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Subjectstatm_pgd_range() sucks!
Okay, I have *had it* with statm_pgd_range()!

So I started up top(1) to monitor a benchmark. And the idiot thing
couldn't get out of the kernel fast enough to refresh the screen every
5s and ate an entire cpu (well, I did have 15 to spare, but still). I
pulled the plug and applied the following, which fixed the situation,
albeit with slight changes to the semantics of what's reported.

(1) shared, lib, text, & total are now reported as what's mapped
instead of what's resident. This actually fixes two bugs:
(1A) Misreporting the total size of what's mapped by a task
when third-level pagetables haven't been instantiated
for regions (i.e. incorrect VSZ).
(1B) Misreporting "shared" because elevated reference counts
do not represent sharing of pages between processes,
such things as presence on the LRU and buffers foil it.
shared is now the size of the regions mapped as MAP_SHARED,
which is different, but at least meaningful.

(2) dirty and reported as 0. It wasn't used by userspace anyway, and
frankly that information is too expensive to collect at all.

(3) The hardcoded i386 a.out format hack (0x60000000) is removed
and lib pages reported as 0. This value is not used by
userspace and has not been reliable or meaningful for some time,
as library pages have no special meaning to ELF. It can only be
faithfully reported by means of awareness of both the executable
format and architectural address space layout.

(4) The horrifically expensive pagetable walk is gone, and so
/proc/$PID/statm will disturb the system far less in general.
The reduction in cpu consumption observed was immense.
B = Before, A = After:

PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
B: 2813 wli 25 0 1624 1624 744 R 99.9 0.0 0:38 top
A: 259 wli 15 0 2452 1616 0 R 6.1 0.0 3:45 top

Those cpus are 700MHz P-III Xeons. This efficiency gain is immense.

Profile of a stock kernel from a UP "desktop" system where I
continually run top(1) to catch runaway memory consumers:

Before (running for 20 days or so):
1492108515 total 1102.3391
1340420417 default_idle 27925425.3542
13822366 statm_pgd_range 29789.5819
7922620 number 11515.4360
7052090 handle_IRQ_event 88151.1250
6041850 fget 94403.9062
5077128 __generic_copy_to_user 63464.1000
4278107 csum_partial_copy_generic 16711.3555
3968287 vsnprintf 3757.8475
3336038 proc_pid_stat 4255.1505

After (running for 24 hours or so):
9921095 total 7.3620
9710998 default_idle 202312.4583
20253 number 18.6149
13130 proc_getdata 23.4464
12696 __generic_copy_to_user 158.7000
7454 vsnprintf 7.0587
6963 handle_IRQ_event 72.5312
6112 fast_clear_page 54.5714
4568 proc_pid_stat 6.3444
3931 kmem_cache_free 27.2986


statm_pgd_range() was eating buttloads of cpu for exactly zero
gain. Cutting it down to size is worthwhile even for "desktops".
This UP box is under heavy load with general network I/O,
serving up nfsroots to ancient NetBSD toasters, Mozilla/GNOME
gunk, and by some braindeath top(1) manages to dominate the
profile. It's not difficult to understand that a real-life
end-user interactive load is being dominated, and perhaps even
its progress hindered, by the cost of collecting information to
report to userspace in a grossly inefficient manner. And yes, I
do need to know what's eating cpu & mem on my box at all times.
(Before telling me to get more memory, I see enough highmem to
know I don't want highmem at home, and it's already got 768MB.)

(5) The code fits into 80x24 and is generally easier on the eyes.

(6) Less code:
$ diffstat no_statm_pgd_range-2.5.31-4
array.c | 128 ++++++++++------------------------------------------------------
1 files changed, 21 insertions(+), 107 deletions(-)


Against 2.5.31, applies cleanly to 2.5.32.


