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SubjectRe: [PATCH] perf, x86: Optimize intel_pmu_pebs_fixup_ip()
On Thu, Oct 17, 2013 at 11:08:16PM +0200, Peter Zijlstra wrote:
> I did a patch that avoids the page count mucking about, Don didn't see
> any significant improvements from it.

On top of which there's another patch -- which could as easily be done
without it, that adds some state to the copy_from_user_nmi() and avoids
re-doing the page walk and memcpy().

Don also tried a vraiant of the below for the pebs fixup code; that
turned out to be slower than the single copy all at once.

The below -- as stated in the changelog -- might still improve the
frame-pointer chase, but I've no numbers what so ever atm.

---
Subject: perf, x86: Optimize copy_from_user_nmi()
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu, 17 Oct 2013 11:41:45 +0200

In an attempt to speed up the frame-pointer chase we introduce a new
copy_from_user_nmi() interface:

struct copy_from_user_nmi_state;

void *copy_from_user_nmi_iter(void *to, void __user *from, int s,
struct copy_from_user_nmi_state *state);
void copy_from_user_nmi_end(struct copy_from_user_nmi_state *state);

The _iter() method returns a pointer to the memory requested; if this
is entirely contained within one page it simply returns a pointer into
the kmap and avoids the copy. Otherwise it will copy into the buffer
provided in the @to argument and return a pointer thereto.

Because we potentially need to keep the kmap alive, we need the
closing _end() function and the @state variable.

Since we keep state, we also avoid the page-table walk for consecutive
accesses to the same page.

Using this we (hopefully) reduce the number of page-table walks (and
kmap on i386) operations.

Cc: Don Zickus <dzickus@redhat.com>
Cc: dave.hansen@linux.intel.com
Cc: eranian@google.com
Cc: jmario@redhat.com
Cc: acme@infradead.org
Cc: mingo@kernel.org
Cc: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20131017094145.GE3364@laptop.programming.kicks-ass.net
---
arch/x86/include/asm/uaccess.h | 13 ++++++
arch/x86/kernel/cpu/perf_event.c | 32 ++++++---------
arch/x86/lib/usercopy.c | 79 +++++++++++++++++++++++++++++++++++++++
3 files changed, 106 insertions(+), 18 deletions(-)

--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -516,6 +516,19 @@ struct __large_struct { unsigned long bu

extern unsigned long
copy_from_user_nmi(void *to, const void __user *from, unsigned long n);
+
+struct copy_from_user_nmi_state {
+ void *map;
+ unsigned long address;
+ unsigned long flags;
+};
+
+extern void *
+copy_from_user_nmi_iter(void *to, const void __user *from,
+ unsigned long n, struct copy_from_user_nmi_state *state);
+extern void
+copy_from_user_nmi_end(struct copy_from_user_nmi_state *state);
+
extern __must_check long
strncpy_from_user(char *dst, const char __user *src, long count);

--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -1979,8 +1979,9 @@ static inline int
perf_callchain_user32(struct pt_regs *regs, struct perf_callchain_entry *entry)
{
/* 32-bit process in 64-bit kernel. */
+ struct copy_from_user_nmi_state state = { NULL };
unsigned long ss_base, cs_base;
- struct stack_frame_ia32 frame;
+ struct stack_frame_ia32 frame, *f;
const void __user *fp;

if (!test_thread_flag(TIF_IA32))
@@ -1991,20 +1992,17 @@ perf_callchain_user32(struct pt_regs *re

fp = compat_ptr(ss_base + regs->bp);
while (entry->nr < PERF_MAX_STACK_DEPTH) {
- unsigned long bytes;
- frame.next_frame = 0;
- frame.return_address = 0;
-
- bytes = copy_from_user_nmi(&frame, fp, sizeof(frame));
- if (bytes != sizeof(frame))
+ f = copy_from_user_nmi_iter(&frame, fp, sizeof(frame), &state);
+ if (!f)
break;

if (!valid_user_frame(fp, sizeof(frame)))
break;

