linux-kernel - Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation

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Message-ID: <51f7c840-f892-fa98-632b-4c06b76fb32a@linux.alibaba.com>
Date:   Tue, 5 Jul 2022 21:17:48 +0800
From:   "guanghui.fgh" <guanghuifeng@...ux.alibaba.com>
To:     Mike Rapoport <rppt@...nel.org>, Will Deacon <will@...nel.org>
Cc:     Ard Biesheuvel <ardb@...nel.org>, baolin.wang@...ux.alibaba.com,
        catalin.marinas@....com, akpm@...ux-foundation.org,
        david@...hat.com, jianyong.wu@....com, james.morse@....com,
        quic_qiancai@...cinc.com, christophe.leroy@...roup.eu,
        jonathan@...ek.ca, mark.rutland@....com,
        thunder.leizhen@...wei.com, anshuman.khandual@....com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        geert+renesas@...der.be, linux-mm@...ck.org,
        yaohongbo@...ux.alibaba.com, alikernel-developer@...ux.alibaba.com
Subject: Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance
 degradation



在 2022/7/5 20:56, Mike Rapoport 写道:
> On Tue, Jul 05, 2022 at 01:11:16PM +0100, Will Deacon wrote:
>> On Tue, Jul 05, 2022 at 08:07:07PM +0800, guanghui.fgh wrote:
>>>
>>> 1.The rodata full is harm to the performance and has been disabled in-house.
>>>
>>> 2.When using crashkernel with rodata non full, the kernel also will use non
>>> block/section mapping which cause high d-TLB miss and degrade performance
>>> greatly.
>>> This patch fix it to use block/section mapping as far as possible.
>>>
>>> bool can_set_direct_map(void)
>>> {
>>> 	return rodata_full || debug_pagealloc_enabled();
>>> }
>>>
>>> map_mem:
>>> if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
>>> 	flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>>>
>>> 3.When rodata full is disabled, crashkernel also need protect(keep
>>> arch_kexec_[un]protect_crashkres using).
>>> I think crashkernel should't depend on radata full(Maybe other architecture
>>> don't support radata full now).
>>
>> I think this is going round in circles :/
>>
>> As a first step, can we please leave the crashkernel mapped unless
>> rodata=full? It should be a much simpler patch to write, review and maintain
>> and it gives you the performance you want when crashkernel is being used.
> 
> Since we are talking about large systems, what do you think about letting
> them set CRASH_ALIGN to PUD_SIZE, then
> 
> 	unmap(crashkernel);
> 	__create_pgd_mapping(crashkernel, NO_BLOCK_MAPPINGS);
> 
> should be enough to make crash kernel mapped with base pages.
>   
>> Will
> 
Thanks.

1.When kernel boots with crashkernel, the crashkernel parameters format is:
0M-2G:0M,2G-256G:256M,256G-1024G:320M,1024G-:384M which is self-adaption 
to multiple system.

2.As mentioned above, the crashkernel mem size maybe less than 
PUD_SIZE(Not multiple time to PUD_SIZE).
So, maybe we also need use some non block/section mappings.