cpumask: reduce cpumask_size

Now we're sure noone is using old cpumask operators, nor *cpumask, we can allocate less bits safely. This reduces the memory usage of off-stack cpumasks when CONFIG_CPUMASK_OFFSTACK=y but we don't have NR_CPUS actual cpus.

Signed-off-by Rusty Russell <[email protected]>

Cc Arnd Bergmann <[email protected]>

Cc [email protected]

Cc KOSAKI Motohiro <[email protected]>

Cc Mike Travis <[email protected]>

---
 include/linux/cpumask.h |    6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

include/linux/cpumask.h +2 -4

@@ -1014,13 +1014,11 @@ static inline int cpulist_parse(const ch
 /**
  * cpumask_size - size to allocate for a 'struct cpumask' in bytes
  *
- * This will eventually be a runtime variable, depending on nr_cpu_ids.
+ * This can be a runtime variable, depending on nr_cpu_ids.
  */
 static inline size_t cpumask_size(void)
 {
-	/* FIXME: Once all cpumask assignments are eliminated, this
-	 * can be nr_cpumask_bits */
-	return BITS_TO_LONGS(NR_CPUS) * sizeof(long);
+	return BITS_TO_LONGS(nr_cpumask_bits) * sizeof(long);
 }
 
 /*

Quoted 4 lines

Now we're sure noone is using old cpumask operators, nor *cpumask, we can allocate less bits safely. This reduces the memory usage of off-stack cpumasks when CONFIG_CPUMASK_OFFSTACK=y but we don't have NR_CPUS actual cpus.

I have to say I'm sorry. Probably I broke your assumption. If this patch applied, we reintroduce exposing nr_cpu_ids issue and break libnuma again. I think following change is necessary too.

Or, Am I missing something?

Quoted 10 lines

Quoted 4 lines

Now we're sure noone is using old cpumask operators, nor *cpumask, we can allocate less bits safely. This reduces the memory usage of off-stack cpumasks when CONFIG_CPUMASK_OFFSTACK=y but we don't have NR_CPUS actual cpus.

I have to say I'm sorry. Probably I broke your assumption. If this patch applied, we reintroduce exposing nr_cpu_ids issue and break libnuma again. I think following change is necessary too.

Or, Am I missing something?

I cc'd you because I remembered you being involved in that libnuma issue and couldn't remember the details.

Unfortunately, this solution doesn't work:

Quoted 6 lines

kernel/sched.c +0 -0

index 18faf4d..c14acad 100644
@@ -4823,7 +4823,9 @@ SYSCALL_DEFINE3(sched_getaffinity, pid_t, pid, unsigned int, len,
 

kernel/sched.c SYSCALL_DEFINE3(sched_getaffinity, pid_t, pid, unsigned int, len, · line 4828

Quoted 6 lines

kernel/sched.c +2 -1

        ret = sched_getaffinity(pid, mask);
        if (ret == 0) {
-               size_t retlen = min_t(size_t, len, cpumask_size());
+               size_t retlen = min_t(size_t, len,
+                                     BITS_TO_LONGS(NR_CPUS) * sizeof(long));
 

Since mask is a cpumask_var_t, only cpumask_size() is allocated. We can't copy NR_CPUS bits.

But I think it's OK, anyway. libnuma is broken because it gets upset if the number of cpus it reads from /sys/.../cpumap is more than the cpumask size returned from sys_sched_getaffinity.

Currently, getaffinity returns cpumask_size() (ie. based on NR_CPUS), and the printing routines use nr_cpumask_bits (ie. based on NR_CPUS for !CPUMASK_OFFSTACK, nr_cpu_ids for CPUMASK_OFFSTACK).

(libnuma is OK on CONFIG_CPUMASK_OFFSTACK=y because the sysfs output is *shorter* than expected. I checked the code).

With this patch, cpumask_size() becomes based on nr_cpumask_bits, so both getaffinity and sysfs are using the same basis.

Do you agree?