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

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Date:   Mon, 10 Apr 2023 19:47:30 +0000
From:   Nadav Amit <namit@...are.com>
To:     Huang Ying <ying.huang@...el.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



> 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.

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()?

In addition, if this goes into stable (based on the Fixes tag), consider
breaking it into 2 patches, when only one would be backported.