Re: scalable kmap (was Re: vm lock contention reduction)

Andrew Morton
2002-07-09T05:28:06Z

"Martin J. Bligh" wrote:
>
> > It's a bit weird that copy_strings is the heaviest user of kmap().
>
> I can dig out the whole acg, and see what was calling copy_strings,
> that might help give us a clue.

Well, it'll be exec().

> > I bet it's kmapping the same page over and over. A little cache
> > there will help. I'll fix that up.
>
> Sounds like a plan ...

Updated patch below. We don't need an atomic kmap in copy_strings
at all. kmap is the right thing to do, but just be smarter about it.
Hanging onto the existing kmap in there reduces the number of kmap()
calls by a factor of 32 across a kernel compile.

But still no aggregate speedup.

> > So here's the patch. Seems to work. Across a `make -j6 bzImage'
> > the number of calls to kmap_high() went from 490,429 down to 41,174.
> >
> > And guess what? Zero change in wallclock time.
>
> Well, I was going to try to bench this tonight, but am having a
> problem with 2.5.25 right now (we've been on 2.4 for a while,
> but are shifting). Hopefully get you some numbers tommorow, and
> will get some other benchmarks done by people here on various
> machines.

Don't tell me those NUMAQ's are using IDE ;)

But seriously, what's the problem? We really do need the big
boxes to be able to test 2.5 right now, and any blockage needs
to be cleared away.

> > Any theories?
>
> Maybe the cost of the atomic kmap counters the gain? Having to do a
> single line tlbflush every time ... a per cpu pool might help if that
> is the problem, but would have to make it a reasonable size to counter
> the cost. I'll do some more measurements first, and get some profile
> data to see if the number of ticks changes down in one function and
> up in the other?

Well, certainly it's dumb to perform an atomic_kunmap() when we're
just about to atomic_kmap() the same page again. But no change.

arch/i386/lib/usercopy.c | 10 +++++
arch/i386/mm/fault.c | 40 ++++++++++++++++++++
fs/exec.c | 72 ++++++++++++++++++++++++++++--------
include/asm-i386/kmap_types.h | 3 +
include/asm-i386/processor.h | 2 +
include/asm-ppc/kmap_types.h | 1
include/asm-sparc/kmap_types.h | 1
include/asm-x86_64/kmap_types.h | 1
include/linux/highmem.h | 78 ++++++++++++++++++++++++++++++++++++++++
include/linux/sched.h | 5 ++
mm/filemap.c | 11 +++--
11 files changed, 202 insertions(+), 22 deletions(-)

--- 2.5.25/arch/i386/mm/fault.c~linus-copy_user-hack Mon Jul 8 21:44:21 2002
+++ 2.5.25-akpm/arch/i386/mm/fault.c Mon Jul 8 21:44:21 2002
@@ -18,6 +18,7 @@
#include <linux/ptrace.h>
#include <linux/mman.h>
#include <linux/mm.h>
+#include <linux/highmem.h>
#include <linux/smp.h>
#include <linux/smp_lock.h>
#include <linux/interrupt.h>
@@ -138,6 +139,39 @@ void bust_spinlocks(int yes)
}
}

+#ifdef CONFIG_HIGHMEM
+static inline void highmem_remove_atomic_kmap(struct pt_regs *regs)
+{
+ struct copy_user_state *cus = current->copy_user_state;
+
+ if (cus)
+ kunmap_atomic(cus->kaddr, cus->type);
+}
+
+static inline void highmem_restore_atomic_kmap(struct pt_regs *regs)
+{
+ struct copy_user_state *cus = current->copy_user_state;
+
+ if (cus) {
+ long *reg;
+ unsigned offset;
+
+ cus->kaddr = kmap_atomic(cus->page, cus->type);
+ if (cus->src)
+ reg = &regs->esi;
+ else
+ reg = &regs->edi;
+ offset = *reg & (PAGE_SIZE - 1);
+ *reg = ((long)cus->kaddr) | offset;
+ }
+}
+#else
+static inline void highmem_remove_atomic_kmap(struct pt_regs *regs)
+{}
+static inline void highmem_restore_atomic_kmap(struct pt_regs *regs)
+{}
+#endif
+
asmlinkage void do_invalid_op(struct pt_regs *, unsigned long);
extern unsigned long idt;

@@ -206,6 +240,8 @@ asmlinkage void do_page_fault(struct pt_
}
#endif

+ highmem_remove_atomic_kmap(regs);
+
down_read(&mm->mmap_sem);

vma = find_vma(mm, address);
@@ -267,8 +303,10 @@ good_area:
tsk->maj_flt++;
break;
case VM_FAULT_SIGBUS:
+ highmem_restore_atomic_kmap(regs);
goto do_sigbus;
case VM_FAULT_OOM:
+ highmem_restore_atomic_kmap(regs);
goto out_of_memory;
default:
BUG();
@@ -283,6 +321,7 @@ good_area:
tsk->thread.screen_bitmap |= 1 << bit;
}
up_read(&mm->mmap_sem);
+ highmem_restore_atomic_kmap(regs);
return;

/*
@@ -291,6 +330,7 @@ good_area:
*/
bad_area:
up_read(&mm->mmap_sem);
+ highmem_restore_atomic_kmap(regs);

/* User mode accesses just cause a SIGSEGV */
if (error_code & 4) {
--- 2.5.25/include/asm-i386/kmap_types.h~linus-copy_user-hack Mon Jul 8 21:44:21 2002
+++ 2.5.25-akpm/include/asm-i386/kmap_types.h Mon Jul 8 21:44:21 2002
@@ -19,7 +19,8 @@ D(5) KM_BIO_SRC_IRQ,
D(6) KM_BIO_DST_IRQ,
D(7) KM_PTE0,
D(8) KM_PTE1,
-D(9) KM_TYPE_NR
+D(9) KM_FILEMAP,
+D(10) KM_TYPE_NR
};

#undef D
--- 2.5.25/include/asm-ppc/kmap_types.h~linus-copy_user-hack Mon Jul 8 21:44:21 2002
+++ 2.5.25-akpm/include/asm-ppc/kmap_types.h Mon Jul 8 21:44:21 2002
@@ -15,6 +15,7 @@ enum km_type {
KM_BIO_DST_IRQ,
KM_PTE0,
KM_PTE1,
+ KM_FILEMAP,
KM_TYPE_NR
};

--- 2.5.25/include/asm-sparc/kmap_types.h~linus-copy_user-hack Mon Jul 8 21:44:21 2002
+++ 2.5.25-akpm/include/asm-sparc/kmap_types.h Mon Jul 8 21:44:21 2002
@@ -9,6 +9,7 @@ enum km_type {
KM_USER1,
KM_BIO_SRC_IRQ,
KM_BIO_DST_IRQ,
+ KM_FILEMAP,
KM_TYPE_NR
};

--- 2.5.25/include/asm-x86_64/kmap_types.h~linus-copy_user-hack Mon Jul 8 21:44:21 2002
+++ 2.5.25-akpm/include/asm-x86_64/kmap_types.h Mon Jul 8 21:44:21 2002
@@ -9,6 +9,7 @@ enum km_type {
KM_USER1,
KM_BIO_SRC_IRQ,
KM_BIO_DST_IRQ,
+ KM_FILEMAP,
KM_TYPE_NR
};

--- 2.5.25/include/linux/highmem.h~linus-copy_user-hack Mon Jul 8 21:44:21 2002
+++ 2.5.25-akpm/include/linux/highmem.h Mon Jul 8 21:44:21 2002
@@ -3,6 +3,7 @@

#include <linux/config.h>
#include <linux/fs.h>
+#include <asm/processor.h>
#include <asm/cacheflush.h>

#ifdef CONFIG_HIGHMEM
@@ -10,6 +11,7 @@
extern struct page *highmem_start_page;

#include <asm/highmem.h>
+#include <asm/kmap_types.h>

/* declarations for linux/mm/highmem.c */
unsigned int nr_free_highpages(void);
@@ -72,4 +74,80 @@ static inline void copy_user_highpage(st
kunmap_atomic(vto, KM_USER1);
}

