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SubjectRe: [PATCH] perf, x86: Optimize intel_pmu_pebs_fixup_ip()
On Thu, Oct 17, 2013 at 11:26:23AM -0700, Linus Torvalds wrote:
> On Thu, Oct 17, 2013 at 9:30 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > So avoid having to call copy_from_user_nmi() for every instruction.
> > Since we already limit the max basic block size, we can easily
> > pre-allocate a piece of memory to copy the entire thing into in one
> > go.
>
> copy_from_user_nmi() itself is all kinds of nasty.
>
> Using __get_user_pages_fast() for a single page is quite expensive,
> and mucks around with the page counts etc.
>
> If copy_from_user_nmi() just did the (simple) page table walk by hand,
> it could avoid *all* of that. No page count stuff - just have
> interrupts disabled over not just the page walk, but the copy too - to
> guarantee that no cross-CPU TLB flush can come in.
>
> So instead of trying to improve __get_user_pages_fast() - which is
> impossible because the interface fundamentally means that it has to
> iterate over things and check page counts - you could simplify the
> caller instead.
>
> That is, if we really care any more. Maybe this "do the
> copy_from_user_nmi() just once" is already good enough that nobody
> much cares.

I did a patch that avoids the page count mucking about, Don didn't see
any significant improvements from it.

---
Subject: x86: Optimize copy_from_user_nmi()
From: Peter Zijlstra <peterz@infradead.org>
Date: Wed Oct 16 10:55:59 CEST 2013

Since copy_from_user_nmi() is pretty much always called from either
IRQ or NMI context there's no need to take and release a reference on
the page.

Provide yet another __gup_fast() interface: '___gup_fast()' which
assumes the called has disabled IRQs and which makes the taking of
page count references optional.

Then change copy_from_user_nmi() to use this new variant to avoid
taking and releasing page references, thereby avoiding a number of
atomic ops.

This can be esp. useful when profiling threaded apps that run mostly
the same code; in that case intel_pmu_pebs_fixup_ip() can call
copy_form_user_nmi() a lot on the same few text pages from many CPUs
at the same time.

Cc: eranian@google.com
Cc: Don Zickus <dzickus@redhat.com>
Cc: jmario@redhat.com
Cc: acme@infradead.org
Cc: mingo@kernel.org
Cc: dave.hansen@linux.intel.com
Suggested-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20131015150736.GZ26785@twins.programming.kicks-ass.net
---
arch/x86/lib/usercopy.c | 12 ++++++---
arch/x86/mm/gup.c | 63 ++++++++++++++++++++++++++++++------------------
2 files changed, 49 insertions(+), 26 deletions(-)

--- a/arch/x86/lib/usercopy.c
+++ b/arch/x86/lib/usercopy.c
@@ -10,6 +10,8 @@
#include <asm/word-at-a-time.h>
#include <linux/sched.h>

+extern int ___get_user_pages_fast(unsigned long start, int nr_pages, int flags,
+ struct page **pages);
/*
* best effort, GUP based copy_from_user() that is NMI-safe
*/
@@ -18,6 +20,7 @@ copy_from_user_nmi(void *to, const void
{
unsigned long offset, addr = (unsigned long)from;
unsigned long size, len = 0;
+ unsigned long flags;
struct page *page;
void *map;
int ret;
@@ -26,9 +29,12 @@ copy_from_user_nmi(void *to, const void
return len;

do {
- ret = __get_user_pages_fast(addr, 1, 0, &page);
- if (!ret)
+ local_irq_save(flags);
+ ret = ___get_user_pages_fast(addr, 1, 0, &page);
+ if (!ret) {
+ local_irq_restore(flags);
break;
+ }

offset = addr & (PAGE_SIZE - 1);
size = min(PAGE_SIZE - offset, n - len);
@@ -36,7 +42,7 @@ copy_from_user_nmi(void *to, const void
map = kmap_atomic(page);
memcpy(to, map+offset, size);
kunmap_atomic(map);
- put_page(page);
+ local_irq_restore(flags);

len += size;
to += size;
--- a/arch/x86/mm/gup.c
+++ b/arch/x86/mm/gup.c
@@ -63,19 +63,22 @@ static inline pte_t gup_get_pte(pte_t *p
#endif
}

+#define GUPF_GET 0x01
+#define GUPF_WRITE 0x02
+
/*
* The performance critical leaf functions are made noinline otherwise gcc
* inlines everything into a single function which results in too much
* register pressure.
*/
static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
- unsigned long end, int write, struct page **pages, int *nr)
+ unsigned long end, int flags, struct page **pages, int *nr)
{
unsigned long mask;
pte_t *ptep;

mask = _PAGE_PRESENT|_PAGE_USER;
- if (write)
+ if (flags & GUPF_WRITE)
mask |= _PAGE_RW;

ptep = pte_offset_map(&pmd, addr);
@@ -89,7 +92,8 @@ static noinline int gup_pte_range(pmd_t
}
VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
page = pte_page(pte);
- get_page(page);
+ if (flags & GUPF_GET)
+ get_page(page);
SetPageReferenced(page);
pages[*nr] = page;
(*nr)++;
@@ -109,7 +113,7 @@ static inline void get_head_page_multipl
}

static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
- unsigned long end, int write, struct page **pages, int *nr)
+ unsigned long end, int flags, struct page **pages, int *nr)
{
unsigned long mask;
pte_t pte = *(pte_t *)&pmd;
@@ -117,7 +121,7 @@ static noinline int gup_huge_pmd(pmd_t p
int refs;

