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Message-ID: <8e4cd345-2530-4ace-a596-a0fa6911a35e@redhat.com>
Date: Thu, 6 Jun 2024 10:49:45 +0200
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Subject: Re: [v4 PATCH] arm64: mm: force write fault for atomic RMW
 instructions
To: Yang Shi <yang@os.amperecomputing.com>, catalin.marinas@arm.com,
 will@kernel.org, anshuman.khandual@arm.com, scott@os.amperecomputing.com,
 cl@gentwo.org
Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
References: <20240605203723.643329-1-yang@os.amperecomputing.com>
From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <20240605203723.643329-1-yang@os.amperecomputing.com>
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On 05.06.24 22:37, Yang Shi wrote:
> The atomic RMW instructions, for example, ldadd, actually does load +
> add + store in one instruction, it will trigger two page faults per the
> ARM64 architecture spec, the first fault is a read fault, the second
> fault is a write fault.
> 
> Some applications use atomic RMW instructions to populate memory, for
> example, openjdk uses atomic-add-0 to do pretouch (populate heap memory
> at launch time) between v18 and v22 in order to permit use of memory
> concurrently with pretouch.
> 
> But the double page fault has some problems:
> 
> 1. Noticeable TLB overhead.  The kernel actually installs zero page with
>     readonly PTE for the read fault.  The write fault will trigger a
>     write-protection fault (CoW).  The CoW will allocate a new page and
>     make the PTE point to the new page, this needs TLB invalidations.  The
>     tlb invalidation and the mandatory memory barriers may incur
>     significant overhead, particularly on the machines with many cores.
> 
> 2. Break up huge pages.  If THP is on the read fault will install huge
>     zero pages.  The later CoW will break up the huge page and allocate
>     base pages instead of huge page.  The applications have to rely on
>     khugepaged (kernel thread) to collapse huge pages asynchronously.
>     This also incurs noticeable performance penalty.
> 
> 3. 512x page faults with huge page.  Due to #2, the applications have to
>     have page faults for every 4K area for the write, this makes the speed
>     up by using huge page actually gone.

All interesting and valid points.

As raised, the app likely really should be using MADV_POPULATE_WRITE.

Acked-by: David Hildenbrand <david@redhat.com>


-- 
Cheers,

David / dhildenb