Reset nr_hugepages to zero before the start of the test.
If a non-zero number of hugepages is already set before the start of the
test, the following problems arise:
- The probability of the test getting OOM-killed increases.
Proof: The test wants to run on 80% of available memory to prevent
OOM-killing (see original code comments). Let the value of mem_free at the
start of the test, when nr_hugepages = 0, be x. In the other case, when
nr_hugepages > 0, let the memory consumed by hugepages be y. In the former
case, the test operates on 0.8 * x of memory. In the latter, the test
operates on 0.8 * (x - y) of memory, with y already filled, hence, memory
consumed is y + 0.8 * (x - y) = 0.8 * x + 0.2 * y > 0.8 * x. Q.E.D
- The probability of a bogus test success increases.
Proof: Let the memory consumed by hugepages be greater than 25% of x,
with x and y defined as above. The definition of compaction_index is
c_index = (x - y)/z where z is the memory consumed by hugepages after
trying to increase them again. In check_compaction(), we set the number
of hugepages to zero, and then increase them back; the probability that
they will be set back to consume at least y amount of memory again is
very high (since there is not much delay between the two attempts of
changing nr_hugepages). Hence, z >= y > (x/4) (by the 25% assumption).
Therefore,
c_index = (x - y)/z <= (x - y)/y = x/y - 1 < 4 - 1 = 3
hence, c_index can always be forced to be less than 3, thereby the test
succeeding always. Q.E.D
NOTE: This patch depends on the previous one.
Fixes: bd67d5c15cc1 ("Test compaction of mlocked memory")
Cc: [email protected]
Signed-off-by: Dev Jain <[email protected]>
---
tools/testing/selftests/mm/compaction_test.c | 72 ++++++++++++++------
1 file changed, 50 insertions(+), 22 deletions(-)
diff --git a/tools/testing/selftests/mm/compaction_test.c b/tools/testing/selftests/mm/compaction_test.c
index c5be395f8363..2ae059989771 100644
--- a/tools/testing/selftests/mm/compaction_test.c
+++ b/tools/testing/selftests/mm/compaction_test.c
@@ -82,12 +82,15 @@ int prereq(void)
return -1;
}
-int check_compaction(unsigned long mem_free, unsigned int hugepage_size)
+int check_compaction(unsigned long mem_free, unsigned int hugepage_size,
+ int initial_nr_hugepages)
{
int fd, ret = -1;
int compaction_index = 0;
- char initial_nr_hugepages[10] = {0};
char nr_hugepages[10] = {0};
+ char init_nr_hugepages[10] = {0};
+
+ sprintf(init_nr_hugepages, "%d", initial_nr_hugepages);
/* We want to test with 80% of available memory. Else, OOM killer comes
in to play */
@@ -101,23 +104,6 @@ int check_compaction(unsigned long mem_free, unsigned int hugepage_size)
goto out;
}
- if (read(fd, initial_nr_hugepages, sizeof(initial_nr_hugepages)) <= 0) {
- ksft_print_msg("Failed to read from /proc/sys/vm/nr_hugepages: %s\n",
- strerror(errno));
- goto close_fd;
- }
-
- lseek(fd, 0, SEEK_SET);
-
- /* Start with the initial condition of 0 huge pages*/
- if (write(fd, "0", sizeof(char)) != sizeof(char)) {
- ksft_print_msg("Failed to write 0 to /proc/sys/vm/nr_hugepages: %s\n",
- strerror(errno));
- goto close_fd;
- }
-
- lseek(fd, 0, SEEK_SET);
-
/* Request a large number of huge pages. The Kernel will allocate
as much as it can */
if (write(fd, "100000", (6*sizeof(char))) != (6*sizeof(char))) {
@@ -140,8 +126,8 @@ int check_compaction(unsigned long mem_free, unsigned int hugepage_size)
lseek(fd, 0, SEEK_SET);
- if (write(fd, initial_nr_hugepages, strlen(initial_nr_hugepages))
- != strlen(initial_nr_hugepages)) {
+ if (write(fd, init_nr_hugepages, strlen(init_nr_hugepages))
+ != strlen(init_nr_hugepages)) {
ksft_print_msg("Failed to write value to /proc/sys/vm/nr_hugepages: %s\n",
strerror(errno));
goto close_fd;
@@ -165,6 +151,42 @@ int check_compaction(unsigned long mem_free, unsigned int hugepage_size)
return ret;
}
+int set_zero_hugepages(int *initial_nr_hugepages)
+{
+ int fd, ret = -1;
+ char nr_hugepages[10] = {0};