Cheers,
Bill


===== fs/proc/array.c 1.26 vs edited =====
--- 1.26/fs/proc/array.c Wed Jul 24 18:36:09 2002
+++ edited/fs/proc/array.c Tue Aug 20 09:12:37 2002
@@ -394,120 +394,34 @@
return res;
}

-static inline void statm_pte_range(pmd_t * pmd, unsigned long address, unsigned long size, int * pages, int * shared, int * dirty, int * total)
-{
- unsigned long end, pmd_end;
- pte_t *pte;
-
- if (pmd_none(*pmd))
- return;
- if (pmd_bad(*pmd)) {
- pmd_ERROR(*pmd);
- pmd_clear(pmd);
- return;
- }
- preempt_disable();
- pte = pte_offset_map(pmd, address);
- end = address + size;
- pmd_end = (address + PMD_SIZE) & PMD_MASK;
- if (end > pmd_end)
- end = pmd_end;
- do {
- pte_t page = *pte;
- struct page *ptpage;
- unsigned long pfn;
-
- address += PAGE_SIZE;
- pte++;
- if (pte_none(page))
- continue;
- ++*total;
- if (!pte_present(page))
- continue;
- pfn = pte_pfn(page);
- if (!pfn_valid(pfn))
- continue;
- ptpage = pfn_to_page(pfn);
- if (PageReserved(ptpage))
- continue;
- ++*pages;
- if (pte_dirty(page))
- ++*dirty;
- if (page_count(pte_page(page)) > 1)
- ++*shared;
- } while (address < end);
- pte_unmap(pte - 1);
- preempt_enable();
-}
-
-static inline void statm_pmd_range(pgd_t * pgd, unsigned long address, unsigned long size,
- int * pages, int * shared, int * dirty, int * total)
-{
- pmd_t * pmd;
- unsigned long end;
-
- if (pgd_none(*pgd))
- return;
- if (pgd_bad(*pgd)) {
- pgd_ERROR(*pgd);
- pgd_clear(pgd);
- return;
- }
- pmd = pmd_offset(pgd, address);
- address &= ~PGDIR_MASK;
- end = address + size;
- if (end > PGDIR_SIZE)
- end = PGDIR_SIZE;
- do {
- statm_pte_range(pmd, address, end - address, pages, shared, dirty, total);
- address = (address + PMD_SIZE) & PMD_MASK;
- pmd++;
- } while (address < end);
-}
-
-static void statm_pgd_range(pgd_t * pgd, unsigned long address, unsigned long end,
- int * pages, int * shared, int * dirty, int * total)
-{
- while (address < end) {
- statm_pmd_range(pgd, address, end - address, pages, shared, dirty, total);
- address = (address + PGDIR_SIZE) & PGDIR_MASK;
- pgd++;
- }
-}
-
int proc_pid_statm(struct task_struct *task, char * buffer)
{
- int size=0, resident=0, share=0, trs=0, lrs=0, drs=0, dt=0;
+ int size, resident, shared, text, lib, data, dirty = 0;
struct mm_struct *mm = get_task_mm(task);
+ struct vm_area_struct * vma;

- if (mm) {
- struct vm_area_struct * vma;
- down_read(&mm->mmap_sem);
- vma = mm->mmap;
- while (vma) {
- pgd_t *pgd = pgd_offset(mm, vma->vm_start);
- int pages = 0, shared = 0, dirty = 0, total = 0;
+ size = resident = shared = text = lib = data = 0;

- statm_pgd_range(pgd, vma->vm_start, vma->vm_end, &pages, &shared, &dirty, &total);
- resident += pages;
- share += shared;
- dt += dirty;
- size += total;
- if (vma->vm_flags & VM_EXECUTABLE)
- trs += pages; /* text */
- else if (vma->vm_flags & VM_GROWSDOWN)
- drs += pages; /* stack */
- else if (vma->vm_end > 0x60000000)
- lrs += pages; /* library */
- else
- drs += pages;
- vma = vma->vm_next;
- }
- up_read(&mm->mmap_sem);
- mmput(mm);
+ if (!mm)
+ goto out;
+
+ down_read(&mm->mmap_sem);
+ resident = mm->rss;
+ for (vma = mm->mmap; vma; vma = vma->vm_next) {
+ int pages = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
+ size += pages;
+ if (vma->vm_flags & VM_SHARED)
+ shared += pages;
+ if (vma->vm_flags & VM_EXECUTABLE)
+ text += pages;
+ else
+ data += pages;
}
+ up_read(&mm->mmap_sem);
+ mmput(mm);
+out:
return sprintf(buffer,"%d %d %d %d %d %d %d\n",
- size, resident, share, trs, lrs, drs, dt);
+ size, resident, shared, text, lib, data, dirty);
}

/*
-
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