2023-06-03 02:29:10

by John Hubbard

Subject: [PATCH v2 02/11] selftests/mm: fix unused variable warnings in hugetlb-madvise.c, migration.c

Dummy variables are required in order to make these two (similar)
routines work, so in both cases, declare the variables as volatile in
order to avoid the clang compiler warning.

Furthermore, in order to ensure that each test actually does what is
intended, add an asm volatile invocation (thanks to David Hildenbrand
for the suggestion), with a clarifying comment so that it survives
future maintenance.

Cc: David Hildenbrand <[email protected]>
Cc: Peter Xu <[email protected]>
Signed-off-by: John Hubbard <[email protected]>
---
tools/testing/selftests/mm/hugetlb-madvise.c | 8 ++++++--
tools/testing/selftests/mm/migration.c | 5 ++++-
2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/mm/hugetlb-madvise.c b/tools/testing/selftests/mm/hugetlb-madvise.c
index 28426e30d9bc..d55322df4b73 100644
--- a/tools/testing/selftests/mm/hugetlb-madvise.c
+++ b/tools/testing/selftests/mm/hugetlb-madvise.c
@@ -65,11 +65,15 @@ void write_fault_pages(void *addr, unsigned long nr_pages)

void read_fault_pages(void *addr, unsigned long nr_pages)
{
- unsigned long dummy = 0;
+ volatile unsigned long dummy = 0;
unsigned long i;

- for (i = 0; i < nr_pages; i++)
+ for (i = 0; i < nr_pages; i++) {
dummy += *((unsigned long *)(addr + (i * huge_page_size)));
+
+ /* Prevent the compiler from optimizing out the entire loop: */
+ asm volatile("" : "+r" (dummy));
+ }
}

int main(int argc, char **argv)
diff --git a/tools/testing/selftests/mm/migration.c b/tools/testing/selftests/mm/migration.c
index 1cec8425e3ca..379581567f27 100644
--- a/tools/testing/selftests/mm/migration.c
+++ b/tools/testing/selftests/mm/migration.c
@@ -95,12 +95,15 @@ int migrate(uint64_t *ptr, int n1, int n2)

void *access_mem(void *ptr)
{
- uint64_t y = 0;
+ volatile uint64_t y = 0;
volatile uint64_t *x = ptr;

while (1) {
pthread_testcancel();
y += *x;
+
+ /* Prevent the compiler from optimizing out the writes to y: */
+ asm volatile("" : "+r" (y));
}

return NULL;
--
2.40.1

2023-06-05 11:41:29

by David Hildenbrand

Subject: Re: [PATCH v2 02/11] selftests/mm: fix unused variable warnings in hugetlb-madvise.c, migration.c

On 03.06.23 04:15, John Hubbard wrote:
> Dummy variables are required in order to make these two (similar)
> routines work, so in both cases, declare the variables as volatile in
> order to avoid the clang compiler warning.
>
> Furthermore, in order to ensure that each test actually does what is
> intended, add an asm volatile invocation (thanks to David Hildenbrand
> for the suggestion), with a clarifying comment so that it survives
> future maintenance.
>
> Cc: David Hildenbrand <[email protected]>
> Cc: Peter Xu <[email protected]>
> Signed-off-by: John Hubbard <[email protected]>
> ---
> tools/testing/selftests/mm/hugetlb-madvise.c | 8 ++++++--
> tools/testing/selftests/mm/migration.c | 5 ++++-
> 2 files changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/hugetlb-madvise.c b/tools/testing/selftests/mm/hugetlb-madvise.c
> index 28426e30d9bc..d55322df4b73 100644
> --- a/tools/testing/selftests/mm/hugetlb-madvise.c
> +++ b/tools/testing/selftests/mm/hugetlb-madvise.c
> @@ -65,11 +65,15 @@ void write_fault_pages(void *addr, unsigned long nr_pages)
>
> void read_fault_pages(void *addr, unsigned long nr_pages)
> {
> - unsigned long dummy = 0;
> + volatile unsigned long dummy = 0;
> unsigned long i;
>
> - for (i = 0; i < nr_pages; i++)
> + for (i = 0; i < nr_pages; i++) {
> dummy += *((unsigned long *)(addr + (i * huge_page_size)));
> +
> + /* Prevent the compiler from optimizing out the entire loop: */
> + asm volatile("" : "+r" (dummy));
> + }
> }
>
> int main(int argc, char **argv)
> diff --git a/tools/testing/selftests/mm/migration.c b/tools/testing/selftests/mm/migration.c
> index 1cec8425e3ca..379581567f27 100644
> --- a/tools/testing/selftests/mm/migration.c
> +++ b/tools/testing/selftests/mm/migration.c
> @@ -95,12 +95,15 @@ int migrate(uint64_t *ptr, int n1, int n2)
>
> void *access_mem(void *ptr)
> {
> - uint64_t y = 0;
> + volatile uint64_t y = 0;
> volatile uint64_t *x = ptr;
>
> while (1) {
> pthread_testcancel();
> y += *x;
> +
> + /* Prevent the compiler from optimizing out the writes to y: */
> + asm volatile("" : "+r" (y));
> }
>
> return NULL;

With the asm, I think the "volatile" might be completely unnecessary.
But it doesn't hurt.

Reviewed-by: David Hildenbrand <[email protected]>

--
Cheers,

David / dhildenb

2023-06-05 16:02:38

by Peter Xu

Subject: Re: [PATCH v2 02/11] selftests/mm: fix unused variable warnings in hugetlb-madvise.c, migration.c

On Fri, Jun 02, 2023 at 07:15:49PM -0700, John Hubbard wrote:
> Dummy variables are required in order to make these two (similar)
> routines work, so in both cases, declare the variables as volatile in
> order to avoid the clang compiler warning.
>
> Furthermore, in order to ensure that each test actually does what is
> intended, add an asm volatile invocation (thanks to David Hildenbrand
> for the suggestion), with a clarifying comment so that it survives
> future maintenance.
>
> Cc: David Hildenbrand <[email protected]>
> Cc: Peter Xu <[email protected]>
> Signed-off-by: John Hubbard <[email protected]>

Reviewed-by: Peter Xu <[email protected]>

--
Peter Xu