Messages in this thread Patch in this message | | | Subject | Re: >Re: [RFC] should VM_BUG_ON(cond) really evaluate cond | From | Eric Dumazet <> | Date | Sun, 30 Oct 2011 09:52:34 +0100 |
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Le samedi 29 octobre 2011 à 10:34 -0700, Linus Torvalds a écrit : > On Fri, Oct 28, 2011 at 7:55 AM, Linus Torvalds > <torvalds@linux-foundation.org> wrote: > > > > Comments? I think I'm open to tested patches.. > > Here's a *untested* patch. > > In particular, I worry that I'd need to add a "#include > <linux/compiler.h>" to some header file, although I suspect it gets > included some way regardless. > > And when I say "untested", I mean it. I verified that this makes > *some* difference to the generated code, but I didn't actually check > if it really matters, or if it actually compiles and works in general. > > Caveat tester, >
Since jetlag strikes me again, I took your patch and had to change it a bit, since :
1) x86 uses its own atomic_read() definition in arch/x86/include/asm/atomic.h
2) We can use a const pointer in ACCESS_AT_MOST_ONCE(*ptr), so I had to change a bit your implementation, I hope I did not mess it.
Tested (built/booted) on x86 and x86_64
We could logically split this patch in three parts, but hey, maybe I can try to sleep after all ;)
Thanks
[PATCH] atomic: introduce ACCESS_AT_MOST_ONCE() helper
In commit 4e60c86bd9e (gcc-4.6: mm: fix unused but set warnings) Andi forced VM_BUG_ON(cond) to evaluate cond, even if CONFIG_DEBUG_VM is not set :
#ifdef CONFIG_DEBUG_VM #define VM_BUG_ON(cond) BUG_ON(cond) #else #define VM_BUG_ON(cond) do { (void)(cond); } while (0) #endif
As a side effect, get_page()/put_page_testzero() are performing more bus transactions on contended cache line on some workloads (tcp_sendmsg() for example, where a page is acting as a shared buffer)
0,05 : ffffffff815e4775: je ffffffff815e4970 <tcp_sendmsg+0xc80> 0,05 : ffffffff815e477b: mov 0x1c(%r9),%eax // useless 3,32 : ffffffff815e477f: mov (%r9),%rax // useless 0,51 : ffffffff815e4782: lock incl 0x1c(%r9) 3,87 : ffffffff815e4787: mov (%r9),%rax 0,00 : ffffffff815e478a: test $0x80,%ah 0,00 : ffffffff815e478d: jne ffffffff815e49f2 <tcp_sendmsg+0xd02>
Thats because both atomic_read() and constant_test_bit() use a volatile attribute and thus compiler is forced to perform a read, even if the result is optimized away.
Linus suggested using an asm("") trick and place it in a variant of ACCESS_ONCE(), allowing compiler to omit reading memory if result is unused.
This patch introduces ACCESS_AT_MOST_ONCE() helper and use it in the x86 implementation of atomic_read() and constant_test_bit()
on x86_64, we thus reduce vmlinux text a bit (if CONFIG_DEBUG_VM=n)
# size vmlinux.old vmlinux.new text data bss dec hex filename 10706848 2894216 1540096 15141160 e70928 vmlinux.old 10704680 2894216 1540096 15138992 e700b0 vmlinux.new
Based on a prior patch from Linus
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com> --- arch/x86/include/asm/atomic.h | 2 +- arch/x86/include/asm/bitops.h | 7 +++++-- include/asm-generic/atomic.h | 2 +- include/linux/compiler.h | 10 ++++++++++ 4 files changed, 17 insertions(+), 4 deletions(-)
diff --git a/arch/x86/include/asm/atomic.h b/arch/x86/include/asm/atomic.h index 58cb6d4..b1f0c6b 100644 --- a/arch/x86/include/asm/atomic.h +++ b/arch/x86/include/asm/atomic.h @@ -22,7 +22,7 @@ */ static inline int atomic_read(const atomic_t *v) { - return (*(volatile int *)&(v)->counter); + return ACCESS_AT_MOST_ONCE(v->counter); } /** diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h index 1775d6e..e30a190 100644 --- a/arch/x86/include/asm/bitops.h +++ b/arch/x86/include/asm/bitops.h @@ -308,8 +308,11 @@ static inline int test_and_change_bit(int nr, volatile unsigned long *addr) static __always_inline int constant_test_bit(unsigned int nr, const volatile unsigned long *addr) { - return ((1UL << (nr % BITS_PER_LONG)) & - (addr[nr / BITS_PER_LONG])) != 0; + const unsigned long *word = (const unsigned long *)addr + + (nr / BITS_PER_LONG); + unsigned long bit = 1UL << (nr % BITS_PER_LONG); + + return (bit & ACCESS_AT_MOST_ONCE(*word)) != 0; } static inline int variable_test_bit(int nr, volatile const unsigned long *addr) diff --git a/include/asm-generic/atomic.h b/include/asm-generic/atomic.h index e37963c..c05e21f 100644 --- a/include/asm-generic/atomic.h +++ b/include/asm-generic/atomic.h @@ -39,7 +39,7 @@ * Atomically reads the value of @v. */ #ifndef atomic_read -#define atomic_read(v) (*(volatile int *)&(v)->counter) +#define atomic_read(v) ACCESS_AT_MOST_ONCE((v)->counter) #endif /** diff --git a/include/linux/compiler.h b/include/linux/compiler.h index 320d6c9..21f102d 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -308,4 +308,14 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect); */ #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x)) +/* + * Like ACCESS_ONCE, but can be optimized away if nothing uses the value, + * and/or merged with previous non-ONCE accesses. + */ +#define ACCESS_AT_MOST_ONCE(x) \ + ({ unsigned long __y; \ + asm("":"=r" (__y):"0" (x)); \ + (__force __typeof__(x)) __y; \ + }) + #endif /* __LINUX_COMPILER_H */
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