Re: [PATCH 2/2] powerpc/64s/hash: add torture_hpt kernel boot option to increase hash faults

Nicholas Piggin [off-list ref] writes:

This option increases the number of hash misses by limiting the number of
kernel HPT entries. This helps stress test difficult to hit paths in the
kernel.

It would nice if we can explain in commit message how we are limiting
the number of HPT entries.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 .../admin-guide/kernel-parameters.txt         |  9 +++
 arch/powerpc/include/asm/book3s/64/mmu-hash.h | 10 +++
 arch/powerpc/mm/book3s64/hash_4k.c            |  3 +
 arch/powerpc/mm/book3s64/hash_64k.c           |  8 +++
 arch/powerpc/mm/book3s64/hash_utils.c         | 66 ++++++++++++++++++-
 5 files changed, 95 insertions(+), 1 deletion(-)

....

  

+void hpt_do_torture(unsigned long ea, unsigned long access,
+		    unsigned long rflags, unsigned long hpte_group)
+{
+	unsigned long last_group;
+	int cpu = raw_smp_processor_id();
+
+	last_group = torture_hpt_last_group[cpu];
+	if (last_group != -1UL) {
+		while (mmu_hash_ops.hpte_remove(last_group) != -1)
+			;
+		torture_hpt_last_group[cpu] = -1UL;
+	}
+
+#define QEMU_WORKAROUND	0
+
+	if (ea >= PAGE_OFFSET) {
+		if (!QEMU_WORKAROUND && (access & (_PAGE_READ|_PAGE_WRITE)) &&
+		    !(rflags & (HPTE_R_I|HPTE_R_G))) {
+			/* prefetch / prefetchw does not seem to set up a TLB
+			 * entry with the powerpc systemsim (mambo) emulator,
+			 * though it works with real hardware. An alternative
+			 * approach that would work more reliably on quirky
+			 * emulators like QEMU may be to remember the last
+			 * insertion and remove that, rather than removing the
+			 * current insertion. Then no prefetch is required.
+			 */
+			if ((access & _PAGE_WRITE) && (access & _PAGE_READ))
+				atomic_add(0, (atomic_t *)(ea & ~0x3));
+			else if (access & _PAGE_READ)
+				*(volatile char *)ea;
+
+			mb();
+
+			while (mmu_hash_ops.hpte_remove(hpte_group) != -1)
+				;

Do we get similar hpte faults rate, if we remove everything except the
current inserted entry?. If so that would largely simplify the code.

+		} else {
+			/* Can't prefetch cache-inhibited so clear next time. */
+			torture_hpt_last_group[cpu] = hpte_group;
+		}
+	}
+}
+
+
 #ifdef CONFIG_DEBUG_PAGEALLOC
 static void kernel_map_linear_page(unsigned long vaddr, unsigned long lmi)
 {
-- 
2.23.0

-aneesh