+#if defined(CONFIG_HIGHMEM) && defined(ARCH_HAS_KMAP_FIXUP)
+/*
+ * Used when performing a copy_*_user while holding an atomic kmap
+ */
+struct copy_user_state {
+ struct page *page; /* The page which is kmap_atomiced */
+ void *kaddr; /* Its mapping */
+ enum km_type type; /* Its offset */
+ int src; /* 1: fixup ESI. 0: Fixup EDI */
+};
+
+/*
+ * `src' is true if the kmap_atomic virtual address is the source of the copy.
+ */
+static inline void *
+kmap_copy_user(struct copy_user_state *cus, struct page *page,
+ enum km_type type, int src)
+{
+ cus->page = page;
+ cus->kaddr = kmap_atomic(page, type);
+ if (PageHighMem(page)) {
+ cus->type = type;
+ cus->src = src;
+ BUG_ON(current->copy_user_state != NULL);
+ current->copy_user_state = cus;
+ }
+ return cus->kaddr;
+}
+
+static inline void kunmap_copy_user(struct copy_user_state *cus)
+{
+ if (PageHighMem(cus->page)) {
+ BUG_ON(current->copy_user_state != cus);
+ kunmap_atomic(cus->kaddr, cus->type);
+ current->copy_user_state = NULL;
+ cus->page = NULL; /* debug */
+ }
+}
+
+/*
+ * After a copy_*_user, the kernel virtual address may be different. So
+ * use kmap_copy_user_addr() to get the new value.
+ */
+static inline void *kmap_copy_user_addr(struct copy_user_state *cus)
+{
+ return cus->kaddr;
+}
+
+#else
+
+struct copy_user_state {
+ struct page *page;
+};
+
+/*
+ * This must be a macro because `type' may be undefined
+ */
+
+#define kmap_copy_user(cus, page, type, src) \
+ ({ \
+ (cus)->page = (page); \
+ kmap(page); \
+ })
+
+static inline void kunmap_copy_user(struct copy_user_state *cus)
+{
+ kunmap(cus->page);
+}
+
+static inline void *kmap_copy_user_addr(struct copy_user_state *cus)
+{
+ return page_address(cus->page);
+}
+
+#endif
+
#endif /* _LINUX_HIGHMEM_H */
--- 2.5.25/include/linux/sched.h~linus-copy_user-hack Mon Jul 8 21:44:21 2002
+++ 2.5.25-akpm/include/linux/sched.h Mon Jul 8 21:44:21 2002
@@ -248,6 +248,8 @@ extern struct user_struct root_user;

typedef struct prio_array prio_array_t;

+struct copy_user_state;
+
struct task_struct {
volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */
struct thread_info *thread_info;
@@ -365,6 +367,9 @@ struct task_struct {
/* journalling filesystem info */
void *journal_info;
struct dentry *proc_dentry;
+#ifdef CONFIG_HIGHMEM
+ struct copy_user_state *copy_user_state;
+#endif
};

extern void __put_task_struct(struct task_struct *tsk);
--- 2.5.25/mm/filemap.c~linus-copy_user-hack Mon Jul 8 21:44:21 2002
+++ 2.5.25-akpm/mm/filemap.c Mon Jul 8 21:44:21 2002
@@ -15,6 +15,7 @@
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/mman.h>
+#include <linux/highmem.h>
#include <linux/pagemap.h>
#include <linux/file.h>
#include <linux/iobuf.h>
@@ -1183,18 +1184,20 @@ static ssize_t generic_file_direct_IO(in
return retval;
}

-int file_read_actor(read_descriptor_t * desc, struct page *page, unsigned long offset, unsigned long size)
+int file_read_actor(read_descriptor_t *desc, struct page *page,
+ unsigned long offset, unsigned long size)
{
char *kaddr;
+ struct copy_user_state copy_user_state;
unsigned long left, count = desc->count;

if (size > count)
size = count;

- kaddr = kmap(page);
+ kaddr = kmap_copy_user(&copy_user_state, page, KM_FILEMAP, 1);
left = __copy_to_user(desc->buf, kaddr + offset, size);
- kunmap(page);
-
+ kunmap_copy_user(&copy_user_state);
+
if (left) {
size -= left;
desc->error = -EFAULT;
--- 2.5.25/include/asm-i386/processor.h~linus-copy_user-hack Mon Jul 8 21:44:21 2002
+++ 2.5.25-akpm/include/asm-i386/processor.h Mon Jul 8 21:44:21 2002
@@ -485,4 +485,6 @@ extern inline void prefetchw(const void

