[PATCH] x86/mm/tlb: Remove flush_tlb_info from the stack

[Nadav Amit]

Remove flush_tlb_info variables from 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):

			base		off-stack
			----		---------
avg (per operation)	1.629		1.580	(-3%)
stddev			0.007		0.012

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
 arch/x86/mm/tlb.c | 75 ++++++++++++++++++++++++++++++++++-------------
 1 file changed, 54 insertions(+), 21 deletions(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 487b8474c01c..c4ac66dfb34e 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 +722,62 @@ 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
+
 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;
 	int cpu;
 
-	struct flush_tlb_info info = {
-		.mm = mm,
-		.stride_shift = stride_shift,
-		.freed_tables = freed_tables,
-	};
-
 	cpu = get_cpu();
+	info = this_cpu_ptr(&flush_tlb_info);
+
+	/*
+	 * 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.
+	 */
+#ifdef CONFIG_DEBUG_VM
+	BUG_ON(this_cpu_inc_return(flush_tlb_info_idx) != 1);
+#endif
 
 	/* This is also a barrier that synchronizes with switch_mm(). */
-	info.new_tlb_gen = inc_mm_tlb_gen(mm);
+	info->new_tlb_gen = inc_mm_tlb_gen(mm);
+	info->mm = mm;
+	info->stride_shift = stride_shift;
+	info->freed_tables = freed_tables;
 
 	/* 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;
+		info->start = start;
+		info->end = end;
 	} else {
-		info.start = 0UL;
-		info.end = TLB_FLUSH_ALL;
+		info->start = 0UL;
+		info->end = TLB_FLUSH_ALL;
 	}
 
 	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);
 
+#ifdef CONFIG_DEBUG_VM
+	barrier();
+	this_cpu_dec(flush_tlb_info_idx);
+#endif
 	put_cpu();
 }
 
@@ -787,22 +806,36 @@ 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();
+
+#ifdef CONFIG_DEBUG_VM
+		BUG_ON(this_cpu_inc_return(flush_tlb_info_idx) != 1);
+#endif
+
+		info = this_cpu_ptr(&flush_tlb_info);
+		info->start = start;
+		info->end = end;
+
+		on_each_cpu(do_kernel_range_flush, info, 1);
+
+#ifdef CONFIG_DEBUG_VM
+		barrier();
+		this_cpu_dec(flush_tlb_info_idx);
+#endif
+		preempt_enable();
 	}
 }
 
 void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
 {
-	struct flush_tlb_info info = {
+	static const struct flush_tlb_info info = {
 		.mm = NULL,
 		.start = 0UL,
 		.end = TLB_FLUSH_ALL,
-- 
2.19.1