From: Alastair D'Silva <redacted>

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 <redacted>
---
 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 cff947cb2a84..a2758e473d58 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 += FLUSH_CHUNK_SIZE) {

+               flush_dcache_range(i, min(stop, start + FLUSH_CHUNK_SIZE));
+               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