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

[Nadav Amit]

> On Apr 11, 2023, at 6:50 PM, Huang, Ying <ying.huang@xxxxxxxxx> wrote:
> 
> !! External Email
> 
> Nadav Amit <namit@xxxxxxxxxx> writes:
> 
>>> On Apr 10, 2023, at 6:31 PM, Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>> 
>>> !! External Email
>>> 
>>> Hi, Amit,
>>> 
>>> Thank you very much for review!
>>> 
>>> Nadav Amit <namit@xxxxxxxxxx> writes:
>>> 
>>>>> On Apr 10, 2023, at 12:52 AM, Huang Ying <ying.huang@xxxxxxxxx> 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?

You didn’t miss anything. I went over the comment on
inc_tlb_flush_pending() and you follow the scheme.