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Date:   Thu, 15 Oct 2020 11:55:43 +0100
From:   Will Deacon <will@kernel.org>
To:     Kalesh Singh <kaleshsingh@google.com>
Cc:     surenb@google.com, minchan@google.com, joelaf@google.com,
        lokeshgidra@google.com, kernel-team@android.com,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Shuah Khan <shuah@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Kees Cook <keescook@chromium.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
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Subject: Re: [PATCH v4 2/5] arm64: mremap speedup - Enable HAVE_MOVE_PMD
Message-ID: <20201015105542.GA5110@willie-the-truck>
References: <20201014005320.2233162-1-kaleshsingh@google.com>
 <20201014005320.2233162-3-kaleshsingh@google.com>
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On Wed, Oct 14, 2020 at 12:53:07AM +0000, Kalesh Singh wrote:
> HAVE_MOVE_PMD enables remapping pages at the PMD level if both the
> source and destination addresses are PMD-aligned.
> 
> HAVE_MOVE_PMD is already enabled on x86. The original patch [1] that
> introduced this config did not enable it on arm64 at the time because
> of performance issues with flushing the TLB on every PMD move. These
> issues have since been addressed in more recent releases with
> improvements to the arm64 TLB invalidation and core mmu_gather code as
> Will Deacon mentioned in [2].
> 
> From the data below, it can be inferred that there is approximately
> 8x improvement in performance when HAVE_MOVE_PMD is enabled on arm64.
> 
> --------- Test Results ----------
> 
> The following results were obtained on an arm64 device running a 5.4
> kernel, by remapping a PMD-aligned, 1GB sized region to a PMD-aligned
> destination. The results from 10 iterations of the test are given below.
> All times are in nanoseconds.
> 
> Control    HAVE_MOVE_PMD
> 
> 9220833    1247761
> 9002552    1219896
> 9254115    1094792
> 8725885    1227760
> 9308646    1043698
> 9001667    1101771
> 8793385    1159896
> 8774636    1143594
> 9553125    1025833
> 9374010    1078125
> 
> 9100885.4  1134312.6    <-- Mean Time in nanoseconds
> 
> Total mremap time for a 1GB sized PMD-aligned region drops from
> ~9.1 milliseconds to ~1.1 milliseconds. (~8x speedup).
> 
> [1] https://lore.kernel.org/r/20181108181201.88826-3-joelaf@google.com
> [2] https://www.mail-archive.com/linuxppc-dev@lists.ozlabs.org/msg140837.html
> 
> Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> ---
> Changes in v4:
>   - Add Kirill's Acked-by.

Argh, I thought we already enabled this for PMDs back in 2018! Looks like
that we forgot to actually do that after I improved the performance of
the TLB invalidation.

I'll pick this one patch up for 5.10.

Will