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Subjectlow-latency patch
This is pretty much "it".  I ran it for six hours on a "typical
desktop", running X, netscape, Apache, StarOffice, imapd, ftpd,
sendmail, fetchmail, etc and the worst-case latency was 3-4 millisecs:

0-1 millisecs: 99.999%
1-2 0.0004%
2-3 0.00009%
3-4 0.0005%
Yes, there are probably still ways it can be tripped up, but there's no
point in addressing these unless someone can demonstrate a problem which
doesn't have an acceptable workaround. I can't, apart from waking up
kswapd.

There are now nine rescheduling points which handle:

sys_unlink()
sys_msync()
sys_read()
sys_madvise(MADV_DONTNEED)
sys_write()
sys_sync()
sys_munmap()
sys_exit() (__exit_mm)

It has been given a withering 20-hour stress test on a dual-CPU box.
I'm quite impressed that 2.4 stood up to it, actually.

The patch is quite a bit longer than ten lines because it does some
interface consolidation on zap_page_range().

There may be a buglet in vmtruncate(): the flushes which surround the
second call to zap_page_range() can flush more memory than is being
zapped. However this has been left alone.

It's not clear to this author why some callers to zap_page_range() do
both tlb and cache flushing, but others do just one or neither.

Next time around I'll probably rename conditional_schedule() to
ll_conditional_schedule(), because there are already about 50 instances
of

if (current->need_resched)
schedule();
in the kernel. These should be called conditional_schedule()...

I'll continue to maintain the "Don't do that" list.

Filesystems other than ext2 and NFS client have not been tested.

Patch against 2.4.0-test3 is attached.--- linux-2.4.0-test3/include/linux/sched.h Tue Jul 11 22:21:17 2000
+++ linux-akpm/include/linux/sched.h Wed Jul 12 20:36:37 2000
@@ -146,6 +146,8 @@
extern signed long FASTCALL(schedule_timeout(signed long timeout));
asmlinkage void schedule(void);

+#define conditional_schedule() do { if (current->need_resched) schedule(); } while (0)
+
/*
* The default fd array needs to be at least BITS_PER_LONG,
* as this is the granularity returned by copy_fdset().
--- linux-2.4.0-test3/include/linux/mm.h Tue Jul 11 22:21:17 2000
+++ linux-akpm/include/linux/mm.h Wed Jul 12 20:36:37 2000
@@ -178,6 +178,11 @@
/* bits 21-30 unused */
#define PG_reserved 31

+/* Actions for zap_page_range() */
+#define ZPR_FLUSH_CACHE 1 /* Do flush_cache_range() prior to releasing pages */
+#define ZPR_FLUSH_TLB 2 /* Do flush_tlb_range() after releasing pages */
+#define ZPR_DEFER_FREE_PAGE 4 /* Defer passing of pages to free_page until after flush_tlb_range() */
+#define ZPR_COND_RESCHED 8 /* Do a conditional_schedule() occasionally */

/* Make it prettier to test the above... */
#define Page_Uptodate(page) test_bit(PG_uptodate, &(page)->flags)
@@ -399,7 +404,7 @@

extern int map_zero_setup(struct vm_area_struct *);

-extern void zap_page_range(struct mm_struct *mm, unsigned long address, unsigned long size);
+extern void zap_page_range(struct mm_struct *mm, unsigned long address, unsigned long size, int actions);
extern int copy_page_range(struct mm_struct *dst, struct mm_struct *src, struct vm_area_struct *vma);
extern int remap_page_range(unsigned long from, unsigned long to, unsigned long size, pgprot_t prot);
extern int zeromap_page_range(unsigned long from, unsigned long size, pgprot_t prot);
--- linux-2.4.0-test3/mm/filemap.c Tue Jul 11 22:21:17 2000
+++ linux-akpm/mm/filemap.c Wed Jul 12 20:50:40 2000
@@ -160,6 +160,7 @@
start = (lstart + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;

repeat:
+ conditional_schedule(); /* sys_unlink() */
head = &mapping->pages;
spin_lock(&pagecache_lock);
curr = head->next;
@@ -450,6 +451,7 @@

page_cache_get(page);
spin_unlock(&pagecache_lock);
+ conditional_schedule(); /* sys_msync() */
lock_page(page);

/* The buffers could have been free'd while we waited for the page lock */
@@ -1081,6 +1083,8 @@
* "pos" here (the actor routine has to update the user buffer
* pointers and the remaining count).
*/
+ conditional_schedule(); /* sys_read() */
+
nr = actor(desc, page, offset, nr);
offset += nr;
index += offset >> PAGE_CACHE_SHIFT;
@@ -1533,6 +1537,7 @@
* vma/file is guaranteed to exist in the unmap/sync cases because
* mmap_sem is held.
*/
+ conditional_schedule(); /* sys_msync() */
return page->mapping->a_ops->writepage(file, page);
}

@@ -2022,9 +2027,8 @@
if (vma->vm_flags & VM_LOCKED)
return -EINVAL;

- flush_cache_range(vma->vm_mm, start, end);
- zap_page_range(vma->vm_mm, start, end - start);
- flush_tlb_range(vma->vm_mm, start, end);
+ zap_page_range(vma->vm_mm, start, end - start,
+ ZPR_FLUSH_CACHE|ZPR_FLUSH_TLB|ZPR_COND_RESCHED); /* sys_madvise(MADV_DONTNEED) */
return 0;
}

@@ -2487,6 +2491,8 @@
while (count) {
unsigned long bytes, index, offset;
char *kaddr;
+
+ conditional_schedule(); /* sys_write() */

