On Tue, Jan 21, 2025 at 10:55:07AM +0100, Peter Zijlstra wrote:
> On Sun, Jan 19, 2025 at 09:40:17PM -0500, Rik van Riel wrote:
> > +/*
> > + * Figure out whether to assign a global ASID to a process.
> > + * We vary the threshold by how empty or full global ASID space is.
> > + * 1/4 full: >= 4 active threads
> > + * 1/2 full: >= 8 active threads
> > + * 3/4 full: >= 16 active threads
> > + * 7/8 full: >= 32 active threads
> > + * etc
> > + *
> > + * This way we should never exhaust the global ASID space, even on very
> > + * large systems, and the processes with the largest number of active
> > + * threads should be able to use broadcast TLB invalidation.
> > + */
> > +#define HALFFULL_THRESHOLD 8
> > +static bool meets_global_asid_threshold(struct mm_struct *mm)
> > +{
> > + int avail = global_asid_available;
> > + int threshold = HALFFULL_THRESHOLD;
> > +
> > + if (!avail)
> > + return false;
> > +
> > + if (avail > MAX_ASID_AVAILABLE * 3 / 4) {
> > + threshold = HALFFULL_THRESHOLD / 4;
> > + } else if (avail > MAX_ASID_AVAILABLE / 2) {
> > + threshold = HALFFULL_THRESHOLD / 2;
> > + } else if (avail < MAX_ASID_AVAILABLE / 3) {
> > + do {
> > + avail *= 2;
> > + threshold *= 2;
> > + } while ((avail + threshold) < MAX_ASID_AVAILABLE / 2);
> > + }
> > +
> > + return mm_active_cpus_exceeds(mm, threshold);
> > +}
>
> I'm still very much disliking this. Why do we need this? Yes, running
> out of ASID space is a pain, but this increasing threshold also makes
> things behave weird.
>
> Suppose our most used processes starts slow, and ends up not getting an
> ASID because too much irrelevant crap gets started before it spawns
> enough threads and then no longer qualifies.
>
> Can't we just start with a very simple constant test and poke at things
> if/when its found to not work?
Something like so perhaps?
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -268,7 +268,7 @@ static inline u16 mm_global_asid(struct
if (!cpu_feature_enabled(X86_FEATURE_INVLPGB))
return 0;
- asid = READ_ONCE(mm->context.global_asid);
+ asid = smp_load_acquire(&mm->context.global_asid);
/* mm->context.global_asid is either 0, or a global ASID */
VM_WARN_ON_ONCE(is_dyn_asid(asid));
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -308,13 +308,18 @@ static void reset_global_asid_space(void
static u16 get_global_asid(void)
{
lockdep_assert_held(&global_asid_lock);
+ bool done_reset = false;
do {
u16 start = last_global_asid;
u16 asid = find_next_zero_bit(global_asid_used, MAX_ASID_AVAILABLE, start);
- if (asid >= MAX_ASID_AVAILABLE) {
+ if (asid > MAX_ASID_AVAILABLE) {
+ if (done_reset)
+ return asid;
+
reset_global_asid_space();
+ done_reset = true;
continue;
}
@@ -392,6 +398,12 @@ static bool mm_active_cpus_exceeds(struc
*/
static void use_global_asid(struct mm_struct *mm)
{
+ u16 asid;
+
+ /* This process is already using broadcast TLB invalidation. */
+ if (mm->context.global_asid)
+ return;
+
guard(raw_spinlock_irqsave)(&global_asid_lock);
/* This process is already using broadcast TLB invalidation. */
@@ -402,58 +414,25 @@ static void use_global_asid(struct mm_st
if (!global_asid_available)
return;
+ asid = get_global_asid();
+ if (asid > MAX_ASID_AVAILABLE)
+ return;
+
/*
- * The transition from IPI TLB flushing, with a dynamic ASID,
- * and broadcast TLB flushing, using a global ASID, uses memory
- * ordering for synchronization.
- *
- * While the process has threads still using a dynamic ASID,
- * TLB invalidation IPIs continue to get sent.
- *
- * This code sets asid_transition first, before assigning the
- * global ASID.
- *
- * The TLB flush code will only verify the ASID transition
- * after it has seen the new global ASID for the process.
+ * Notably flush_tlb_mm_range() -> broadcast_tlb_flush() ->
+ * finish_asid_transition() needs to observe asid_transition == true
+ * once it observes global_asid.
*/
- WRITE_ONCE(mm->context.asid_transition, true);
- WRITE_ONCE(mm->context.global_asid, get_global_asid());
+ mm->context.asid_transition = true;
+ smp_store_release(&mm->context.global_asid, asid);
}
-/*
- * Figure out whether to assign a global ASID to a process.
- * We vary the threshold by how empty or full global ASID space is.
- * 1/4 full: >= 4 active threads
- * 1/2 full: >= 8 active threads
- * 3/4 full: >= 16 active threads
- * 7/8 full: >= 32 active threads
- * etc
- *
- * This way we should never exhaust the global ASID space, even on very
- * large systems, and the processes with the largest number of active
- * threads should be able to use broadcast TLB invalidation.
- */
-#define HALFFULL_THRESHOLD 8
static bool meets_global_asid_threshold(struct mm_struct *mm)
{
- int avail = global_asid_available;
- int threshold = HALFFULL_THRESHOLD;
-
- if (!avail)
+ if (!global_asid_available)
return false;
- if (avail > MAX_ASID_AVAILABLE * 3 / 4) {
- threshold = HALFFULL_THRESHOLD / 4;
- } else if (avail > MAX_ASID_AVAILABLE / 2) {
- threshold = HALFFULL_THRESHOLD / 2;
- } else if (avail < MAX_ASID_AVAILABLE / 3) {
- do {
- avail *= 2;
- threshold *= 2;
- } while ((avail + threshold) < MAX_ASID_AVAILABLE / 2);
- }
-
- return mm_active_cpus_exceeds(mm, threshold);
+ return mm_active_cpus_exceeds(mm, 4);
}
static void consider_global_asid(struct mm_struct *mm)
