[PATCH v2 3/9] x86/mm/tlb: Open-code on_each_cpu_cond_mask() for tlb_is_not_lazy()

Nadav Amit
2019-07-03T07:14:40Z

Open-code on_each_cpu_cond_mask() in native_flush_tlb_others() to
optimize the code. Open-coding eliminates the need for the indirect branch
that is used to call is_lazy(), and in CPUs that are vulnerable to
Spectre v2, it eliminates the retpoline. In addition, it allows to use a
preallocated cpumask to compute the CPUs that should be.

This would later allow us not to adapt on_each_cpu_cond_mask() to
support local and remote functions.

Note that calling tlb_is_not_lazy() for every CPU that needs to be
flushed, as done in native_flush_tlb_multi() might look ugly, but it is
equivalent to what is currently done in on_each_cpu_cond_mask().
Actually, native_flush_tlb_multi() does it more efficiently since it
avoids using an indirect branch for the matter.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
arch/x86/mm/tlb.c | 40 ++++++++++++++++++++++++++++++++++------
1 file changed, 34 insertions(+), 6 deletions(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 8a957b58525f..5c9b1607191d 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -658,11 +658,13 @@ 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)
+static bool tlb_is_not_lazy(int cpu)
{
return !per_cpu(cpu_tlbstate.is_lazy, cpu);
}

+static DEFINE_PER_CPU(cpumask_t, flush_tlb_mask);
+
void native_flush_tlb_others(const struct cpumask *cpumask,
const struct flush_tlb_info *info)
{
@@ -706,12 +708,38 @@ void native_flush_tlb_others(const struct cpumask *cpumask,
* up on the new contents of what used to be page tables, while
* doing a speculative memory access.
*/
- if (info->freed_tables)
+ 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);
+ } else {
+ /*
+ * Although we could have used on_each_cpu_cond_mask(),
+ * open-coding it has performance advantages, as it eliminates
+ * the need for indirect calls or retpolines. In addition, it
+ * allows to use a designated cpumask for evaluating the
+ * condition, instead of allocating one.
+ *
+ * This code works under the assumption that there are no nested
+ * TLB flushes, an assumption that is already made in
+ * flush_tlb_mm_range().
+ *
+ * cond_cpumask is logically a stack-local variable, but it is
+ * more efficient to have it off the stack and not to allocate
+ * it on demand. Preemption is disabled and this code is
+ * non-reentrant.
+ */
+ struct cpumask *cond_cpumask = this_cpu_ptr(&flush_tlb_mask);
+ int cpu;
+
+ cpumask_clear(cond_cpumask);
+
+ for_each_cpu(cpu, cpumask) {
+ if (tlb_is_not_lazy(cpu))
+ __cpumask_set_cpu(cpu, cond_cpumask);
+ }
+ smp_call_function_many(cond_cpumask, flush_tlb_func_remote,
+ (void *)info, 1);
+ }
}

/*
@@ -865,7 +893,7 @@ void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
if (cpumask_test_cpu(cpu, &batch->cpumask)) {
lockdep_assert_irqs_enabled();
local_irq_disable();
- flush_tlb_func_local(&full_flush_tlb_info);
+ flush_tlb_func_local((void *)&full_flush_tlb_info);
local_irq_enable();
}

--
2.17.1