from_dmi(struct screen_info *si, const char *opt)

static inline void efi_set_pgd(struct mm_struct *mm)
{
- __switch_mm(mm);
+ __switch_mm(mm, smp_processor_id());

if (system_uses_ttbr0_pan()) {
if (mm != current->active_mm) {
diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
index e4d862420bb4..9072fd7bc5f8 100644
--- a/arch/arm64/include/asm/mmu.h
+++ b/arch/arm64/include/asm/mmu.h
@@ -20,6 +20,7 @@ typedef struct {
atomic64_t id;
void *vdso;
unsigned long flags;
+ atomic_t nr_active_mm;
} mm_context_t;

/*
@@ -27,7 +28,8 @@ typedef struct {
* ASID change and therefore doesn't need to reload the counter using
* atomic64_read.
*/
-#define ASID(mm) ((mm)->context.id.counter & 0xffff)
+#define __ASID(asid) ((asid) & 0xffff)
+#define ASID(mm) __ASID((mm)->context.id.counter)

extern bool arm64_use_ng_mappings;

diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
index 3827ff4040a3..9c66fe317e2f 100644
--- a/arch/arm64/include/asm/mmu_context.h
+++ b/arch/arm64/include/asm/mmu_context.h
@@ -175,7 +175,10 @@ static inline void cpu_replace_ttbr1(pgd_t *pgdp)
#define destroy_context(mm) do { } while(0)
void check_and_switch_context(struct mm_struct *mm, unsigned int cpu);

-#define init_new_context(tsk,mm) ({ atomic64_set(&(mm)->context.id, 0); 0; })
+#define init_new_context(tsk,mm) \
+ ({ atomic64_set(&(mm)->context.id, 0); \
+ atomic_set(&(mm)->context.nr_active_mm, 0); \
+ 0; })

#ifdef CONFIG_ARM64_SW_TTBR0_PAN
static inline void update_saved_ttbr0(struct task_struct *tsk,
@@ -203,6 +206,15 @@ static inline void update_saved_ttbr0(struct task_struct *tsk,
static inline void
enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
{
+ unsigned int cpu = smp_processor_id();
+ if (per_cpu(cpu_not_lazy_tlb, cpu) &&
+ is_idle_task(tsk)) {
+ per_cpu(cpu_not_lazy_tlb, cpu) = false;
+ if (!system_uses_ttbr0_pan())
+ cpu_set_reserved_ttbr0();
+ atomic_dec(&mm->context.nr_active_mm);
+ }
+ VM_WARN_ON(atomic_read(&mm->context.nr_active_mm) < 0);
/*
* We don't actually care about the ttbr0 mapping, so point it at the
* zero page.
@@ -210,10 +222,8 @@ enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
update_saved_ttbr0(tsk, &init_mm);
}

-static inline void __switch_mm(struct mm_struct *next)
+static inline void __switch_mm(struct mm_struct *next, unsigned int cpu)
{
- unsigned int cpu = smp_processor_id();
-
/*
* init_mm.pgd does not contain any user mappings and it is always
* active for kernel addresses in TTBR1. Just set the reserved TTBR0.
@@ -230,8 +240,19 @@ static inline void
switch_mm(struct mm_struct *prev, struct mm_struct *next,
struct task_struct *tsk)
{
- if (prev != next)
- __switch_mm(next);
+ unsigned int cpu = smp_processor_id();
+
+ if (!per_cpu(cpu_not_lazy_tlb, cpu)) {
+ per_cpu(cpu_not_lazy_tlb, cpu) = true;
+ atomic_inc(&next->context.nr_active_mm);
+ __switch_mm(next, cpu);
+ } else if (prev != next) {
+ atomic_inc(&next->context.nr_active_mm);
+ __switch_mm(next, cpu);
+ atomic_dec(&prev->context.nr_active_mm);
+ }
+ VM_WARN_ON(!atomic_read(&next->context.nr_active_mm));
+ VM_WARN_ON(atomic_read(&prev->context.nr_active_mm) < 0);

