Commit 4e57b946 authored by Andy Lutomirski's avatar Andy Lutomirski Committed by Ingo Molnar

x86/mm: Tidy up "x86/mm: Flush more aggressively in lazy TLB mode"

Due to timezones, commit:

  b956575b ("x86/mm: Flush more aggressively in lazy TLB mode")

was an outdated patch that well tested and fixed the bug but didn't
address Borislav's review comments.

Tidy it up:

 - The name "tlb_use_lazy_mode()" was highly confusing.  Change it to
   "tlb_defer_switch_to_init_mm()", which describes what it actually

 - Move the static_branch crap into a helper.

 - Improve comments.

Actually removing the debugfs option is in the next patch.
Reported-by: default avatarBorislav Petkov <>
Signed-off-by: default avatarAndy Lutomirski <>
Cc: Linus Torvalds <>
Cc: Peter Zijlstra <>
Cc: Thomas Gleixner <>
Fixes: b956575b ("x86/mm: Flush more aggressively in lazy TLB mode")
Link: default avatarIngo Molnar <>
parent e8b9b0cc
......@@ -87,7 +87,12 @@ static inline u64 inc_mm_tlb_gen(struct mm_struct *mm)
* to init_mm when we switch to a kernel thread (e.g. the idle thread). If
* it's false, then we immediately switch CR3 when entering a kernel thread.
static inline bool tlb_defer_switch_to_init_mm(void)
return static_branch_unlikely(&__tlb_defer_switch_to_init_mm);
* 6 because 6 should be plenty and struct tlb_state will fit in
......@@ -30,7 +30,7 @@
atomic64_t last_mm_ctx_id = ATOMIC64_INIT(1);
static void choose_new_asid(struct mm_struct *next, u64 next_tlb_gen,
u16 *new_asid, bool *need_flush)
......@@ -213,6 +213,9 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
* Please ignore the name of this function. It should be called
* switch_to_kernel_thread().
* enter_lazy_tlb() is a hint from the scheduler that we are entering a
* kernel thread or other context without an mm. Acceptable implementations
* include doing nothing whatsoever, switching to init_mm, or various clever
......@@ -227,7 +230,7 @@ void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
if (this_cpu_read(cpu_tlbstate.loaded_mm) == &init_mm)
if (static_branch_unlikely(&tlb_use_lazy_mode)) {
if (tlb_defer_switch_to_init_mm()) {
* There's a significant optimization that may be possible
* here. We have accurate enough TLB flush tracking that we
......@@ -632,7 +635,8 @@ static ssize_t tlblazy_read_file(struct file *file, char __user *user_buf,
char buf[2];
buf[0] = static_branch_likely(&tlb_use_lazy_mode) ? '1' : '0';
buf[0] = static_branch_likely(&__tlb_defer_switch_to_init_mm)
? '1' : '0';
buf[1] = '\n';
return simple_read_from_buffer(user_buf, count, ppos, buf, 2);
......@@ -647,9 +651,9 @@ static ssize_t tlblazy_write_file(struct file *file,
return -EINVAL;
if (val)
return count;
......@@ -660,23 +664,25 @@ static const struct file_operations fops_tlblazy = {
.llseek = default_llseek,
static int __init init_tlb_use_lazy_mode(void)
static int __init init_tlblazy(void)
if (boot_cpu_has(X86_FEATURE_PCID)) {
* Heuristic: with PCID on, switching to and from
* init_mm is reasonably fast, but remote flush IPIs
* as expensive as ever, so turn off lazy TLB mode.
* If we have PCID, then switching to init_mm is reasonably
* fast. If we don't have PCID, then switching to init_mm is
* quite slow, so we default to trying to defer it in the
* hopes that we can avoid it entirely. The latter approach
* runs the risk of receiving otherwise unnecessary IPIs.
* We can't do this in setup_pcid() because static keys
* haven't been initialized yet, and it would blow up
* badly.
debugfs_create_file("tlb_use_lazy_mode", S_IRUSR | S_IWUSR,
debugfs_create_file("tlb_defer_switch_to_init_mm", S_IRUSR | S_IWUSR,
arch_debugfs_dir, NULL, &fops_tlblazy);
return 0;
Markdown is supported
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment