Subject: [RFC PATCH 2/3] powerpc/64s/radix: reset mm_cpumask for single thread process when possible

From: Nicholas Piggin <[email-masked]>
Date: Thu, 29 Mar 2018 01:29:50 +1000

When a single-threaded process has a non-local mm_cpumask and requires
a full PID tlbie invalidation, use that as an opportunity to reset the
cpumask back to the current CPU we're running on.

No other thread can concurrently switch to this mm, because it must
have had a reference on mm_users before it could use_mm. mm_users can
be asynchronously incremented e.g., by mmget_not_zero, but those
users should not be doing use_mm.
---
 arch/powerpc/include/asm/mmu_context.h | 33 +++++++++++++++-----
 arch/powerpc/include/asm/tlb.h         |  7 +++++
 arch/powerpc/mm/tlb-radix.c            | 57 +++++++++++++++++++++++++---------
 3 files changed, 75 insertions(+), 22 deletions(-)

diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
index 3a15b6db9501..9d373c8fe9fa 100644
--- a/arch/powerpc/include/asm/mmu_context.h
+++ b/arch/powerpc/include/asm/mmu_context.h
@@ -6,6 +6,7 @@
 #include <linux/kernel.h>
 #include <linux/mm.h>
 #include <linux/sched.h>
+#include <linux/sched/mm.h>
 #include <linux/spinlock.h>
 #include <asm/mmu.h>	
 #include <asm/cputable.h>
@@ -102,13 +103,6 @@ static inline void mm_context_add_copro(struct mm_struct *mm)
 
 static inline void mm_context_remove_copro(struct mm_struct *mm)
 {
-	int c;
-
-	c = atomic_dec_if_positive(&mm->context.copros);
-
-	/* Detect imbalance between add and remove */
-	WARN_ON(c < 0);
-
 	/*
 	 * Need to broadcast a global flush of the full mm before
 	 * decrementing active_cpus count, as the next TLBI may be
@@ -119,10 +113,15 @@ static inline void mm_context_remove_copro(struct mm_struct *mm)
 	 * for the time being. Invalidations will remain global if
 	 * used on hash.
 	 */
-	if (c == 0 && radix_enabled()) {
+	if (radix_enabled()) {
 		flush_all_mm(mm);
 		dec_mm_active_cpus(mm);
 	}
+
+	/* Detect imbalance between add and remove */
+	WARN_ON(!atomic_read(&mm->context.copros));
+	atomic_dec(&mm->context.copros);
+
 }
 #else
 static inline void inc_mm_active_cpus(struct mm_struct *mm) { }
@@ -162,6 +161,24 @@ static inline void activate_mm(struct mm_struct *prev, struct mm_struct *next)
 static inline void enter_lazy_tlb(struct mm_struct *mm,
 				  struct task_struct *tsk)
 {
+#ifdef CONFIG_PPC_BOOK3S_64
+	/*
+	 * Under radix, we do not want to keep lazy PIDs around because
+	 * even if the CPU does not access userspace, it can still bring
+	 * in translations through speculation and prefetching.
+	 *
+	 * Switching away here allows us to trim back the mm_cpumask in
+	 * cases where we know the process is not running on some CPUs
+	 * (see mm/tlb-radix.c).
+	 */
+	if (radix_enabled() && mm != &init_mm) {
+		mmgrab(&init_mm);
+		tsk->active_mm = &init_mm;
+		switch_mm_irqs_off(mm, tsk->active_mm, tsk);
+		mmdrop(mm);
+	}
+#endif
+
 	/* 64-bit Book3E keeps track of current PGD in the PACA */
 #ifdef CONFIG_PPC_BOOK3E_64
 	get_paca()->pgd = NULL;
diff --git a/arch/powerpc/include/asm/tlb.h b/arch/powerpc/include/asm/tlb.h
index a7eabff27a0f..c4f43bcc6494 100644
--- a/arch/powerpc/include/asm/tlb.h
+++ b/arch/powerpc/include/asm/tlb.h
@@ -76,6 +76,13 @@ static inline int mm_is_thread_local(struct mm_struct *mm)
 		return false;
 	return cpumask_test_cpu(smp_processor_id(), mm_cpumask(mm));
 }
+static inline void mm_reset_thread_local(struct mm_struct *mm)
+{
+	WARN_ON(!(atomic_read(&mm->mm_users) == 1 && current->mm == mm));
+	atomic_set(&mm->context.active_cpus, 1);
+	cpumask_clear(mm_cpumask(mm));
+	cpumask_set_cpu(smp_processor_id(), mm_cpumask(mm));
+}
 #else /* CONFIG_PPC_BOOK3S_64 */
 static inline int mm_is_thread_local(struct mm_struct *mm)
 {
diff --git a/arch/powerpc/mm/tlb-radix.c b/arch/powerpc/mm/tlb-radix.c
index a07f5372a4bf..f00acacf48f1 100644
--- a/arch/powerpc/mm/tlb-radix.c
+++ b/arch/powerpc/mm/tlb-radix.c
@@ -341,6 +341,15 @@ void radix__local_flush_tlb_page(struct vm_area_struct *vma, unsigned long vmadd
 }
 EXPORT_SYMBOL(radix__local_flush_tlb_page);
 
