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From:   "Alastair D'Silva" <alastair@au1.ibm.com>
To:     alastair@d-silva.org
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Qian Cai <cai@lca.pw>, Allison Randal <allison@lohutok.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Nicholas Piggin <npiggin@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        David Hildenbrand <david@redhat.com>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: [PATCH 4/6] powerpc: Chunk calls to flush_dcache_range in arch_*_memory
Date:   Thu, 15 Aug 2019 14:10:49 +1000
In-Reply-To: <20190815041057.13627-1-alastair@au1.ibm.com>
References: <20190815041057.13627-1-alastair@au1.ibm.com>
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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 16GB 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 | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
index 5400da87a804..fb0d5e9aa11b 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 (16ull * 1024ull * 1024ull * 1024ull)
+
 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;
+	unsigned long i;
 	int rc;
 
 	resize_hpt_for_hotplug(memblock_phys_mem_size());
@@ -120,7 +123,11 @@ 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));
+		cond_resched();
+	}
 
 	return __add_pages(nid, start_pfn, nr_pages, restrictions);
 }
@@ -131,13 +138,18 @@ 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);
+	unsigned long 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();
+	}
+
 	ret = remove_section_mapping(start, start + size);
 	WARN_ON_ONCE(ret);
 
-- 
2.21.0