Re: [PATCH] mm,unmap: avoid flushing TLB in batch if PTE is inaccessible

["Huang, Ying" <ying.huang@intel.com>]

Nadav Amit <namit@vmware.com> writes:

>> On Apr 10, 2023, at 6:31 PM, Huang, Ying <ying.huang@intel.com> wrote:
>> 
>> !! External Email
>> 
>> Hi, Amit,
>> 
>> Thank you very much for review!
>> 
>> Nadav Amit <namit@vmware.com> writes:
>> 
>>>> On Apr 10, 2023, at 12:52 AM, Huang Ying <ying.huang@intel.com> wrote:
>>>> 
>>>> 0Day/LKP reported a performance regression for commit
>>>> 7e12beb8ca2a ("migrate_pages: batch flushing TLB"). In the commit, the
>>>> TLB flushing during page migration is batched.  So, in
>>>> try_to_migrate_one(), ptep_clear_flush() is replaced with
>>>> set_tlb_ubc_flush_pending().  In further investigation, it is found
>>>> that the TLB flushing can be avoided in ptep_clear_flush() if the PTE
>>>> is inaccessible.  In fact, we can optimize in similar way for the
>>>> batched TLB flushing too to improve the performance.
>>>> 
>>>> So in this patch, we check pte_accessible() before
>>>> set_tlb_ubc_flush_pending() in try_to_unmap/migrate_one().  Tests show
>>>> that the benchmark score of the anon-cow-rand-mt test case of
>>>> vm-scalability test suite can improve up to 2.1% with the patch on a
>>>> Intel server machine.  The TLB flushing IPI can reduce up to 44.3%.
>>> 
>>> LGTM.
>> 
>> Thanks!
>> 
>>> I know it’s meaningless for x86 (but perhaps ARM would use this infra
>>> too): do we need smp_mb__after_atomic() after ptep_get_and_clear() and
>>> before pte_accessible()?
>> 
>> Why do we need the memory barrier?  IIUC, the PTL is locked, so PTE
>> value will not be changed under us.  Anything else?
>
> I was thinking about the ordering with respect to
> atomic_read(&mm->tlb_flush_pending), which is not protected by the PTL.
> I guess you can correctly argue that because of other control-flow
> dependencies, the barrier is not necessary.

For ordering between ptep_get_and_clear() and
atomic_read(&mm->tlb_flush_pending), I think PTL has provided the
necessary protection already.  The code path to write
mm->tlb_flush_pending is,

  tlb_gather_mmu
    inc_tlb_flush_pending       a)
  lock PTL
  change PTE                    b)
  unlock PTL
  tlb_finish_mmu
    dec_tlb_flush_pending       c)

While code path of try_to_unmap/migrate_one is,

  lock PTL
  read and change PTE           d)
  read mm->tlb_flush_pending    e)
  unlock PTL

Even if e) occurs before d), they cannot occur at the same time of b).
Do I miss anything?

Best Regards,
Huang, Ying

[snip]