On Thu, Dec 13, 2007 at 05:13:19PM -0800, H. Peter Anvin wrote: > When the conversion factor between jiffies and milli- or microseconds > is not a single multiply or divide, as for the case of HZ == 300, we > currently do a multiply followed by a divide. The intervening > result, however, is subject to overflows, especially since the > fraction is not simplified (for HZ == 300, we multiply by 300 and > divide by 1000). > > This is exposed to the user when passing a large timeout to poll(), > for example. > > This patch replaces the multiply-divide with a reciprocal > multiplication on 32-bit platforms. When the input is an unsigned > long, there is no portable way to do this on 64-bit platforms there is > no portable way to do this since it requires a 128-bit intermediate > result (which gcc does support on 64-bit platforms but may generate > libgcc calls, e.g. on 64-bit s390), but since the output is a 32-bit > integer in the cases affected, just simplify the multiply-divide > (*3/10 instead of *300/1000). > > The reciprocal multiply used can have off-by-one errors in the upper > half of the valid output range. This could be avoided at the expense > of having to deal with a potential 65-bit intermediate result. Since > the intent is to avoid overflow problems and most of the other time > conversions are only semiexact, the off-by-one errors were considered > an acceptable tradeoff. > > NOTE: This patch uses a bc-based shell script to compute the > appropriate constants. This script should be run by hand if new HZ > values are created, as doing it automatically introduces a dependency > on bc, and has been shown to be unreliable in some environments. The > flipside, unfortunately, is that this may cause problems for "make > randconfig" on MIPS and OMAP, which appear to allow arbitrary values > to be entered into CONFIG_HZ. I understood that bc is unreliable. How about creating a small .c program that does to necessary computation and integrate it in the build process. It should be much cleaner than this approach. Sam - To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
