linux-kernel - Re: [PATCH v2 6/6] powerpc: Don't flush caches when adding memory

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Date:   Tue, 3 Sep 2019 08:23:28 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Alastair D'Silva <alastair@....ibm.com>, alastair@...ilva.org
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, Qian Cai <cai@....pw>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nicholas Piggin <npiggin@...il.com>,
        Allison Randal <allison@...utok.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/6] powerpc: Don't flush caches when adding memory



Le 03/09/2019 à 07:24, Alastair D'Silva a écrit :
> From: Alastair D'Silva <alastair@...ilva.org>
> 
> This operation takes a significant amount of time when hotplugging
> large amounts of memory (~50 seconds with 890GB of persistent memory).
> 
> This was orignally in commit fb5924fddf9e
> ("powerpc/mm: Flush cache on memory hot(un)plug") to support memtrace,
> but the flush on add is not needed as it is flushed on remove.
> 
> Signed-off-by: Alastair D'Silva <alastair@...ilva.org>
> ---
>   arch/powerpc/mm/mem.c | 7 -------
>   1 file changed, 7 deletions(-)
> 
> diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
> index 854aaea2c6ae..2a14b5b93e19 100644
> --- a/arch/powerpc/mm/mem.c
> +++ b/arch/powerpc/mm/mem.c
> @@ -111,7 +111,6 @@ int __ref arch_add_memory(int nid, u64 start, u64 size,
>   {
>   	unsigned long start_pfn = start >> PAGE_SHIFT;
>   	unsigned long nr_pages = size >> PAGE_SHIFT;
> -	u64 i;
>   	int rc;
>   
>   	resize_hpt_for_hotplug(memblock_phys_mem_size());
> @@ -124,12 +123,6 @@ int __ref arch_add_memory(int nid, u64 start, u64 size,
>   		return -EFAULT;
>   	}
>   
> -	for (i = 0; i < size; i += FLUSH_CHUNK_SIZE) {
> -		flush_dcache_range(start + i,
> -				   min(start + size, start + i + FLUSH_CHUNK_SIZE));
> -		cond_resched();
> -	}
> -

So you are removing the code you added in patch 4. Why not move this one 
before patch 4 ?

>   	return __add_pages(nid, start_pfn, nr_pages, restrictions);
>   }
>   
> 

Christophe