- perf_callchain_store(entry, cs_base + frame.return_address);
- fp = compat_ptr(ss_base + frame.next_frame);
+ perf_callchain_store(entry, cs_base + f->return_address);
+ fp = compat_ptr(ss_base + f->next_frame);
}
+ copy_from_user_nmi_end(&state);
return 1;
}
#else
@@ -2018,7 +2016,8 @@ perf_callchain_user32(struct pt_regs *re
void
perf_callchain_user(struct perf_callchain_entry *entry, struct pt_regs *regs)
{
- struct stack_frame frame;
+ struct copy_from_user_nmi_state state = { NULL };
+ struct stack_frame frame, *f;
const void __user *fp;

if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
@@ -2043,20 +2042,17 @@ perf_callchain_user(struct perf_callchai
return;

while (entry->nr < PERF_MAX_STACK_DEPTH) {
- unsigned long bytes;
- frame.next_frame = NULL;
- frame.return_address = 0;
-
- bytes = copy_from_user_nmi(&frame, fp, sizeof(frame));
- if (bytes != sizeof(frame))
+ f = copy_from_user_nmi_iter(&frame, fp, sizeof(frame), &state);
+ if (!f)
break;

if (!valid_user_frame(fp, sizeof(frame)))
break;

- perf_callchain_store(entry, frame.return_address);
- fp = frame.next_frame;
+ perf_callchain_store(entry, f->return_address);
+ fp = f->next_frame;
}
+ copy_from_user_nmi_end(&state);
}

/*
--- a/arch/x86/lib/usercopy.c
+++ b/arch/x86/lib/usercopy.c
@@ -53,3 +53,82 @@ copy_from_user_nmi(void *to, const void
return len;
}
EXPORT_SYMBOL_GPL(copy_from_user_nmi);
+
+void *copy_from_user_nmi_iter(void *to, const void __user *from,
+ unsigned long n, struct copy_from_user_nmi_state *state)
+{
+ unsigned long offset, addr = (unsigned long)from;
+ unsigned long size, len = 0;
+ unsigned long flags;
+ struct page *page;
+ void *map, *_to = to;
+ int ret;
+
+ if (__range_not_ok(from, n, TASK_SIZE))
+ return NULL;
+
+ if (state->map) {
+ if ((state->address >> PAGE_SHIFT) ==
+ (addr >> PAGE_SHIFT)) {
+ flags = state->flags;
+ map = state->map;
+ goto got_page;
+ }
+ kunmap_atomic(state->map);
+ local_irq_restore(state->flags);
+ }
+
+ for (;;) {
+ local_irq_save(flags);
+ ret = ___get_user_pages_fast(addr, 1, 0, &page);
+ if (!ret) {
+ local_irq_restore(flags);
+ state->map = NULL;
+ return NULL;
+ }
+
+ map = kmap_atomic(page);
+
+got_page:
+ offset = addr & (PAGE_SIZE - 1);
+ size = min(PAGE_SIZE - offset, n - len);
+
+ /*
+ * If the entire desired range falls within the one page
+ * avoid the copy and return a pointer into the kmap.
+ */
+ if (size == n) {
+ _to = map + offset;
+ break;
+ }
+
+ memcpy(to, map+offset, size);
+ len += size;
+
+ if (len == n)
+ break;
+
+ to += size;
+ addr += size;
+
+ kunmap_atomic(map);
+ local_irq_restore(flags);
+ }
+
+ state->address = addr;
+ state->flags = flags;
+ state->map = map;
+
+ return _to;
+}
+EXPORT_SYMBOL_GPL(copy_from_user_nmi_iter);
+
+void copy_from_user_nmi_end(struct copy_from_user_nmi_state *state)
+{
+ if (state->map) {
+ kunmap_atomic(state->map);
+ local_irq_restore(state->flags);
+ state->map = NULL;
+ }
+}
+EXPORT_SYMBOL_GPL(copy_from_user_nmi_end);

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