Messages in this thread Patch in this message |  | | | Subject | RE: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression | | Date | Fri, 6 Oct 2006 08:06:37 +0400 | | From | "Ananiev, Leonid I" <> |
> Apparently due to the access to the > newly-added static variable.
The cache miss was increased as a result of instructions reordering but not due to the newly-added static variable. The original patch have added local but not static variable. Newly-added local variable compiler has placed in register. The original patch made reordering of tests-register-and-jump and test-memory-and-jump instructions. That was a side effect of the patch. To watch closely ordering of 'if (a && b)' or 'if (b && a)' becomes more weight than 'unlikely()' using if 'a' is in register and 'b' is in memory. I would say: unlikely() cost is 10:1; ordering cost is 1000:1. Leonid
-----Original Message----- From: Andrew Morton [mailto:akpm@osdl.org] Sent: Friday, October 06, 2006 1:38 AM To: Ananiev, Leonid I Cc: tim.c.chen@linux.intel.com; Jeremy Fitzhardinge; herbert@gondor.apana.org.au; linux-kernel@vger.kernel.org Subject: Re: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression
So how's this look?
I worry a bit that someone's hardware might go and prefetch that static variable even when we didn't ask it to. Can that happen?
Tim and Ananiev report that the recent WARN_ON_ONCE changes cause increased cache misses with the tbench workload. Apparently due to the access to the newly-added static variable.
Rearrange the code so that we don't touch that variable unless the warning is going to trigger.
Also rework the logic so that the static variable starts out at zero, so we can move it into bss.
It would seem logical to mark the static variable as __read_mostly too. But it would be wrong, because that would put it back into the vmlinux image, and the kernel will never read from this variable in normal operation anyway. Unless the compiler or hardware go and do some prefetching on us?
For some reason this patch shrinks softirq.o text by 40 bytes.
Cc: Tim Chen <tim.c.chen@intel.com> Cc: Herbert Xu <herbert@gondor.apana.org.au> Cc: "Ananiev, Leonid I" <leonid.i.ananiev@intel.com> Signed-off-by: Andrew Morton <akpm@osdl.org> --- include/asm-generic/bug.h | 16 ++++++++-------- 1 files changed, 8 insertions(+), 8 deletions(-) diff -puN include/asm-generic/bug.h~fix-warn_on--warn_on_once-regression include/asm-generic/bug.h --- a/include/asm-generic/bug.h~fix-warn_on--warn_on_once-regression +++ a/include/asm-generic/bug.h @@ -41,14 +41,14 @@ #endif #endif -#define WARN_ON_ONCE(condition) ({ \ - static int __warn_once = 1; \ - typeof(condition) __ret_warn_once = (condition);\ - \ - if (likely(__warn_once)) \ - if (WARN_ON(__ret_warn_once)) \ - __warn_once = 0; \ - unlikely(__ret_warn_once); \ +#define WARN_ON_ONCE(condition) ({ \ + static int __warned; \ + typeof(condition) __ret_warn_once = (condition); \ + \ + if (unlikely(__ret_warn_once)) \ + if (WARN_ON(!__warned)) \ + __warned = 1; \ + unlikely(__ret_warn_once); \ }) #ifdef CONFIG_SMP _ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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