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

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Message-Id: <864c4b01a9c12f82a32de264fbb0b2acb39592e4.camel@au1.ibm.com>
Date:   Tue, 03 Sep 2019 16:27:15 +1000
From:   "Alastair D'Silva" <alastair@....ibm.com>
To:     Christophe Leroy <christophe.leroy@....fr>
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

On Tue, 2019-09-03 at 08:23 +0200, Christophe Leroy wrote:
> 
> 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 ?
> 

I put them in this order so that if someone did want the flushes in
arch_add_memory, they could drop the later patch, but not trigger RCU
stalls.

-- 
Alastair D'Silva
Open Source Developer
Linux Technology Centre, IBM Australia
mob: 0423 762 819