Re: [kvm-unit-tests PATCH v5 11/11] new: arm/barrier-test for memory barriers

alvise rigo
2015-08-03T10:02:44Z

Hi Alex,

Nice set of tests, they are proving to be helpful.
One question below.

On Fri, Jul 31, 2015 at 5:54 PM, Alex Bennée <alex.bennee@linaro.org> wrote:
> From: Alex Bennée <alex@bennee.com>
>
> This test has been written mainly to stress multi-threaded TCG behaviour
> but will demonstrate failure by default on real hardware. The test takes
> the following parameters:
>
> - "lock" use GCC's locking semantics
> - "excl" use load/store exclusive semantics
> - "acqrel" use acquire/release semantics
>
> Currently excl/acqrel lock up on MTTCG
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
> arm/barrier-test.c | 206 +++++++++++++++++++++++++++++++++++++++++++
> config/config-arm-common.mak | 2 +
> 2 files changed, 208 insertions(+)
> create mode 100644 arm/barrier-test.c
>
> diff --git a/arm/barrier-test.c b/arm/barrier-test.c
> new file mode 100644
> index 0000000..53d690b
> --- /dev/null
> +++ b/arm/barrier-test.c
> @@ -0,0 +1,206 @@
> +#include <libcflat.h>
> +#include <asm/smp.h>
> +#include <asm/cpumask.h>
> +#include <asm/barrier.h>
> +#include <asm/mmu.h>
> +
> +#include <prng.h>
> +
> +#define MAX_CPUS 4
> +
> +/* How many increments to do */
> +static int increment_count = 10000000;
> +
> +
> +/* shared value, we use the alignment to ensure the global_lock value
> + * doesn't share a page */
> +static unsigned int shared_value;
> +
> +/* PAGE_SIZE * uint32_t means we span several pages */
> +static uint32_t memory_array[PAGE_SIZE];
> +
> +__attribute__((aligned(PAGE_SIZE))) static unsigned int per_cpu_value[MAX_CPUS];
> +__attribute__((aligned(PAGE_SIZE))) static cpumask_t smp_test_complete;
> +__attribute__((aligned(PAGE_SIZE))) static int global_lock;
> +
> +struct isaac_ctx prng_context[MAX_CPUS];
> +
> +void (*inc_fn)(void);
> +
> +static void lock(int *lock_var)
> +{
> + while (__sync_lock_test_and_set(lock_var, 1));
> +}
> +static void unlock(int *lock_var)
> +{
> + __sync_lock_release(lock_var);
> +}
> +
> +static void increment_shared(void)
> +{
> + shared_value++;
> +}
> +
> +static void increment_shared_with_lock(void)
> +{
> + lock(&global_lock);
> + shared_value++;
> + unlock(&global_lock);
> +}
> +
> +static void increment_shared_with_excl(void)
> +{
> +#if defined (__LP64__) || defined (_LP64)
> + asm volatile(
> + "1: ldxr w0, [%[sptr]]\n"
> + " add w0, w0, #0x1\n"
> + " stxr w1, w0, [%[sptr]]\n"
> + " cbnz w1, 1b\n"
> + : /* out */
> + : [sptr] "r" (&shared_value) /* in */
> + : "w0", "w1", "cc");
> +#else
> + asm volatile(
> + "1: ldrex r0, [%[sptr]]\n"
> + " add r0, r0, #0x1\n"
> + " strexeq r1, r0, [%[sptr]]\n"
> + " cmpeq r1, #0\n"

Why are we calling these last two instructions with the 'eq' suffix?
Shouldn't we just strex r1, r0, [sptr] and then cmp r1, #0?

