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Subject[PATCH v3] x86/mm/tlb: Remove flush_tlb_info from the stack
Date
Move flush_tlb_info variables off the stack. This allows to align
flush_tlb_info to cache-line and avoid potentially unnecessary cache
line movements. It also allows to have a fixed virtual-to-physical
translation of the variables, which reduces TLB misses.

Use per-CPU struct for flush_tlb_mm_range() and
flush_tlb_kernel_range(). Add debug assertions to ensure there are
no nested TLB flushes that might overwrite the per-CPU data. For
arch_tlbbatch_flush() use a const struct.

Results when running a microbenchmarks that performs 10^6 MADV_DONTEED
operations and touching a page, in which 3 additional threads run a
busy-wait loop (5 runs, PTI and retpolines are turned off):

base off-stack
---- ---------
avg (usec/op) 1.629 1.570 (-3%)
stddev 0.014 0.009

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Nadav Amit <namit@vmware.com>

---

v2->v3:
- Remove redundant assignment operations
- Formatting, comments, adding full_flush_tlb_info [Ingo]

v1->v2:
- Initialize all flush_tlb_info fields [Andy]
---
arch/x86/mm/tlb.c | 116 ++++++++++++++++++++++++++++++++--------------
1 file changed, 82 insertions(+), 34 deletions(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 487b8474c01c..7f61431c75fb 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -634,7 +634,7 @@ static void flush_tlb_func_common(const struct flush_tlb_info *f,
this_cpu_write(cpu_tlbstate.ctxs[loaded_mm_asid].tlb_gen, mm_tlb_gen);
}

-static void flush_tlb_func_local(void *info, enum tlb_flush_reason reason)
+static void flush_tlb_func_local(const void *info, enum tlb_flush_reason reason)
{
const struct flush_tlb_info *f = info;

@@ -722,43 +722,81 @@ void native_flush_tlb_others(const struct cpumask *cpumask,
*/
unsigned long tlb_single_page_flush_ceiling __read_mostly = 33;

+static DEFINE_PER_CPU_SHARED_ALIGNED(struct flush_tlb_info, flush_tlb_info);
+
+#ifdef CONFIG_DEBUG_VM
+static DEFINE_PER_CPU(unsigned int, flush_tlb_info_idx);
+#endif
+
+static inline struct flush_tlb_info *get_flush_tlb_info(struct mm_struct *mm,
+ unsigned long start, unsigned long end,
+ unsigned int stride_shift, bool freed_tables,
+ u64 new_tlb_gen)
+{
+ struct flush_tlb_info *info = this_cpu_ptr(&flush_tlb_info);
+
+#ifdef CONFIG_DEBUG_VM
+ /*
+ * Ensure that the following code is non-reentrant and flush_tlb_info
+ * is not overwritten. This means no TLB flushing is initiated by
+ * interrupt handlers and machine-check exception handlers.
+ */
+ BUG_ON(this_cpu_inc_return(flush_tlb_info_idx) != 1);
+#endif
+
+ info->start = start;
+ info->end = end;
+ info->mm = mm;
+ info->stride_shift = stride_shift;
+ info->freed_tables = freed_tables;
+ info->new_tlb_gen = new_tlb_gen;
+
+ return info;
+}
+
+static inline void put_flush_tlb_info(void)
+{
+#ifdef CONFIG_DEBUG_VM
+ /* Complete reentrency prevention checks */
+ barrier();
+ this_cpu_dec(flush_tlb_info_idx);
+#endif
+}
+
void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
unsigned long end, unsigned int stride_shift,
bool freed_tables)
{
+ struct flush_tlb_info *info;
+ u64 new_tlb_gen;
int cpu;

- struct flush_tlb_info info = {
- .mm = mm,
- .stride_shift = stride_shift,
- .freed_tables = freed_tables,
- };
-
cpu = get_cpu();