+
+ fd = open("/proc/sys/vm/nr_hugepages", O_RDWR | O_NONBLOCK);
+ if (fd < 0) {
+ ksft_print_msg("Failed to open /proc/sys/vm/nr_hugepages: %s\n",
+ strerror(errno));
+ goto out;
+ }
+
+ if (read(fd, nr_hugepages, sizeof(nr_hugepages)) <= 0) {
+ ksft_print_msg("Failed to read from /proc/sys/vm/nr_hugepages: %s\n",
+ strerror(errno));
+ goto close_fd;
+ }
+
+ lseek(fd, 0, SEEK_SET);
+
+ /* Start with the initial condition of 0 huge pages */
+ if (write(fd, "0", sizeof(char)) != sizeof(char)) {
+ ksft_print_msg("Failed to write 0 to /proc/sys/vm/nr_hugepages: %s\n",
+ strerror(errno));
+ goto close_fd;
+ }
+
+ *initial_nr_hugepages = atoi(nr_hugepages);
+ ret = 0;
+
+ close_fd:
+ close(fd);
+
+ out:
+ return ret;
+}
int main(int argc, char **argv)
{
@@ -175,6 +197,7 @@ int main(int argc, char **argv)
unsigned long mem_free = 0;
unsigned long hugepage_size = 0;
long mem_fragmentable_MB = 0;
+ int initial_nr_hugepages;
ksft_print_header();
@@ -183,6 +206,10 @@ int main(int argc, char **argv)
ksft_set_plan(1);
+ /* start the test without hugepages reducing mem_free */
+ if (set_zero_hugepages(&initial_nr_hugepages))
+ return ksft_exit_fail();
+
lim.rlim_cur = RLIM_INFINITY;
lim.rlim_max = RLIM_INFINITY;
if (setrlimit(RLIMIT_MEMLOCK, &lim))
@@ -226,7 +253,8 @@ int main(int argc, char **argv)
entry = entry->next;
}
- if (check_compaction(mem_free, hugepage_size) == 0)
+ if (check_compaction(mem_free, hugepage_size,
+ initial_nr_hugepages) == 0)
return ksft_exit_pass();
return ksft_exit_fail();
--
2.30.2
On Wed, 15 May 2024 15:06:33 +0530 Dev Jain <[email protected]> wrote:
> Reset nr_hugepages to zero before the start of the test.
>
> If a non-zero number of hugepages is already set before the start of the
> test, the following problems arise:
>
> - The probability of the test getting OOM-killed increases.
> Proof: The test wants to run on 80% of available memory to prevent
> OOM-killing (see original code comments). Let the value of mem_free at the
> start of the test, when nr_hugepages = 0, be x. In the other case, when
> nr_hugepages > 0, let the memory consumed by hugepages be y. In the former
> case, the test operates on 0.8 * x of memory. In the latter, the test
> operates on 0.8 * (x - y) of memory, with y already filled, hence, memory
> consumed is y + 0.8 * (x - y) = 0.8 * x + 0.2 * y > 0.8 * x. Q.E.D
>
> - The probability of a bogus test success increases.
> Proof: Let the memory consumed by hugepages be greater than 25% of x,
> with x and y defined as above. The definition of compaction_index is
> c_index = (x - y)/z where z is the memory consumed by hugepages after
> trying to increase them again. In check_compaction(), we set the number
> of hugepages to zero, and then increase them back; the probability that
> they will be set back to consume at least y amount of memory again is
> very high (since there is not much delay between the two attempts of
> changing nr_hugepages). Hence, z >= y > (x/4) (by the 25% assumption).
> Therefore,
> c_index = (x - y)/z <= (x - y)/y = x/y - 1 < 4 - 1 = 3
> hence, c_index can always be forced to be less than 3, thereby the test
> succeeding always. Q.E.D
>
> NOTE: This patch depends on the previous one.
>
> -int check_compaction(unsigned long mem_free, unsigned int hugepage_size)
> +int check_compaction(unsigned long mem_free, unsigned int hugepage_size,
> + int initial_nr_hugepages)
> {
> int fd, ret = -1;
> int compaction_index = 0;
> - char initial_nr_hugepages[10] = {0};
> char nr_hugepages[10] = {0};
> + char init_nr_hugepages[10] = {0};
> +
> + sprintf(init_nr_hugepages, "%d", initial_nr_hugepages);
Well, [10] isn't really large enough. "-1111111111" requires 12 chars,
with the trailing \0. And I'd suggest an unsigned type and a %u -
negative initial_nr_hugepages doesn't make a lot of sense.
>
> +int set_zero_hugepages(int *initial_nr_hugepages)
> +{
> + int fd, ret = -1;
> + char nr_hugepages[10] = {0};
Ditto?
