Messages in this thread Patch in this message | | | Date | Thu, 1 Jun 2006 23:08:25 -0400 | From | Jeff Dike <> | Subject | non-scalar ktime addition and subtraction broken |
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The use of 64-bit additions and subtractions on something which is nominally a struct containing 32-bit second and nanosecond field is broken when a negative time is involved. When the structure is treated as a 64-bit integer, the increment of the upper 32 bits that's part of two's-complement subtraction is lost. This leaves the end result off by one second.
This manifested itself with sleeps inside UML lasting about 1 second shorter than expected.
The patch below is more a problem statement than a real fix. People thought about performance, and I don't know what this does to that work.
I'm not sure why the hrtimer.c part is needed - I had done that before tracking down the ktime_add problem. I see short sleeps without it, so it is needed somehow.
The ktime_sub piece was done for completeness - UML compiles and boots with no apparent ill effects, but it's otherwise untested.
As an aside, I fail to see how it can be correct for ktime_sub to add NSEC_PER_SEC to something without compensating somewhere else for it.
Andrew - please don't drop this into -mm without an OK from Thomas or someone else who's familiar with this code :-)
Jeff
Index: linux-2.6.17-mm/include/linux/ktime.h =================================================================== --- linux-2.6.17-mm.orig/include/linux/ktime.h 2006-06-01 22:15:44.000000000 -0400 +++ linux-2.6.17-mm/include/linux/ktime.h 2006-06-01 23:00:32.000000000 -0400 @@ -148,7 +148,8 @@ static inline ktime_t ktime_sub(const kt { ktime_t res; - res.tv64 = lhs.tv64 - rhs.tv64; + res.tv.sec = lhs.tv.sec - rhs.tv.sec; + res.tv.nsec = lhs.tv.nsec - rhs.tv.nsec; if (res.tv.nsec < 0) res.tv.nsec += NSEC_PER_SEC; @@ -167,7 +168,8 @@ static inline ktime_t ktime_add(const kt { ktime_t res; - res.tv64 = add1.tv64 + add2.tv64; + res.tv.sec = add1.tv.sec + add2.tv.sec; + res.tv.nsec = add1.tv.nsec + add2.tv.nsec; /* * performance trick: the (u32) -NSEC gives 0x00000000Fxxxxxxx * so we subtract NSEC_PER_SEC and add 1 to the upper 32 bit. Index: linux-2.6.17-mm/kernel/hrtimer.c =================================================================== --- linux-2.6.17-mm.orig/kernel/hrtimer.c 2006-06-01 22:15:44.000000000 -0400 +++ linux-2.6.17-mm/kernel/hrtimer.c 2006-06-01 22:57:24.000000000 -0400 @@ -627,7 +627,8 @@ static inline void run_hrtimer_queue(str int restart; timer = rb_entry(node, struct hrtimer, node); - if (base->softirq_time.tv64 <= timer->expires.tv64) + if(ktime_to_ns(base->softirq_time) <= + ktime_to_ns(timer->expires)) break; fn = timer->function; - 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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