- /* This is also a barrier that synchronizes with switch_mm(). */
- info.new_tlb_gen = inc_mm_tlb_gen(mm);
-
/* Should we flush just the requested range? */
- if ((end != TLB_FLUSH_ALL) &&
- ((end - start) >> stride_shift) <= tlb_single_page_flush_ceiling) {
- info.start = start;
- info.end = end;
- } else {
- info.start = 0UL;
- info.end = TLB_FLUSH_ALL;
+ if ((end == TLB_FLUSH_ALL) ||
+ ((end - start) >> stride_shift) > tlb_single_page_flush_ceiling) {
+ start = 0;
+ end = TLB_FLUSH_ALL;
}

+ /* This is also a barrier that synchronizes with switch_mm(). */
+ new_tlb_gen = inc_mm_tlb_gen(mm);
+
+ info = get_flush_tlb_info(mm, start, end, stride_shift, freed_tables,
+ new_tlb_gen);
+
if (mm == this_cpu_read(cpu_tlbstate.loaded_mm)) {
- VM_WARN_ON(irqs_disabled());
+ lockdep_assert_irqs_enabled();
local_irq_disable();
- flush_tlb_func_local(&info, TLB_LOCAL_MM_SHOOTDOWN);
+ flush_tlb_func_local(info, TLB_LOCAL_MM_SHOOTDOWN);
local_irq_enable();
}

if (cpumask_any_but(mm_cpumask(mm), cpu) < nr_cpu_ids)
- flush_tlb_others(mm_cpumask(mm), &info);
+ flush_tlb_others(mm_cpumask(mm), info);

+ put_flush_tlb_info();
put_cpu();
}

@@ -787,38 +825,48 @@ static void do_kernel_range_flush(void *info)

void flush_tlb_kernel_range(unsigned long start, unsigned long end)
{
-
/* Balance as user space task's flush, a bit conservative */
if (end == TLB_FLUSH_ALL ||
(end - start) > tlb_single_page_flush_ceiling << PAGE_SHIFT) {
on_each_cpu(do_flush_tlb_all, NULL, 1);
} else {
- struct flush_tlb_info info;
- info.start = start;
- info.end = end;
- on_each_cpu(do_kernel_range_flush, &info, 1);
+ struct flush_tlb_info *info;
+
+ preempt_disable();
+ info = get_flush_tlb_info(NULL, start, end, 0, false, 0);
+
+ on_each_cpu(do_kernel_range_flush, info, 1);
+
+ put_flush_tlb_info();
+ preempt_enable();
}
}

+/*
+ * arch_tlbbatch_flush() performs a full TLB flush regardless of the active mm.
+ * This means that the 'struct flush_tlb_info' that describes which mappings to
+ * flush is actually fixed. We therefore set a single fixed struct and use it in
+ * arch_tlbbatch_flush().
+ */
+static const struct flush_tlb_info full_flush_tlb_info = {
+ .mm = NULL,
+ .start = 0,
+ .end = TLB_FLUSH_ALL,
+};
+
void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
{
- struct flush_tlb_info info = {
- .mm = NULL,
- .start = 0UL,
- .end = TLB_FLUSH_ALL,
- };
-
int cpu = get_cpu();

if (cpumask_test_cpu(cpu, &batch->cpumask)) {
- VM_WARN_ON(irqs_disabled());
+ lockdep_assert_irqs_enabled();
local_irq_disable();
- flush_tlb_func_local(&info, TLB_LOCAL_SHOOTDOWN);
+ flush_tlb_func_local(&full_flush_tlb_info, TLB_LOCAL_SHOOTDOWN);
local_irq_enable();
}

if (cpumask_any_but(&batch->cpumask, cpu) < nr_cpu_ids)
- flush_tlb_others(&batch->cpumask, &info);
+ flush_tlb_others(&batch->cpumask, &full_flush_tlb_info);

cpumask_clear(&batch->cpumask);

--
2.19.1
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