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mm: get rid of 'vmalloc_info' from /proc/meminfo 

It turns out that at least some versions of glibc end up reading
/proc/meminfo at every single startup, because glibc wants to know the
amount of memory the machine has.  And while that's arguably insane,
it's just how things are.

And it turns out that it's not all that expensive most of the time, but
the vmalloc information statistics (amount of virtual memory used in the
vmalloc space, and the biggest remaining chunk) can be rather expensive
to compute.

The 'get_vmalloc_info()' function actually showed up on my profiles as
4% of the CPU usage of "make test" in the git source repository, because
the git tests are lots of very short-lived shell-scripts etc.

It turns out that apparently this same silly vmalloc info gathering
shows up on the facebook servers too, according to Dave Jones.  So it's
not just "make test" for git.

We had two patches to just cache the information (one by me, one by
Ingo) to mitigate this issue, but the whole vmalloc information of of
rather dubious value to begin with, and people who *actually* want to
know what the situation is wrt the vmalloc area should just look at the
much more complete /proc/vmallocinfo instead.

In fact, according to my testing - and perhaps more importantly,
according to that big search engine in the sky: Google - there is
nothing out there that actually cares about those two expensive fields:
VmallocUsed and VmallocChunk.

So let's try to just remove them entirely.  Actually, this just removes
the computation and reports the numbers as zero for now, just to try to
be minimally intrusive.

If this breaks anything, we'll obviously have to re-introduce the code
to compute this all and add the caching patches on top.  But if given
the option, I'd really prefer to just remove this bad idea entirely
rather than add even more code to work around our historical mistake
that likely nobody really cares about.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Linus Torvalds 2015-11-01 17:09:15 -08:00
parent 2e00266297
commit a5ad88ce8c
3 changed files with 2 additions and 64 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
fs/proc/meminfo.c
View File

@ -27,7 +27,6 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
{
struct sysinfo i;
unsigned long committed;
struct vmalloc_info vmi;
long cached;
long available;
unsigned long pagecache;
@ -49,8 +48,6 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
if (cached < 0)
cached = 0;
get_vmalloc_info(&vmi);
for (lru = LRU_BASE; lru < NR_LRU_LISTS; lru++)
pages[lru] = global_page_state(NR_LRU_BASE + lru);
@ -191,8 +188,8 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
K(vm_commit_limit()),
K(committed),
(unsigned long)VMALLOC_TOTAL >> 10,
vmi.used >> 10,
vmi.largest_chunk >> 10
0ul, // used to be vmalloc 'used'
0ul // used to be vmalloc 'largest_chunk'
#ifdef CONFIG_MEMORY_FAILURE
, atomic_long_read(&num_poisoned_pages) << (PAGE_SHIFT - 10)
#endif

12
include/linux/vmalloc.h
View File

@ -182,22 +182,10 @@ pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
# endif
#endif
struct vmalloc_info {
unsigned long used;
unsigned long largest_chunk;
};
#ifdef CONFIG_MMU
#define VMALLOC_TOTAL (VMALLOC_END - VMALLOC_START)
extern void get_vmalloc_info(struct vmalloc_info *vmi);
#else
#define VMALLOC_TOTAL 0UL
#define get_vmalloc_info(vmi) \
do { \
(vmi)->used = 0; \
(vmi)->largest_chunk = 0; \
} while (0)
#endif
#endif /* _LINUX_VMALLOC_H */

47
mm/vmalloc.c
View File

@ -2688,52 +2688,5 @@ static int __init proc_vmalloc_init(void)
}
module_init(proc_vmalloc_init);
void get_vmalloc_info(struct vmalloc_info *vmi)
{
struct vmap_area *va;
unsigned long free_area_size;
unsigned long prev_end;
vmi->used = 0;
vmi->largest_chunk = 0;
prev_end = VMALLOC_START;
rcu_read_lock();
if (list_empty(&vmap_area_list)) {
vmi->largest_chunk = VMALLOC_TOTAL;
goto out;
}
list_for_each_entry_rcu(va, &vmap_area_list, list) {
unsigned long addr = va->va_start;
/*
* Some archs keep another range for modules in vmalloc space
*/
if (addr < VMALLOC_START)
continue;
if (addr >= VMALLOC_END)
break;
if (va->flags & (VM_LAZY_FREE | VM_LAZY_FREEING))
continue;
vmi->used += (va->va_end - va->va_start);
free_area_size = addr - prev_end;
if (vmi->largest_chunk < free_area_size)
vmi->largest_chunk = free_area_size;
prev_end = va->va_end;
}
if (VMALLOC_END - prev_end > vmi->largest_chunk)
vmi->largest_chunk = VMALLOC_END - prev_end;
out:
rcu_read_unlock();
}
#endif