Messages in this thread | | | Date | Wed, 03 May 2006 20:50:49 -0700 | From | George Anzinger <> | Subject | Re: sched_clock() uses are broken |
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Nicolas Pitre wrote: > On Tue, 2 May 2006, Russell King wrote: > > >>On Tue, May 02, 2006 at 06:43:45PM +0200, Andi Kleen wrote: >> >>>Russell King <rmk+lkml@arm.linux.org.uk> writes: >>> >>>>However, this is not the case. On x86 with TSC, it returns a 54 bit >>>>number. This means that when t1 < t0, time_passed_ns becomes a very >>>>large number which no longer represents the amount of time. >>> >>>Good point. For a 1Ghz system this would happen every ~0.57 years. >>> >>>The problem is there is AFAIK no non destructive[1] way to find out how >>>many bits the TSC has >>> >>>Destructive would be to overwrite it with -1 and see how many stick. >>> >>> >>>>All uses in kernel/sched.c seem to be aflicted by this problem. >>>> >>>>There are several solutions to this - the most obvious being that we >>>>need a function which returns the nanosecond difference between two >>>>sched_clock() return values, and this function needs to know how to >>>>handle the case where sched_clock() has wrapped. >>> >>>Ok it can be done with a simple test. > > > Better yet the sched_clock() implementation just needs to return a value > shifted left so the wrap around always happens on 64 bits and the > difference between two consecutive samples is always right. > > >>>>IOW: >>>> >>>> t0 = sched_clock(); >>>> /* do something */ >>>> t1 = sched_clock(); >>>> >>>> time_passed = sched_clock_diff(t1, t0); >>>> >>>>Comments? >>> >>>Agreed it's a problem, but probably a small one. At worst you'll get >>>a small scheduling hickup every half year, which should be hardly >>>that big an issue. > > > ... on x86 that is. > > >>>Might chose to just ignore it with a big fat comment? >> >>You're right assuming you have a 64-bit TSC, but ARM has at best a >>32-bit cycle counter which rolls over about every 179 seconds - with >>gives a range of values from sched_clock from 0 to 178956970625 or >>0x29AAAAAA81. >> >>That's rather more of a problem than having it happen every 208 days. > > > Yet that counter isn't necessarily nanosecond based. So rescaling the > returned value to nanosecs requires expensive divisions which could be > done only once within sched_clock_diff() instead of twice as often in > each sched_clock() calls.
Oh phooey!! Scaling can be done with a mpy and a shift. See the new clock code where the TSC (or what ever) is scaled to ns.
-- George Anzinger george@wildturkeyranch.net HRT (High-res-timers): http://sourceforge.net/projects/high-res-timers/ - 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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