/*
* Update the saved TTBR0_EL1 of the scheduled-in task as the previous
diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
index bc3949064725..0bd987ff9cbd 100644
--- a/arch/arm64/include/asm/tlbflush.h
+++ b/arch/arm64/include/asm/tlbflush.h
@@ -136,6 +136,15 @@ static inline void local_flush_tlb_all(void)
isb();
}

+static inline void local_flush_tlb_asid(unsigned long asid)
+{
+ asid = __TLBI_VADDR(0, __ASID(asid));
+ dsb(nshst);
+ __tlbi(aside1, asid);
+ __tlbi_user(aside1, asid);
+ dsb(nsh);
+}
+
static inline void flush_tlb_all(void)
{
dsb(ishst);
@@ -144,9 +153,38 @@ static inline void flush_tlb_all(void)
isb();
}

+DECLARE_PER_CPU(bool, cpu_not_lazy_tlb);
+
+enum tlb_flush_types {
+ TLB_FLUSH_NO,
+ TLB_FLUSH_LOCAL,
+ TLB_FLUSH_BROADCAST,
+};
+extern enum tlb_flush_types tlb_flush_check(struct mm_struct *mm,
+ unsigned int cpu);
+
static inline void flush_tlb_mm(struct mm_struct *mm)
{
unsigned long asid = __TLBI_VADDR(0, ASID(mm));
+ enum tlb_flush_types flush;
+
+ flush = tlb_flush_check(mm, get_cpu());
+ switch (flush) {
+ case TLB_FLUSH_LOCAL:
+
+ dsb(nshst);
+ __tlbi(aside1, asid);
+ __tlbi_user(aside1, asid);
+ dsb(nsh);
+
+ /* fall through */
+ case TLB_FLUSH_NO:
+ put_cpu();
+ return;
+ case TLB_FLUSH_BROADCAST:
+ break;
+ }
+ put_cpu();

dsb(ishst);
__tlbi(aside1is, asid);
@@ -167,7 +205,31 @@ static inline void flush_tlb_page_nosync(struct vm_area_struct *vma,
static inline void flush_tlb_page(struct vm_area_struct *vma,
unsigned long uaddr)
{
- flush_tlb_page_nosync(vma, uaddr);
+ struct mm_struct *mm = vma->vm_mm;
+ unsigned long addr = __TLBI_VADDR(uaddr, ASID(mm));
+ enum tlb_flush_types flush;
+
+ flush = tlb_flush_check(mm, get_cpu());
+ switch (flush) {
+ case TLB_FLUSH_LOCAL:
+
+ dsb(nshst);
+ __tlbi(vale1, addr);
+ __tlbi_user(vale1, addr);
+ dsb(nsh);
+
+ /* fall through */
+ case TLB_FLUSH_NO:
+ put_cpu();
+ return;
+ case TLB_FLUSH_BROADCAST:
+ break;
+ }
+ put_cpu();
+
+ dsb(ishst);
+ __tlbi(vale1is, addr);
+ __tlbi_user(vale1is, addr);
dsb(ish);
}

@@ -181,14 +243,16 @@ static inline void __flush_tlb_range(struct vm_area_struct *vma,
unsigned long start, unsigned long end,
unsigned long stride, bool last_level)
{
- unsigned long asid = ASID(vma->vm_mm);
+ struct mm_struct *mm = vma->vm_mm;
+ unsigned long asid = ASID(mm);
unsigned long addr;
+ enum tlb_flush_types flush;

start = round_down(start, stride);
end = round_up(end, stride);

if ((end - start) >= (MAX_TLBI_OPS * stride)) {
- flush_tlb_mm(vma->vm_mm);
+ flush_tlb_mm(mm);
return;
}

@@ -198,6 +262,31 @@ static inline void __flush_tlb_range(struct vm_area_struct *vma,
start = __TLBI_VADDR(start, asid);
end = __TLBI_VADDR(end, asid);

