[PATCH v13 10/14] x86/mm: enable broadcast TLB invalidation for multi-threaded processes

Rik van Riel
2025-02-23T19:50:15Z

Use broadcast TLB invalidation, using the INVPLGB instruction.

There is not enough room in the 12-bit ASID address space to hand
out broadcast ASIDs to every process. Only hand out broadcast ASIDs
to processes when they are observed to be simultaneously running
on 4 or more CPUs.

This also allows single threaded process to continue using the
cheaper, local TLB invalidation instructions like INVLPGB.

Combined with the removal of unnecessary lru_add_drain calls
(see https://lkml.org/lkml/2024/12/19/1388) this results in a
nice performance boost for the will-it-scale tlb_flush2_threads
test on an AMD Milan system with 36 cores:

- vanilla kernel: 527k loops/second
- lru_add_drain removal: 731k loops/second
- only INVLPGB: 527k loops/second
- lru_add_drain + INVLPGB: 1157k loops/second

Profiling with only the INVLPGB changes showed while
TLB invalidation went down from 40% of the total CPU
time to only around 4% of CPU time, the contention
simply moved to the LRU lock.

Fixing both at the same time about doubles the
number of iterations per second from this case.

Comparing will-it-scale tlb_flush2_threads with
several different numbers of threads on a 72 CPU
AMD Milan shows similar results. The number
represents the total number of loops per second
across all the threads:

threads tip invlpgb

1 315k 304k
2 423k 424k
4 644k 1032k
8 652k 1267k
16 737k 1368k
32 759k 1199k
64 636k 1094k
72 609k 993k

1 and 2 thread performance is similar with and
without invlpgb, because invlpgb is only used
on processes using 4 or more CPUs simultaneously.

The number is the median across 5 runs.

Some numbers closer to real world performance
can be found at Phoronix, thanks to Michael:

https://www.phoronix.com/news/AMD-INVLPGB-Linux-Benefits

Signed-off-by: Rik van Riel <riel@surriel.com>
Reviewed-by: Nadav Amit <nadav.amit@gmail.com>
Tested-by: Manali Shukla <Manali.Shukla@amd.com>
Tested-by: Brendan Jackman <jackmanb@google.com>
Tested-by: Michael Kelley <mhklinux@outlook.com>
---
arch/x86/mm/tlb.c | 107 +++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 106 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index d8a04e398615..01a5edb51ebe 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -420,6 +420,108 @@ static bool needs_global_asid_reload(struct mm_struct *next, u16 prev_asid)
return false;
}

+/*
+ * x86 has 4k ASIDs (2k when compiled with KPTI), but the largest
+ * x86 systems have over 8k CPUs. Because of this potential ASID
+ * shortage, global ASIDs are handed out to processes that have
+ * frequent TLB flushes and are active on 4 or more CPUs simultaneously.
+ */
+static void consider_global_asid(struct mm_struct *mm)
+{
+ if (!static_cpu_has(X86_FEATURE_INVLPGB))
+ return;
+
+ /* Check every once in a while. */
+ if ((current->pid & 0x1f) != (jiffies & 0x1f))
+ return;
+
+ if (!READ_ONCE(global_asid_available))
+ return;
+
+ /*
+ * Assign a global ASID if the process is active on
+ * 4 or more CPUs simultaneously.
+ */
+ if (mm_active_cpus_exceeds(mm, 3))
+ use_global_asid(mm);
+}
+
+static void finish_asid_transition(struct flush_tlb_info *info)
+{
+ struct mm_struct *mm = info->mm;
+ int bc_asid = mm_global_asid(mm);
+ int cpu;
+
+ if (!READ_ONCE(mm->context.asid_transition))
+ return;
+
+ for_each_cpu(cpu, mm_cpumask(mm)) {
+ /*
+ * The remote CPU is context switching. Wait for that to
+ * finish, to catch the unlikely case of it switching to
+ * the target mm with an out of date ASID.
+ */
+ while (READ_ONCE(per_cpu(cpu_tlbstate.loaded_mm, cpu)) == LOADED_MM_SWITCHING)
+ cpu_relax();
+
+ if (READ_ONCE(per_cpu(cpu_tlbstate.loaded_mm, cpu)) != mm)
+ continue;
+
+ /*
+ * If at least one CPU is not using the global ASID yet,
+ * send a TLB flush IPI. The IPI should cause stragglers
+ * to transition soon.
+ *
+ * This can race with the CPU switching to another task;
+ * that results in a (harmless) extra IPI.
+ */
+ if (READ_ONCE(per_cpu(cpu_tlbstate.loaded_mm_asid, cpu)) != bc_asid) {
+ flush_tlb_multi(mm_cpumask(info->mm), info);
+ return;
+ }
+ }
+
+ /* All the CPUs running this process are using the global ASID. */
+ WRITE_ONCE(mm->context.asid_transition, false);
+}
+
+static void broadcast_tlb_flush(struct flush_tlb_info *info)
+{
+ bool pmd = info->stride_shift == PMD_SHIFT;
+ unsigned long asid = info->mm->context.global_asid;
+ unsigned long addr = info->start;
+
+ /*
+ * TLB flushes with INVLPGB are kicked off asynchronously.
+ * The inc_mm_tlb_gen() guarantees page table updates are done
+ * before these TLB flushes happen.
+ */
+ if (info->end == TLB_FLUSH_ALL) {
+ invlpgb_flush_single_pcid_nosync(kern_pcid(asid));
+ /* Do any CPUs supporting INVLPGB need PTI? */
+ if (static_cpu_has(X86_FEATURE_PTI))
+ invlpgb_flush_single_pcid_nosync(user_pcid(asid));
+ } else do {
+ unsigned long nr = 1;
+
+ if (info->stride_shift <= PMD_SHIFT) {
+ nr = (info->end - addr) >> info->stride_shift;
+ nr = clamp_val(nr, 1, invlpgb_count_max);
+ }
+
+ invlpgb_flush_user_nr_nosync(kern_pcid(asid), addr, nr, pmd);
+ if (static_cpu_has(X86_FEATURE_PTI))
+ invlpgb_flush_user_nr_nosync(user_pcid(asid), addr, nr, pmd);
+
+ addr += nr << info->stride_shift;
+ } while (addr < info->end);
+
+ finish_asid_transition(info);
+
+ /* Wait for the INVLPGBs kicked off above to finish. */
+ __tlbsync();
+}
+
/*
* Given an ASID, flush the corresponding user ASID. We can delay this
* until the next time we switch to it.
@@ -1250,9 +1352,12 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
* a local TLB flush is needed. Optimize this use-case by calling
* flush_tlb_func_local() directly in this case.
*/
- if (cpumask_any_but(mm_cpumask(mm), cpu) < nr_cpu_ids) {
+ if (mm_global_asid(mm)) {
+ broadcast_tlb_flush(info);
+ } else if (cpumask_any_but(mm_cpumask(mm), cpu) < nr_cpu_ids) {
info->trim_cpumask = should_trim_cpumask(mm);
flush_tlb_multi(mm_cpumask(mm), info);
+ consider_global_asid(mm);
} else if (mm == this_cpu_read(cpu_tlbstate.loaded_mm)) {
lockdep_assert_irqs_enabled();
local_irq_disable();
--
2.47.1