/*
* Try to find the page in the cache. If it isn't there,
--- linux-2.4.0-test3/fs/buffer.c Tue Jul 11 22:21:16 2000
+++ linux-akpm/fs/buffer.c Sun Jul 9 23:51:04 2000
@@ -2123,6 +2123,7 @@
__wait_on_buffer(p);
} else if (buffer_dirty(p))
ll_rw_block(WRITE, 1, &p);
+ conditional_schedule(); /* sys_msync() */
} while (tmp != bh);
}

--- linux-2.4.0-test3/mm/memory.c Tue May 16 05:00:33 2000
+++ linux-akpm/mm/memory.c Tue Jul 11 20:57:32 2000
@@ -347,7 +347,7 @@
/*
* remove user pages in a given range.
*/
-void zap_page_range(struct mm_struct *mm, unsigned long address, unsigned long size)
+static void do_zap_page_range(struct mm_struct *mm, unsigned long address, unsigned long size)
{
pgd_t * dir;
unsigned long end = address + size;
@@ -381,6 +381,25 @@
}
}

+#define MAX_ZAP_BYTES 512*PAGE_SIZE /* 1 millisec @ 250 MHz */
+
+void zap_page_range(struct mm_struct *mm, unsigned long address, unsigned long size, int actions)
+{
+ while (size) {
+ unsigned long chunk = size;
+ if (actions & ZPR_COND_RESCHED && chunk > MAX_ZAP_BYTES)
+ chunk = MAX_ZAP_BYTES;
+ if (actions & ZPR_FLUSH_CACHE)
+ flush_cache_range(mm, address, address + chunk);
+ do_zap_page_range(mm, address, chunk);
+ if (actions & ZPR_FLUSH_TLB)
+ flush_tlb_range(mm, address, address + chunk);
+ if (actions & ZPR_COND_RESCHED)
+ conditional_schedule();
+ address += chunk;
+ size -= chunk;
+ }
+}

/*
* Do a quick page-table lookup for a single page.
@@ -961,9 +980,7 @@

/* mapping wholly truncated? */
if (mpnt->vm_pgoff >= pgoff) {
- flush_cache_range(mm, start, end);
- zap_page_range(mm, start, len);
- flush_tlb_range(mm, start, end);
+ zap_page_range(mm, start, len, ZPR_FLUSH_CACHE|ZPR_FLUSH_TLB);
continue;
}

@@ -981,7 +998,7 @@
start = (start + ~PAGE_MASK) & PAGE_MASK;
}
flush_cache_range(mm, start, end);
- zap_page_range(mm, start, len);
+ zap_page_range(mm, start, len, 0);
flush_tlb_range(mm, start, end);
} while ((mpnt = mpnt->vm_next_share) != NULL);
out_unlock:
--- linux-2.4.0-test3/mm/mmap.c Tue Jul 11 22:21:17 2000
+++ linux-akpm/mm/mmap.c Wed Jul 12 20:48:58 2000
@@ -340,9 +340,8 @@
vma->vm_file = NULL;
fput(file);
/* Undo any partial mapping done by a device driver. */
- flush_cache_range(mm, vma->vm_start, vma->vm_end);
- zap_page_range(mm, vma->vm_start, vma->vm_end - vma->vm_start);
- flush_tlb_range(mm, vma->vm_start, vma->vm_end);
+ zap_page_range(mm, vma->vm_start, vma->vm_end - vma->vm_start,
+ ZPR_FLUSH_CACHE|ZPR_FLUSH_TLB);
free_vma:
kmem_cache_free(vm_area_cachep, vma);
return error;
@@ -711,10 +710,8 @@
}
remove_shared_vm_struct(mpnt);
mm->map_count--;
-
- flush_cache_range(mm, st, end);
- zap_page_range(mm, st, size);
- flush_tlb_range(mm, st, end);
+ zap_page_range(mm, st, size,
+ ZPR_FLUSH_CACHE|ZPR_FLUSH_TLB|ZPR_COND_RESCHED); /* sys_munmap() */

/*
* Fix the mapping, and free the old area if it wasn't reused.
@@ -864,7 +861,7 @@
}
mm->map_count--;
remove_shared_vm_struct(mpnt);
- zap_page_range(mm, start, size);
+ zap_page_range(mm, start, size, ZPR_COND_RESCHED); /* sys_exit() */
if (mpnt->vm_file)
fput(mpnt->vm_file);
kmem_cache_free(vm_area_cachep, mpnt);
--- linux-2.4.0-test3/mm/mremap.c Sat Jun 24 15:39:47 2000
+++ linux-akpm/mm/mremap.c Tue Jul 11 21:02:33 2000
@@ -118,8 +118,7 @@
flush_cache_range(mm, new_addr, new_addr + len);
while ((offset += PAGE_SIZE) < len)
move_one_page(mm, new_addr + offset, old_addr + offset);
- zap_page_range(mm, new_addr, len);
- flush_tlb_range(mm, new_addr, new_addr + len);
+ zap_page_range(mm, new_addr, len, ZPR_FLUSH_TLB);
return -1;
}

--- linux-2.4.0-test3/drivers/char/mem.c Sat Jun 24 15:39:43 2000
+++ linux-akpm/drivers/char/mem.c Mon Jul 10 21:58:33 2000
@@ -373,8 +373,7 @@
if (count > size)
count = size;

- flush_cache_range(mm, addr, addr + count);
- zap_page_range(mm, addr, count);
+ zap_page_range(mm, addr, count, ZPR_FLUSH_CACHE);
zeromap_page_range(addr, count, PAGE_COPY);
flush_tlb_range(mm, addr, addr + count);
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