> 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.
>
>> In addition, if this goes into stable (based on the Fixes tag), consider
>> breaking it into 2 patches, when only one would be backported.
>
> The fixed commit (7e12beb8ca2a ("migrate_pages: batch flushing TLB")) is
> merged by v6.3-rc1. So this patch will only be backported to v6.3 and
> later. Is it OK?
Of course. I wasn’t sure when the bug was introduced.
