Re: [kvm-unit-tests RFC PATCH] arm/tlbflush.c: TLB flushing torture test [DEV]

[Andrew Jones <drjones@redhat.com>]

On Fri, Jul 24, 2015 at 02:25:06PM +0100, Alex Bennée wrote:
> This adds a fairly brain dead torture test for TLB flushes intended for
> stressing the MTTCG QEMU build. It takes the usual -smp option for
> multiple CPUs.
> 
> By default it will do a TLBIALL flush after each cycle. If you pass
> -append "page" to the kernel it will take it in turns to flush each of
> the computation functions. At the moment it doesn't do any re-mapping of
> pages but maybe that is something that could be done in the future.
> 
> [DEV VERSION FOR COMMENT]
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
>  arm/tlbflush.c               | 163 +++++++++++++++++++++++++++++++++++++++++++
>  config/config-arm-common.mak |   4 +-
>  lib/arm/asm/mmu.h            |  11 +++
>  3 files changed, 177 insertions(+), 1 deletion(-)
>  create mode 100644 arm/tlbflush.c
> 
> diff --git a/arm/tlbflush.c b/arm/tlbflush.c
> new file mode 100644
> index 0000000..6eeff18
> --- /dev/null
> +++ b/arm/tlbflush.c
> @@ -0,0 +1,163 @@
> +#include <libcflat.h>
> +#include <asm/smp.h>
> +#include <asm/cpumask.h>
> +#include <asm/barrier.h>
> +#include <asm/mmu.h>
> +
> +#define SEQ_LENGTH 10
> +
> +static cpumask_t smp_test_complete;
> +static int flush_count = 100000;
> +static int flush_self = 1;
> +static int flush_page = 0;
> +
> +__attribute__((aligned(0x1000))) unsigned int hash_array(int length, unsigned int *array)

You should use PAGE_SIZE instead of 0x1000 in these attributes, allowing
the test to also work for aarch64, as we're using 64k pages on aarch64.

> +{
> +	int i;
> +	unsigned int sum=0;
> +	for (i=0; i<length; i++)
> +	{
> +		unsigned int val = *array++;
> +		sum ^= val;
> +		sum ^= (val >> (val % 16));
> +		sum ^= (val << (val % 32));
> +	}
> +
> +	return sum;
> +}
> +
> +__attribute__((aligned(0x1000))) void create_fib_sequence(int length, unsigned int *array)
> +{
> +	int i;
> +
> +	/* first two values */
> +	array[0] = 0;
> +	array[1] = 1;
> +	for (i=2; i<length; i++)
> +	{
> +		array[i] = array[i-2] + array[i-1];
> +	}
> +}
> +
> +__attribute__((aligned(0x1000))) unsigned long long factorial(unsigned int n)
> +{
> +	unsigned int i;
> +	unsigned long long fac = 1;
> +	for (i=1; i<=n; i++)
> +	{
> +		fac = fac * i;
> +	}
> +	return fac;
> +}
> +
> +/* do some computationally expensive stuff, return a checksum of the
> + * results */
> +__attribute__((aligned(0x1000))) unsigned int do_computation(void)
> +{
> +	unsigned int fib_array[SEQ_LENGTH];
> +	unsigned long long facfib_array[SEQ_LENGTH];
> +	unsigned int fib_hash, facfib_hash;
> +	int cpu = smp_processor_id();
> +	int i, j;
> +	
> +	create_fib_sequence(SEQ_LENGTH, &fib_array[0]);
> +	fib_hash = hash_array(SEQ_LENGTH, &fib_array[0]);
> +	for (i=0; i<SEQ_LENGTH; i++) {
> +		for (j=0; j<fib_array[i]; j++) {
> +			facfib_array[i] = factorial(fib_array[i]+j);
> +		}
> +	}
> +	facfib_hash = 0;
> +	for (i=0; i<SEQ_LENGTH; i++) {
> +		for (j=0; j<fib_array[i]; j++) {
> +			facfib_hash ^= hash_array(sizeof(facfib_array)/sizeof(unsigned int), (unsigned int *)&facfib_array[0]);
> +		}
> +	}
> +
> +#if 0
> +	printf("CPU:%d FIBSEQ ", cpu);
> +	for (i=0; i<SEQ_LENGTH; i++)
> +		printf("%u,", fib_array[i]);
> +	printf("\n");
> +
> +	printf("CPU:%d FACFIB ", cpu);
> +	for (i=0; i<SEQ_LENGTH; i++)
> +		printf("%llu,", facfib_array[i]);
> +	printf("\n");
> +#endif
> +	
> +	return (fib_hash ^ facfib_hash);
> +}
> +
> +static void * pages[] = {&hash_array, &create_fib_sequence, &factorial, &do_computation};

I can't comment on whether or not the complexity of do_computation is
necessary for your test, but it seems like overkill. Comments explaining
why it's necessary would be good.

