Re: [PATCH v2 2/4] mm: rmap: Allow platforms without mm_cpumask to defer TLB flush

On Tue, Jul 12, 2022 at 1:35 AM Kefeng Wang [off-list ref] wrote:

Hi Barry，

On 2022/7/11 11:46, Barry Song wrote:

quoted

From: Barry Song <redacted>

Platforms like ARM64 have hareware TLB shootdown broadcast. They
don't maintain mm_cpumask but just send tlbi and related sync
instructions for TLB flush. task's mm_cpumask is normally empty
in this case. We also allow deferred TLB flush on this kind of
platforms.

Signed-off-by: Barry Song <redacted>>
---

...

quoted

diff --git a/mm/Kconfig b/mm/Kconfig
index 169e64192e48..7bf54f57ca01 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig

@@ -951,6 +951,9 @@ config ARCH_HAS_CURRENT_STACK_POINTER
        register alias named "current_stack_pointer", this config can be
        selected.

+config ARCH_HAS_MM_CPUMASK
+     bool
+
  config ARCH_HAS_VM_GET_PAGE_PROT
      bool

diff --git a/mm/rmap.c b/mm/rmap.c
index 5bcb334cd6f2..13d4f9a1d4f1 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c

@@ -692,6 +692,10 @@ static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags)
      if (!(flags & TTU_BATCH_FLUSH))
              return false;

+#ifndef CONFIG_ARCH_HAS_MM_CPUMASK
+     return true;
+#endif
+

Here is another option to enable arch's tlbbatch defer

This option is even better than simply having ARCH_HAS_MM_CPUMASK
since arch might make decisions based on specific hardware characters.
for example,
https://lists.ozlabs.org/pipermail/linuxppc-dev/2017-November/165468.html

+bool arch_tlbbatch_should_defer(struct mm_struct *mm)
+{
+     if (!radix_enabled() || cpu_has_feature(CPU_FTR_POWER9_DD1))
+         return false;
+
+     if (!mm_is_thread_local(mm))
+         return true;
+
+     return false;
+}

In this case, having MM_CPUMASK doesn't necessarily mean tlbbatch is needed.

[1]
https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20171101101735.2318-2-khandual@linux.vnet.ibm.com/

quoted

      /* If remote CPUs need to be flushed then defer batch the flush */
      if (cpumask_any_but(mm_cpumask(mm), get_cpu()) < nr_cpu_ids)
              should_defer = true;

Thanks
Barry