#endif

+#define ARCH_HAS_KMAP_FIXUP
+
#endif /* __ASM_I386_PROCESSOR_H */
--- 2.5.25/arch/i386/lib/usercopy.c~linus-copy_user-hack Mon Jul 8 21:44:21 2002
+++ 2.5.25-akpm/arch/i386/lib/usercopy.c Mon Jul 8 21:44:21 2002
@@ -11,6 +11,16 @@

#ifdef CONFIG_X86_USE_3DNOW_AND_WORKS

+/*
+ * We cannot use the mmx functions here with the kmap_atomic fixup
+ * code.
+ *
+ * But CONFIG_X86_USE_3DNOW_AND_WORKS never gets defined anywhere.
+ * Maybe kill this code?
+ */
+
+#error this will not work
+
unsigned long
__generic_copy_to_user(void *to, const void *from, unsigned long n)
{
--- 2.5.25/fs/exec.c~linus-copy_user-hack Mon Jul 8 21:44:21 2002
+++ 2.5.25-akpm/fs/exec.c Mon Jul 8 22:18:12 2002
@@ -182,27 +182,45 @@ static int count(char ** argv, int max)
* memory to free pages in kernel mem. These are in a format ready
* to be put directly into the top of new user memory.
*/
+int akpm_hits;
+int akpm_misses;
+int akpm_kmaps;
+
int copy_strings(int argc,char ** argv, struct linux_binprm *bprm)
{
+ struct page *kmapped_page = NULL;
+ char *kaddr = NULL;
+ int ret;
+
while (argc-- > 0) {
char *str;
int len;
unsigned long pos;

- if (get_user(str, argv+argc) || !(len = strnlen_user(str, bprm->p)))
- return -EFAULT;
- if (bprm->p < len)
- return -E2BIG;
+ if (get_user(str, argv+argc) ||
+ !(len = strnlen_user(str, bprm->p))) {
+ ret = -EFAULT;
+ goto out;
+ }
+
+ if (bprm->p < len) {
+ ret = -E2BIG;
+ goto out;
+ }

bprm->p -= len;
/* XXX: add architecture specific overflow check here. */
-
pos = bprm->p;
+
+ /*
+ * The only sleeping function which we are allowed to call in
+ * this loop is copy_from_user(). Otherwise, copy_user_state
+ * could get trashed.
+ */
while (len > 0) {
- char *kaddr;
int i, new, err;
- struct page *page;
int offset, bytes_to_copy;
+ struct page *page;

offset = pos % PAGE_SIZE;
i = pos/PAGE_SIZE;
@@ -211,32 +229,52 @@ int copy_strings(int argc,char ** argv,
if (!page) {
page = alloc_page(GFP_HIGHUSER);
bprm->page[i] = page;
- if (!page)
- return -ENOMEM;
+ if (!page) {
+ ret = -ENOMEM;
+ goto out;
+ }
new = 1;
}
- kaddr = kmap(page);

+ if (kmapped_page) {
+ if (page == kmapped_page)
+ akpm_hits++;
+ else
+ akpm_misses++;
+ }
+
+ if (page != kmapped_page) {
+ if (kmapped_page)
+ kunmap(kmapped_page);
+ kmapped_page = page;
+ kaddr = kmap(kmapped_page);
+ akpm_kmaps++;
+ }
if (new && offset)
memset(kaddr, 0, offset);
bytes_to_copy = PAGE_SIZE - offset;
if (bytes_to_copy > len) {
bytes_to_copy = len;
if (new)
- memset(kaddr+offset+len, 0, PAGE_SIZE-offset-len);
+ memset(kaddr+offset+len, 0,
+ PAGE_SIZE-offset-len);
+ }
+ err = copy_from_user(kaddr+offset, str, bytes_to_copy);
+ if (err) {
+ ret = -EFAULT;
+ goto out;
}
- err = copy_from_user(kaddr + offset, str, bytes_to_copy);
- kunmap(page);
-
- if (err)
- return -EFAULT;

pos += bytes_to_copy;
str += bytes_to_copy;
len -= bytes_to_copy;
}
}
- return 0;
+ ret = 0;
+out:
+ if (kmapped_page)
+ kunmap(kmapped_page);
+ return ret;
}

/*

-