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mm, x86/mm: Fix performance regression in get_user_pages_fast()

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The 0-day test bot found a performance regression that was tracked down to
switching x86 to the generic get_user_pages_fast() implementation:

  http://lkml.kernel.org/r/20170710024020.GA26389@yexl-desktop

The regression was caused by the fact that we now use local_irq_save() +
local_irq_restore() in get_user_pages_fast() to disable interrupts.
In x86 implementation local_irq_disable() + local_irq_enable() was used.

The fix is to make get_user_pages_fast() use local_irq_disable(),
leaving local_irq_save() for __get_user_pages_fast() that can be called
with interrupts disabled.

Numbers for pinning a gigabyte of memory, one page a time, 20 repeats:

  Before:  Average: 14.91 ms, stddev: 0.45 ms
  After:   Average: 10.76 ms, stddev: 0.18 ms

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Thorsten Leemhuis <regressions@leemhuis.info>
Cc: linux-mm@kvack.org
Fixes: e585513b76 ("x86/mm/gup: Switch GUP to the generic get_user_page_fast() implementation")
Link: http://lkml.kernel.org/r/20170908215603.9189-3-kirill.shutemov@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Kirill A. Shutemov 2017-09-09 00:56:03 +03:00 committed by Ingo Molnar
parent 9e52fc2b50
commit 5b65c4677a
1 changed files with 58 additions and 39 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

97
mm/gup.c
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@ -1618,6 +1618,47 @@ static int gup_p4d_range(pgd_t pgd, unsigned long addr, unsigned long end,
return 1;
}
static void gup_pgd_range(unsigned long addr, unsigned long end,
int write, struct page **pages, int *nr)
{
unsigned long next;
pgd_t *pgdp;
pgdp = pgd_offset(current->mm, addr);
do {
pgd_t pgd = READ_ONCE(*pgdp);
next = pgd_addr_end(addr, end);
if (pgd_none(pgd))
return;
if (unlikely(pgd_huge(pgd))) {
if (!gup_huge_pgd(pgd, pgdp, addr, next, write,
pages, nr))
return;
} else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {
if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,
PGDIR_SHIFT, next, write, pages, nr))
return;
} else if (!gup_p4d_range(pgd, addr, next, write, pages, nr))
return;
} while (pgdp++, addr = next, addr != end);
}
#ifndef gup_fast_permitted
/*
* Check if it's allowed to use __get_user_pages_fast() for the range, or
* we need to fall back to the slow version:
*/
bool gup_fast_permitted(unsigned long start, int nr_pages, int write)
{
unsigned long len, end;
len = (unsigned long) nr_pages << PAGE_SHIFT;
end = start + len;
return end >= start;
}
#endif
/*
* Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
* the regular GUP. It will only return non-negative values.
@ -1625,10 +1666,8 @@ static int gup_p4d_range(pgd_t pgd, unsigned long addr, unsigned long end,
int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
struct page **pages)
{
struct mm_struct *mm = current->mm;
unsigned long addr, len, end;
unsigned long next, flags;
pgd_t *pgdp;
unsigned long flags;
int nr = 0;
start &= PAGE_MASK;
@ -1652,45 +1691,15 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
* block IPIs that come from THPs splitting.
*/
local_irq_save(flags);
pgdp = pgd_offset(mm, addr);
do {
pgd_t pgd = READ_ONCE(*pgdp);
next = pgd_addr_end(addr, end);
if (pgd_none(pgd))
break;
if (unlikely(pgd_huge(pgd))) {
if (!gup_huge_pgd(pgd, pgdp, addr, next, write,
pages, &nr))
break;
} else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {
if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,
PGDIR_SHIFT, next, write, pages, &nr))
break;
} else if (!gup_p4d_range(pgd, addr, next, write, pages, &nr))
break;
} while (pgdp++, addr = next, addr != end);
local_irq_restore(flags);
if (gup_fast_permitted(start, nr_pages, write)) {
local_irq_save(flags);
gup_pgd_range(addr, end, write, pages, &nr);
local_irq_restore(flags);
}
return nr;
}
#ifndef gup_fast_permitted
/*
* Check if it's allowed to use __get_user_pages_fast() for the range, or
* we need to fall back to the slow version:
*/
bool gup_fast_permitted(unsigned long start, int nr_pages, int write)
{
unsigned long len, end;
len = (unsigned long) nr_pages << PAGE_SHIFT;
end = start + len;
return end >= start;
}
#endif
/**
* get_user_pages_fast() - pin user pages in memory
* @start: starting user address
@ -1710,12 +1719,22 @@ bool gup_fast_permitted(unsigned long start, int nr_pages, int write)
int get_user_pages_fast(unsigned long start, int nr_pages, int write,
struct page **pages)
{
unsigned long addr, len, end;
int nr = 0, ret = 0;
start &= PAGE_MASK;
addr = start;
len = (unsigned long) nr_pages << PAGE_SHIFT;
end = start + len;
if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
(void __user *)start, len)))
return 0;
if (gup_fast_permitted(start, nr_pages, write)) {
nr = __get_user_pages_fast(start, nr_pages, write, pages);
local_irq_disable();
gup_pgd_range(addr, end, write, pages, &nr);
local_irq_enable();
ret = nr;
}