Thank you,
alvise

> + " bne 1b\n"
> + : /* out */
> + : [sptr] "r" (&shared_value) /* in */
> + : "r0", "r1", "cc");
> +#endif
> +}
> +
> +static void increment_shared_with_acqrel(void)
> +{
> +#if defined (__LP64__) || defined (_LP64)
> + asm volatile(
> + " ldar w0, [%[sptr]]\n"
> + " add w0, w0, #0x1\n"
> + " str w0, [%[sptr]]\n"
> + : /* out */
> + : [sptr] "r" (&shared_value) /* in */
> + : "w0");
> +#else
> + /* ARMv7 has no acquire/release semantics but we
> + * can ensure the results of the write are propagated
> + * with the use of barriers.
> + */
> + asm volatile(
> + "1: ldrex r0, [%[sptr]]\n"
> + " add r0, r0, #0x1\n"
> + " strexeq r1, r0, [%[sptr]]\n"
> + " cmpeq r1, #0\n"
> + " bne 1b\n"
> + " dmb\n"
> + : /* out */
> + : [sptr] "r" (&shared_value) /* in */
> + : "r0", "r1", "cc");
> +#endif
> +
> +}
> +
> +/* The idea of this is just to generate some random load/store
> + * activity which may or may not race with an un-barried incremented
> + * of the shared counter
> + */
> +static void shuffle_memory(int cpu)
> +{
> + int i;
> + uint32_t lspat = isaac_next_uint32(&prng_context[cpu]);
> + uint32_t seq = isaac_next_uint32(&prng_context[cpu]);
> + int count = seq & 0x1f;
> + uint32_t val=0;
> +
> + seq >>= 5;
> +
> + for (i=0; i<count; i++) {
> + int index = seq & ~PAGE_MASK;
> + if (lspat & 1) {
> + val ^= memory_array[index];
> + } else {
> + memory_array[index] = val;
> + }
> + seq >>= PAGE_SHIFT;
> + seq ^= lspat;
> + lspat >>= 1;
> + }
> +
> +}
> +
> +static void do_increment(void)
> +{
> + int i;
> + int cpu = smp_processor_id();
> +
> + printf("CPU%d online\n", cpu);
> +
> + for (i=0; i < increment_count; i++) {
> + per_cpu_value[cpu]++;
> + inc_fn();
> +
> + shuffle_memory(cpu);
> + }
> +
> + printf("CPU%d: Done, %d incs\n", cpu, per_cpu_value[cpu]);
> +
> + cpumask_set_cpu(cpu, &smp_test_complete);
> + if (cpu != 0)
> + halt();
> +}
> +
> +int main(int argc, char **argv)
> +{
> + int cpu;
> + unsigned int i, sum = 0;
> + static const unsigned char seed[] = "myseed";
> +
> + inc_fn = &increment_shared;
> +
> + isaac_init(&prng_context[0], &seed[0], sizeof(seed));
> +
> + for (i=0; i<argc; i++) {
> + char *arg = argv[i];
> +
> + if (strcmp(arg, "lock") == 0) {
> + inc_fn = &increment_shared_with_lock;
> + report_prefix_push("lock");
> + } else if (strcmp(arg, "excl") == 0) {
> + inc_fn = &increment_shared_with_excl;
> + report_prefix_push("excl");
> + } else if (strcmp(arg, "acqrel") == 0) {
> + inc_fn = &increment_shared_with_acqrel;
> + report_prefix_push("acqrel");
> + } else {
> + isaac_reseed(&prng_context[0], (unsigned char *) arg, strlen(arg));
> + }
> + }
> +
> + /* fill our random page */
> + for (i=0; i<PAGE_SIZE; i++) {
> + memory_array[i] = isaac_next_uint32(&prng_context[0]);
> + }
> +
> + for_each_present_cpu(cpu) {
> + uint32_t seed2 = isaac_next_uint32(&prng_context[0]);
> + if (cpu == 0)
> + continue;
> +
> + isaac_init(&prng_context[cpu], (unsigned char *) &seed2, sizeof(seed2));
> + smp_boot_secondary(cpu, do_increment);
> + }
> +
> + do_increment();
> +
> + while (!cpumask_full(&smp_test_complete))
> + cpu_relax();
> +
> + /* All CPUs done, do we add up */
> + for_each_present_cpu(cpu) {
> + sum += per_cpu_value[cpu];
> + }
> + report("total incs %d", sum == shared_value, shared_value);
> +
> + return report_summary();
> +}
> diff --git a/config/config-arm-common.mak b/config/config-arm-common.mak
> index 67a9dda..af628e6 100644
> --- a/config/config-arm-common.mak
> +++ b/config/config-arm-common.mak
> @@ -12,6 +12,7 @@ endif
> tests-common = $(TEST_DIR)/selftest.flat
> tests-common += $(TEST_DIR)/spinlock-test.flat
> tests-common += $(TEST_DIR)/tlbflush-test.flat
> +tests-common += $(TEST_DIR)/barrier-test.flat
>
> utils-common = $(TEST_DIR)/utils/kvm-query
>
> @@ -80,3 +81,4 @@ utils: $(utils-common)
> $(TEST_DIR)/selftest.elf: $(cstart.o) $(TEST_DIR)/selftest.o
> $(TEST_DIR)/spinlock-test.elf: $(cstart.o) $(TEST_DIR)/spinlock-test.o
> $(TEST_DIR)/tlbflush-test.elf: $(cstart.o) $(TEST_DIR)/tlbflush-test.o
> +$(TEST_DIR)/barrier-test.elf: $(cstart.o) $(TEST_DIR)/barrier-test.o
> --
> 2.5.0
>