> + fd = open("/proc/sys/vm/nr_hugepages", O_RDWR | O_NONBLOCK);
> + if (fd < 0) {
> + ksft_print_msg("Failed to open /proc/sys/vm/nr_hugepages: %s\n",
> + strerror(errno));
> + goto out;
> + }
> +
> + if (read(fd, nr_hugepages, sizeof(nr_hugepages)) <= 0) {
> + ksft_print_msg("Failed to read from /proc/sys/vm/nr_hugepages: %s\n",
> + strerror(errno));
> + goto close_fd;
> + }
> +
> + lseek(fd, 0, SEEK_SET);
> +
> + /* Start with the initial condition of 0 huge pages */
> + if (write(fd, "0", sizeof(char)) != sizeof(char)) {
> + ksft_print_msg("Failed to write 0 to /proc/sys/vm/nr_hugepages: %s\n",
> + strerror(errno));
> + goto close_fd;
> + }
> +
> + *initial_nr_hugepages = atoi(nr_hugepages);
> + ret = 0;
> +
> + close_fd:
> + close(fd);
> +
> + out:
> + return ret;
> +}
On 5/20/24 05:33, Andrew Morton wrote:
> On Wed, 15 May 2024 15:06:33 +0530 Dev Jain <[email protected]> wrote:
>
>> Reset nr_hugepages to zero before the start of the test.
>>
>> If a non-zero number of hugepages is already set before the start of the
>> test, the following problems arise:
>>
>> - The probability of the test getting OOM-killed increases.
>> Proof: The test wants to run on 80% of available memory to prevent
>> OOM-killing (see original code comments). Let the value of mem_free at the
>> start of the test, when nr_hugepages = 0, be x. In the other case, when
>> nr_hugepages > 0, let the memory consumed by hugepages be y. In the former
>> case, the test operates on 0.8 * x of memory. In the latter, the test
>> operates on 0.8 * (x - y) of memory, with y already filled, hence, memory
>> consumed is y + 0.8 * (x - y) = 0.8 * x + 0.2 * y > 0.8 * x. Q.E.D
>>
>> - The probability of a bogus test success increases.
>> Proof: Let the memory consumed by hugepages be greater than 25% of x,
>> with x and y defined as above. The definition of compaction_index is
>> c_index = (x - y)/z where z is the memory consumed by hugepages after
>> trying to increase them again. In check_compaction(), we set the number
>> of hugepages to zero, and then increase them back; the probability that
>> they will be set back to consume at least y amount of memory again is
>> very high (since there is not much delay between the two attempts of
>> changing nr_hugepages). Hence, z >= y > (x/4) (by the 25% assumption).
>> Therefore,
>> c_index = (x - y)/z <= (x - y)/y = x/y - 1 < 4 - 1 = 3
>> hence, c_index can always be forced to be less than 3, thereby the test
>> succeeding always. Q.E.D
>>
>> NOTE: This patch depends on the previous one.
>>
>> -int check_compaction(unsigned long mem_free, unsigned int hugepage_size)
>> +int check_compaction(unsigned long mem_free, unsigned int hugepage_size,
>> + int initial_nr_hugepages)
>> {
>> int fd, ret = -1;
>> int compaction_index = 0;
>> - char initial_nr_hugepages[10] = {0};
>> char nr_hugepages[10] = {0};
>> + char init_nr_hugepages[10] = {0};
>> +
>> + sprintf(init_nr_hugepages, "%d", initial_nr_hugepages);
> Well, [10] isn't really large enough. "-1111111111" requires 12 chars,
> with the trailing \0. And I'd suggest an unsigned type and a %u -
> negative initial_nr_hugepages doesn't make a lot of sense.
>
>>
>> +int set_zero_hugepages(int *initial_nr_hugepages)
>> +{
>> + int fd, ret = -1;
>> + char nr_hugepages[10] = {0};
> Ditto?
Sure, makes sense. I'll just change that to 20 and make it unsigned.
>
>> + fd = open("/proc/sys/vm/nr_hugepages", O_RDWR | O_NONBLOCK);
>> + if (fd < 0) {
>> + ksft_print_msg("Failed to open /proc/sys/vm/nr_hugepages: %s\n",
>> + strerror(errno));
>> + goto out;
>> + }
>> +
>> + if (read(fd, nr_hugepages, sizeof(nr_hugepages)) <= 0) {
>> + ksft_print_msg("Failed to read from /proc/sys/vm/nr_hugepages: %s\n",
>> + strerror(errno));
>> + goto close_fd;
>> + }
>> +
>> + lseek(fd, 0, SEEK_SET);
>> +
>> + /* Start with the initial condition of 0 huge pages */
>> + if (write(fd, "0", sizeof(char)) != sizeof(char)) {
>> + ksft_print_msg("Failed to write 0 to /proc/sys/vm/nr_hugepages: %s\n",
>> + strerror(errno));
>> + goto close_fd;
>> + }
>> +
>> + *initial_nr_hugepages = atoi(nr_hugepages);
>> + ret = 0;
>> +
>> + close_fd:
>> + close(fd);
>> +
>> + out:
>> + return ret;
>> +}