+ flush = tlb_flush_check(mm, get_cpu());
+ switch (flush) {
+ case TLB_FLUSH_LOCAL:
+
+ dsb(nshst);
+ for (addr = start; addr < end; addr += stride) {
+ if (last_level) {
+ __tlbi(vale1, addr);
+ __tlbi_user(vale1, addr);
+ } else {
+ __tlbi(vae1, addr);
+ __tlbi_user(vae1, addr);
+ }
+ }
+ dsb(nsh);
+
+ /* fall through */
+ case TLB_FLUSH_NO:
+ put_cpu();
+ return;
+ case TLB_FLUSH_BROADCAST:
+ break;
+ }
+ put_cpu();
+
dsb(ishst);
for (addr = start; addr < end; addr += stride) {
if (last_level) {
diff --git a/arch/arm64/mm/context.c b/arch/arm64/mm/context.c
index 8ef73e89d514..3152b7f7da12 100644
--- a/arch/arm64/mm/context.c
+++ b/arch/arm64/mm/context.c
@@ -25,6 +25,7 @@ static unsigned long *asid_map;
static DEFINE_PER_CPU(atomic64_t, active_asids);
static DEFINE_PER_CPU(u64, reserved_asids);
static cpumask_t tlb_flush_pending;
+DEFINE_PER_CPU(bool, cpu_not_lazy_tlb);

#define ASID_MASK (~GENMASK(asid_bits - 1, 0))
#define ASID_FIRST_VERSION (1UL << asid_bits)
@@ -191,6 +192,12 @@ static u64 new_context(struct mm_struct *mm)
set_asid:
__set_bit(asid, asid_map);
cur_idx = asid;
+ /*
+ * check_and_switch_context() will change the ASID of this mm
+ * so no need of extra ASID local TLB flushes: the new ASID
+ * isn't stale anymore after the tlb_flush_pending was set.
+ */
+ cpumask_clear(mm_cpumask(mm));
return idx2asid(asid) | generation;
}

@@ -240,6 +247,15 @@ void check_and_switch_context(struct mm_struct *mm, unsigned int cpu)
raw_spin_unlock_irqrestore(&cpu_asid_lock, flags);

switch_mm_fastpath:
+ /*
+ * Enforce CPU ordering between the atomic_inc(nr_active_mm)
+ * in switch_mm() and the below cpumask_test_cpu(mm_cpumask).
+ */
+ smp_mb();
+ if (cpumask_test_cpu(cpu, mm_cpumask(mm))) {
+ cpumask_clear_cpu(cpu, mm_cpumask(mm));
+ local_flush_tlb_asid(asid);
+ }

arm64_apply_bp_hardening();

@@ -251,6 +267,44 @@ void check_and_switch_context(struct mm_struct *mm, unsigned int cpu)
cpu_switch_mm(mm->pgd, mm);
}

+enum tlb_flush_types tlb_flush_check(struct mm_struct *mm, unsigned int cpu)
+{
+ if (atomic_read(&mm->context.nr_active_mm) <= 1) {
+ bool is_local = current->active_mm == mm &&
+ per_cpu(cpu_not_lazy_tlb, cpu);
+ cpumask_t *stale_cpumask = mm_cpumask(mm);
+ unsigned int next_zero = cpumask_next_zero(-1, stale_cpumask);
+ bool local_is_clear = false;
+ if (next_zero < nr_cpu_ids &&
+ (is_local && next_zero == cpu)) {
+ next_zero = cpumask_next_zero(next_zero, stale_cpumask);
+ local_is_clear = true;
+ }
+ if (next_zero < nr_cpu_ids) {
+ cpumask_setall(stale_cpumask);
+ local_is_clear = false;
+ }
+
+ /*
+ * Enforce CPU ordering between the above
+ * cpumask_setall(mm_cpumask) and the below
+ * atomic_read(nr_active_mm).
+ */
+ smp_mb();
+
+ if (likely(atomic_read(&mm->context.nr_active_mm)) <= 1) {
+ if (is_local) {
+ if (!local_is_clear)
+ cpumask_clear_cpu(cpu, stale_cpumask);
+ return TLB_FLUSH_LOCAL;
+ }
+ if (atomic_read(&mm->context.nr_active_mm) == 0)
+ return TLB_FLUSH_NO;
+ }
+ }
+ return TLB_FLUSH_BROADCAST;
+}
+
/* Errata workaround post TTBRx_EL1 update. */
asmlinkage void post_ttbr_update_workaround(void)
{