+static bool mm_is_singlethreaded(struct mm_struct *mm)
+{
+	if (atomic_read(&mm->context.copros) > 0)
+		return false;
+	if (atomic_read(&mm->mm_users) == 1 && current->mm == mm)
+		return true;
+	return false;
+}
+
 static bool mm_needs_flush_escalation(struct mm_struct *mm)
 {
 	/*
@@ -348,7 +357,9 @@ static bool mm_needs_flush_escalation(struct mm_struct *mm)
 	 * caching PTEs and not flushing them properly when
 	 * RIC = 0 for a PID/LPID invalidate
 	 */
-	return atomic_read(&mm->context.copros) != 0;
+	if (atomic_read(&mm->context.copros) > 0)
+		return true;
+	return false;
 }
 
 #ifdef CONFIG_SMP
@@ -362,12 +373,17 @@ void radix__flush_tlb_mm(struct mm_struct *mm)
 
 	preempt_disable();
 	if (!mm_is_thread_local(mm)) {
-		if (mm_needs_flush_escalation(mm))
+		if (mm_is_singlethreaded(mm)) {
 			_tlbie_pid(pid, RIC_FLUSH_ALL);
-		else
+			mm_reset_thread_local(mm);
+		} else if (mm_needs_flush_escalation(mm)) {
+			_tlbie_pid(pid, RIC_FLUSH_ALL);
+		} else {
 			_tlbie_pid(pid, RIC_FLUSH_TLB);
-	} else
+		}
+	} else {
 		_tlbiel_pid(pid, RIC_FLUSH_TLB);
+	}
 	preempt_enable();
 }
 EXPORT_SYMBOL(radix__flush_tlb_mm);
@@ -381,10 +397,13 @@ void radix__flush_all_mm(struct mm_struct *mm)
 		return;
 
 	preempt_disable();
-	if (!mm_is_thread_local(mm))
+	if (!mm_is_thread_local(mm)) {
 		_tlbie_pid(pid, RIC_FLUSH_ALL);
-	else
+		if (mm_is_singlethreaded(mm))
+			mm_reset_thread_local(mm);
+	} else {
 		_tlbiel_pid(pid, RIC_FLUSH_ALL);
+	}
 	preempt_enable();
 }
 EXPORT_SYMBOL(radix__flush_all_mm);
@@ -481,10 +500,14 @@ void radix__flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
 		if (local) {
 			_tlbiel_pid(pid, RIC_FLUSH_TLB);
 		} else {
-			if (mm_needs_flush_escalation(mm))
+			if (mm_is_singlethreaded(mm)) {
+				_tlbie_pid(pid, RIC_FLUSH_ALL);
+				mm_reset_thread_local(mm);
+			} else if (mm_needs_flush_escalation(mm)) {
 				_tlbie_pid(pid, RIC_FLUSH_ALL);
-			else
+			} else {
 				_tlbie_pid(pid, RIC_FLUSH_TLB);
+			}
 		}
 	} else {
 		bool hflush = false;
@@ -596,13 +619,19 @@ static inline void __radix__flush_tlb_range_psize(struct mm_struct *mm,
 	}
 
 	if (full) {
-		if (!local && mm_needs_flush_escalation(mm))
-			also_pwc = true;
-
-		if (local)
+		if (local) {
 			_tlbiel_pid(pid, also_pwc ? RIC_FLUSH_ALL : RIC_FLUSH_TLB);
-		else
-			_tlbie_pid(pid, also_pwc ? RIC_FLUSH_ALL: RIC_FLUSH_TLB);
+		} else {
+			if (mm_is_singlethreaded(mm)) {
+				_tlbie_pid(pid, RIC_FLUSH_ALL);
+				mm_reset_thread_local(mm);
+			} else {
+				if (mm_needs_flush_escalation(mm))
+					also_pwc = true;
+
+				_tlbie_pid(pid, also_pwc ? RIC_FLUSH_ALL : RIC_FLUSH_TLB);
+			}
+		}
 	} else {
 		if (local)
 			_tlbiel_va_range(start, end, pid, page_size, psize, also_pwc);
-- 
2.16.1