> +
> +static void test_flush(void)
> +{
> +	int i, errors = 0;
> +	int cpu = smp_processor_id();
> +
> +	unsigned int ref;
> +
> +	printf("CPU%d online\n", cpu);
> +
> +	ref = do_computation();

What makes you sure that the first time you do the computation
per cpu is correct? I think computing it externally, and saving
the result, i.e. 

#define EXPECTED_RESULT 0x12345678

would be more reliable.

> +
> +	for (i=0; i < flush_count; i++) {
> +		unsigned int this_ref = do_computation();
> +
> +		if (this_ref != ref) {
> +			errors++;
> +			printf("CPU%d: seq%d 0x%x!=0x%x\n",
> +				cpu, i, ref, this_ref);
> +		}
> +
> +		if ((i % 1000) == 0) {
> +			printf("CPU%d: seq%d\n", cpu, i);
> +		}
> +		
> +		if (flush_self) {
> +			if (flush_page) {
> +				int j = (i % (sizeof(pages)/sizeof(void *)));
libcflat.h has the ARRAY_SIZE macro
> +				flush_tlb_page((unsigned long)pages[j]);
> +			} else {
> +				flush_tlb_all();
> +			}
> +		}
> +	}
> +
> +	report("CPU%d: Done - Errors: %d\n", errors == 0, cpu, errors);
> +
> +	cpumask_set_cpu(cpu, &smp_test_complete);
> +	if (cpu != 0)
> +		halt();
> +}
> +
> +int main(int argc, char **argv)
> +{
> +	int cpu, i;
> +	
> +	report_prefix_push("tlbflush");
> +
> +	for (i=0; i<argc; i++) {
> +		char *arg = argv[i];
> +/* 		printf("arg:%d:%s\n", i, arg); */
> +
> +		if (strcmp(arg, "page") == 0) {
> +			report_prefix_push("page");
> +			flush_page = 1;
> +		}
> +	}
> +
> +	for_each_present_cpu(cpu) {
> +		if (cpu == 0)
> +			continue;
> +		smp_boot_secondary(cpu, test_flush);
> +	}
> +
> +	test_flush();
> +
> +	while (!cpumask_full(&smp_test_complete))
> +		cpu_relax();
> +
> +	return report_summary();

As we use the kernel coding style you should run

$KERNEL_SRC/scripts/checkpatch.pl -f arm/tlbflush.c

Also, please rename to tlbflush-test.c to differentiate it
from an implementation of tlbflush support, and to make
the standalone test name (if we commit those patches) more
descriptive.


> +}
> diff --git a/config/config-arm-common.mak b/config/config-arm-common.mak
> index 0674daa..5b14db4 100644
> --- a/config/config-arm-common.mak
> +++ b/config/config-arm-common.mak
> @@ -11,7 +11,8 @@ endif
>  
>  tests-common = \
>  	$(TEST_DIR)/selftest.flat \
> -	$(TEST_DIR)/spinlock-test.flat
> +	$(TEST_DIR)/spinlock-test.flat \
> +        $(TEST_DIR)/tlbflush.flat

As we're adding tests faster now it's becoming clear that the '\' list
isn't so great. To add a new test at the bottom we always have to modify
the last line too. We should either add the new one at the top (right
below the 'test-common =' line), or change this to a '+=' sequence like
some other lists are done.

>  
>  all: test_cases
>  
> @@ -72,3 +73,4 @@ test_cases: $(generated_files) $(tests-common) $(tests)
>  
>  $(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.elf: $(cstart.o) $(TEST_DIR)/tlbflush.o
> diff --git a/lib/arm/asm/mmu.h b/lib/arm/asm/mmu.h
> index c1bd01c..2bb0cde 100644
> --- a/lib/arm/asm/mmu.h
> +++ b/lib/arm/asm/mmu.h
> @@ -14,8 +14,11 @@
>  #define PTE_AF			PTE_EXT_AF
>  #define PTE_WBWA		L_PTE_MT_WRITEALLOC
>  
> +/* See B3.18.7 TLB maintenance operations */
> +
>  static inline void local_flush_tlb_all(void)
>  {
> +	/* TLBIALL */
>  	asm volatile("mcr p15, 0, %0, c8, c7, 0" :: "r" (0));
>  	dsb();
>  	isb();
> @@ -27,6 +30,14 @@ static inline void flush_tlb_all(void)
>  	local_flush_tlb_all();
>  }
>  
> +static inline void flush_tlb_page(unsigned long vaddr)
> +{
> +	/* TLBIMVAA */
> +	asm volatile("mcr p15, 0, %0, c8, c7, 3" :: "r" (vaddr));
> +	dsb();
> +	isb();
> +}
> +
>  #include <asm/mmu-api.h>
>  
>  #endif /* __ASMARM_MMU_H_ */

This mmu.h change looks good, but please add the arm64
flush_tlb_page at the same time. And anyway, I guess you'll
want your test to work for both arm and aarch64?

Thanks,
drew