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Subject: Re: [PATCH v2 4/6] powerpc: Chunk calls to flush_dcache_range in
 arch_*_memory
From:   "Alastair D'Silva" <alastair@au1.ibm.com>
To:     Christophe Leroy <christophe.leroy@c-s.fr>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Qian Cai <cai@lca.pw>, Nicholas Piggin <npiggin@gmail.com>,
        Allison Randal <allison@lohutok.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        David Hildenbrand <david@redhat.com>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Date:   Tue, 03 Sep 2019 16:25:21 +1000
In-Reply-To: <3bde4dbc-5176-0df5-a0bf-993eef2a333b@c-s.fr>
References: <20190903052407.16638-1-alastair@au1.ibm.com>
         <20190903052407.16638-5-alastair@au1.ibm.com>
         <3bde4dbc-5176-0df5-a0bf-993eef2a333b@c-s.fr>
Organization: IBM Australia
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On Tue, 2019-09-03 at 08:19 +0200, Christophe Leroy wrote:
> 
> Le 03/09/2019 à 07:23, Alastair D'Silva a écrit :
> > From: Alastair D'Silva <alastair@d-silva.org>
> > 
> > When presented with large amounts of memory being hotplugged
> > (in my test case, ~890GB), the call to flush_dcache_range takes
> > a while (~50 seconds), triggering RCU stalls.
> > 
> > This patch breaks up the call into 1GB chunks, calling
> > cond_resched() inbetween to allow the scheduler to run.
> > 
> > Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
> > ---
> >   arch/powerpc/mm/mem.c | 18 ++++++++++++++++--
> >   1 file changed, 16 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
> > index cd540123874d..854aaea2c6ae 100644
> > --- a/arch/powerpc/mm/mem.c
> > +++ b/arch/powerpc/mm/mem.c
> > @@ -104,11 +104,14 @@ int __weak remove_section_mapping(unsigned
> > long start, unsigned long end)
> >   	return -ENODEV;
> >   }
> >   
> > +#define FLUSH_CHUNK_SIZE SZ_1G
> 
> Maybe the name is a bit long for a local define. See if we could
> reduce 
> code line splits below by shortening this name.
> 
> > +
> >   int __ref arch_add_memory(int nid, u64 start, u64 size,
> >   			struct mhp_restrictions *restrictions)
> >   {
> >   	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());
> > @@ -120,7 +123,12 @@ int __ref arch_add_memory(int nid, u64 start,
> > u64 size,
> >   			start, start + size, rc);
> >   		return -EFAULT;
> >   	}
> > -	flush_dcache_range(start, start + size);
> > +
> > +	for (i = 0; i < size; i += FLUSH_CHUNK_SIZE) {
> > +		flush_dcache_range(start + i,
> > +				   min(start + size, start + i +
> > FLUSH_CHUNK_SIZE));
> 
> My eyes don't like it.
> 
> What about
> 	for (; i < size; i += FLUSH_CHUNK_SIZE) {
> 		int len = min(size - i, FLUSH_CHUNK_SIZE);
> 
> 		flush_dcache_range(start + i, start + i + len);
> 		cond_resched();
> 	}
> 
> or
> 
> 	end = start + size;
> 	for (; start < end; start += FLUSH_CHUNK_SIZE, size -=
> FLUSH_CHUNK_SIZE) {
> 		int len = min(size, FLUSH_CHUNK_SIZE);
> 
> 		flush_dcache_range(start, start + len);
> 		cond_resched();
> 	}
> 
> > +		cond_resched();
> > +	}
> >   
> >   	return __add_pages(nid, start_pfn, nr_pages, restrictions);
> >   }
> > @@ -131,13 +139,19 @@ void __ref arch_remove_memory(int nid, u64
> > start, u64 size,
> >   	unsigned long start_pfn = start >> PAGE_SHIFT;
> >   	unsigned long nr_pages = size >> PAGE_SHIFT;
> >   	struct page *page = pfn_to_page(start_pfn) +
> > vmem_altmap_offset(altmap);
> > +	u64 i;
> >   	int ret;
> >   
> >   	__remove_pages(page_zone(page), start_pfn, nr_pages, altmap);
> >   
> >   	/* Remove htab bolted mappings for this section of memory */
> >   	start = (unsigned long)__va(start);
> > -	flush_dcache_range(start, start + size);
> > +	for (i = 0; i < size; i += FLUSH_CHUNK_SIZE) {
> > +		flush_dcache_range(start + i,
> > +				   min(start + size, start + i +
> > FLUSH_CHUNK_SIZE));
> > +		cond_resched();
> > +	}
> > +
> 
> This piece of code looks pretty similar to the one before. Can we 
> refactor into a small helper ?
> 

Not much point, it's removed in a subsequent patch.

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