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

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Date:   Tue, 11 Apr 2023 09:31:25 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Nadav Amit <namit@...are.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kernel test robot <yujie.liu@...el.com>,
        "Mel Gorman" <mgorman@...hsingularity.net>,
        Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH] mm,unmap: avoid flushing TLB in batch if PTE is
 inaccessible

Hi, Amit,

Thank you very much for review!

Nadav Amit <namit@...are.com> writes:

>> On Apr 10, 2023, at 12:52 AM, Huang Ying <ying.huang@...el.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?

> 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?

Best Regards,
Huang, Ying