mask = _PAGE_PRESENT|_PAGE_USER;
- if (write)
+ if (flags & GUPF_WRITE)
mask |= _PAGE_RW;
if ((pte_flags(pte) & mask) != mask)
return 0;
@@ -131,19 +135,20 @@ static noinline int gup_huge_pmd(pmd_t p
do {
VM_BUG_ON(compound_head(page) != head);
pages[*nr] = page;
- if (PageTail(page))
+ if ((flags & GUPF_GET) && PageTail(page))
get_huge_page_tail(page);
(*nr)++;
page++;
refs++;
} while (addr += PAGE_SIZE, addr != end);
- get_head_page_multiple(head, refs);
+ if (flags & GUPF_GET)
+ get_head_page_multiple(head, refs);

return 1;
}

static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
- int write, struct page **pages, int *nr)
+ int flags, struct page **pages, int *nr)
{
unsigned long next;
pmd_t *pmdp;
@@ -167,10 +172,10 @@ static int gup_pmd_range(pud_t pud, unsi
if (pmd_none(pmd) || pmd_trans_splitting(pmd))
return 0;
if (unlikely(pmd_large(pmd))) {
- if (!gup_huge_pmd(pmd, addr, next, write, pages, nr))
+ if (!gup_huge_pmd(pmd, addr, next, flags, pages, nr))
return 0;
} else {
- if (!gup_pte_range(pmd, addr, next, write, pages, nr))
+ if (!gup_pte_range(pmd, addr, next, flags, pages, nr))
return 0;
}
} while (pmdp++, addr = next, addr != end);
@@ -179,7 +184,7 @@ static int gup_pmd_range(pud_t pud, unsi
}

static noinline int gup_huge_pud(pud_t pud, unsigned long addr,
- unsigned long end, int write, struct page **pages, int *nr)
+ unsigned long end, int flags, struct page **pages, int *nr)
{
unsigned long mask;
pte_t pte = *(pte_t *)&pud;
@@ -187,7 +192,7 @@ static noinline int gup_huge_pud(pud_t p
int refs;

mask = _PAGE_PRESENT|_PAGE_USER;
- if (write)
+ if (flags & GUPF_WRITE)
mask |= _PAGE_RW;
if ((pte_flags(pte) & mask) != mask)
return 0;
@@ -201,19 +206,20 @@ static noinline int gup_huge_pud(pud_t p
do {
VM_BUG_ON(compound_head(page) != head);
pages[*nr] = page;
- if (PageTail(page))
+ if ((flags & GUPF_GET) && PageTail(page))
get_huge_page_tail(page);
(*nr)++;
page++;
refs++;
} while (addr += PAGE_SIZE, addr != end);
- get_head_page_multiple(head, refs);
+ if (flags & GUPF_GET)
+ get_head_page_multiple(head, refs);

return 1;
}

static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
- int write, struct page **pages, int *nr)
+ int flags, struct page **pages, int *nr)
{
unsigned long next;
pud_t *pudp;
@@ -226,10 +232,10 @@ static int gup_pud_range(pgd_t pgd, unsi
if (pud_none(pud))
return 0;
if (unlikely(pud_large(pud))) {
- if (!gup_huge_pud(pud, addr, next, write, pages, nr))
+ if (!gup_huge_pud(pud, addr, next, flags, pages, nr))
return 0;
} else {
- if (!gup_pmd_range(pud, addr, next, write, pages, nr))
+ if (!gup_pmd_range(pud, addr, next, flags, pages, nr))
return 0;
}
} while (pudp++, addr = next, addr != end);
@@ -241,13 +247,12 @@ static int gup_pud_range(pgd_t pgd, unsi
* Like get_user_pages_fast() except its IRQ-safe in that it won't fall
* back to the regular GUP.
*/
-int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
+int ___get_user_pages_fast(unsigned long start, int nr_pages, int flags,
struct page **pages)
{
struct mm_struct *mm = current->mm;
unsigned long addr, len, end;
unsigned long next;
- unsigned long flags;
pgd_t *pgdp;
int nr = 0;

@@ -255,7 +260,7 @@ int __get_user_pages_fast(unsigned long
addr = start;
len = (unsigned long) nr_pages << PAGE_SHIFT;
end = start + len;
- if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
+ if (unlikely(!access_ok((flags & GUPF_WRITE) ? VERIFY_WRITE : VERIFY_READ,
(void __user *)start, len)))
return 0;

@@ -277,7 +282,6 @@ int __get_user_pages_fast(unsigned long
* (which we do on x86, with the above PAE exception), we can follow the
* address down to the the page and take a ref on it.
*/
- local_irq_save(flags);
pgdp = pgd_offset(mm, addr);
do {
pgd_t pgd = *pgdp;
@@ -285,14 +289,27 @@ int __get_user_pages_fast(unsigned long
next = pgd_addr_end(addr, end);
if (pgd_none(pgd))
break;
- if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
+ if (!gup_pud_range(pgd, addr, next, flags, pages, &nr))
break;
} while (pgdp++, addr = next, addr != end);
- local_irq_restore(flags);

return nr;
}

+int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
+ struct page **pages)
+{
+ unsigned long flags;
+ int ret;
+
+ local_irq_save(flags);
+ ret = ___get_user_pages_fast(start, nr_pages,
+ GUPF_GET | (write ? GUPF_WRITE : 0), pages);
+ local_irq_restore(flags);
+
+ return ret;
+}
+
/**
* get_user_pages_fast() - pin user pages in memory
* @start: starting user address

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