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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>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>, Qian Cai <cai@lca.pw>,
        Nicholas Piggin <npiggin@gmail.com>,
        Allison Randal <allison@lohutok.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: [PATCH v5 5/6] powerpc: Chunk calls to flush_dcache_range in arch_*_memory
Date:   Mon,  4 Nov 2019 13:32:57 +1100
In-Reply-To: <20191104023305.9581-1-alastair@au1.ibm.com>
References: <20191104023305.9581-1-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 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 | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
index 54d61ba15e93..a7b662fc02c8 100644
--- a/arch/powerpc/mm/mem.c
+++ b/arch/powerpc/mm/mem.c
@@ -104,6 +104,27 @@ int __weak remove_section_mapping(unsigned long start, unsigned long end)
 	return -ENODEV;
 }
 
+#define FLUSH_CHUNK_SIZE SZ_1G
+/**
+ * flush_dcache_range_chunked(): Write any modified data cache blocks out to
+ * memory and invalidate them, in chunks of up to FLUSH_CHUNK_SIZE
+ * Does not invalidate the corresponding instruction cache blocks.
+ *
+ * @start: the start address
+ * @stop: the stop address (exclusive)
+ * @chunk: the max size of the chunks
+ */
+static void flush_dcache_range_chunked(unsigned long start, unsigned long stop,
+				       unsigned long chunk)
+{
+	unsigned long i;
+
+	for (i = start; i < stop; i += chunk) {
+		flush_dcache_range(i, min(stop, start + chunk));
+		cond_resched();
+	}
+}
+
 int __ref arch_add_memory(int nid, u64 start, u64 size,
 			struct mhp_restrictions *restrictions)
 {
@@ -120,7 +141,8 @@ int __ref arch_add_memory(int nid, u64 start, u64 size,
 			start, start + size, rc);
 		return -EFAULT;
 	}
-	flush_dcache_range(start, start + size);
+
+	flush_dcache_range_chunked(start, start + size, FLUSH_CHUNK_SIZE);
 
 	return __add_pages(nid, start_pfn, nr_pages, restrictions);
 }
@@ -137,7 +159,8 @@ void __ref arch_remove_memory(int nid, u64 start, u64 size,
 
 	/* Remove htab bolted mappings for this section of memory */
 	start = (unsigned long)__va(start);
-	flush_dcache_range(start, start + size);
+	flush_dcache_range_chunked(start, start + size, FLUSH_CHUNK_SIZE);
+
 	ret = remove_section_mapping(start, start + size);
 	WARN_ON_ONCE(ret);
 
-- 
2.21.0