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Subject[PATCH 7/7] x86/mm/tlb: Make lazy TLB mode lazier
Date
Lazy TLB mode can result in an idle CPU being woken up by a TLB flush,
when all it really needs to do is reload %CR3 at the next context switch,
assuming no page table pages got freed.

Memory ordering is used to prevent race conditions between switch_mm_irqs_off,
which checks whether .tlb_gen changed, and the TLB invalidation code, which
increments .tlb_gen whenever page table entries get invalidated.

The atomic increment in inc_mm_tlb_gen is its own barrier; the context
switch code adds an explicit barrier between reading tlbstate.is_lazy and
next->context.tlb_gen.

CPUs in lazy TLB mode remain part of the mm_cpumask(mm), both because
that allows TLB flush IPIs to be sent at page table freeing time, and
because the cache line bouncing on the mm_cpumask(mm) was responsible
for about half the CPU use in switch_mm_irqs_off().

Tested-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Rik van Riel <riel@surriel.com>
---
arch/x86/mm/tlb.c | 67 ++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 58 insertions(+), 9 deletions(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 5d73bde0e4c8..280b4cc4c28c 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -156,6 +156,7 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
{
struct mm_struct *real_prev = this_cpu_read(cpu_tlbstate.loaded_mm);
u16 prev_asid = this_cpu_read(cpu_tlbstate.loaded_mm_asid);
+ bool was_lazy = this_cpu_read(cpu_tlbstate.is_lazy);
unsigned cpu = smp_processor_id();
u64 next_tlb_gen;
bool need_flush;
@@ -207,17 +208,40 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
next->context.ctx_id);

/*
- * We don't currently support having a real mm loaded without
- * our cpu set in mm_cpumask(). We have all the bookkeeping
- * in place to figure out whether we would need to flush
- * if our cpu were cleared in mm_cpumask(), but we don't
- * currently use it.
+ * Even in lazy TLB mode, the CPU should stay set in the
+ * mm_cpumask. The TLB shootdown code can figure out from
+ * from cpu_tlbstate.is_lazy whether or not to send an IPI.
*/
if (WARN_ON_ONCE(real_prev != &init_mm &&
!cpumask_test_cpu(cpu, mm_cpumask(next))))
cpumask_set_cpu(cpu, mm_cpumask(next));

- return;
+ /*
+ * If the CPU is not in lazy TLB mode, we are just switching
+ * from one thread in a process to another thread in the same
+ * process. No TLB flush required.
+ */
+ if (!was_lazy)
+ return;
+
+ /*
+ * Read the tlb_gen to check whether a flush is needed.
+ * If the TLB is up to date, just use it.
+ * The barrier synchronizes with the tlb_gen increment in
+ * the TLB shootdown code.
+ */
+ smp_mb();
+ next_tlb_gen = atomic64_read(&next->context.tlb_gen);
+ if (this_cpu_read(cpu_tlbstate.ctxs[prev_asid].tlb_gen) ==
+ next_tlb_gen)
+ return;
+
+ /*
+ * TLB contents went out of date while we were in lazy
+ * mode. Fall through to the TLB switching code below.
+ */
+ new_asid = prev_asid;
+ need_flush = true;
} else {
u64 last_ctx_id = this_cpu_read(cpu_tlbstate.last_ctx_id);

@@ -308,8 +332,10 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
this_cpu_write(cpu_tlbstate.loaded_mm, next);
this_cpu_write(cpu_tlbstate.loaded_mm_asid, new_asid);

- load_mm_cr4(next);
- switch_ldt(real_prev, next);
+ if (next != real_prev) {
+ load_mm_cr4(next);
+ switch_ldt(real_prev, next);
+ }
}

/*
@@ -416,6 +442,9 @@ static void flush_tlb_func_common(const struct flush_tlb_info *f,
* paging-structure cache to avoid speculatively reading
* garbage into our TLB. Since switching to init_mm is barely
* slower than a minimal flush, just switch to init_mm.
+ *
+ * This should be rare, with native_flush_tlb_others skipping
+ * IPIs to lazy TLB mode CPUs.
*/
switch_mm_irqs_off(NULL, &init_mm, NULL);
return;
@@ -519,6 +548,11 @@ static void flush_tlb_func_remote(void *info)
flush_tlb_func_common(f, false, TLB_REMOTE_SHOOTDOWN);
}

+static bool tlb_is_not_lazy(int cpu, void *data)
+{
+ return !per_cpu(cpu_tlbstate.is_lazy, cpu);
+}
+
void native_flush_tlb_others(const struct cpumask *cpumask,
const struct flush_tlb_info *info)
{
@@ -554,8 +588,23 @@ void native_flush_tlb_others(const struct cpumask *cpumask,
(void *)info, 1);
return;
}
- smp_call_function_many(cpumask, flush_tlb_func_remote,
+
+ /*
+ * If no page tables were freed, we can skip sending IPIs to
+ * CPUs in lazy TLB mode. They will flush the CPU themselves
+ * at the next context switch.
+ *
+ * However, if page tables are getting freed, we need to send the
+ * IPI everywhere, to prevent CPUs in lazy TLB mode from tripping
+ * up on the new contents of what used to be page tables, while
+ * doing a speculative memory access.
+ */
+ if (info->freed_tables)
+ smp_call_function_many(cpumask, flush_tlb_func_remote,
(void *)info, 1);
+ else
+ on_each_cpu_cond_mask(tlb_is_not_lazy, flush_tlb_func_remote,
+ (void *)info, 1, GFP_ATOMIC, cpumask);
}

/*
--
2.17.1
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