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

["Alex Bennée" <alex.bennee@linaro.org>]

alvise rigo <a.rigo@virtualopensystems.com> writes:

> 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?

Possibly, my armv7 is a little rusty. I'm just looking at tweaking this
test now so I'll try and clean that up.

>
> 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
>>

-- 
Alex Bennée