[ Not relevant upstream, therefore no upstream commit. ]
To fix, use jiffies64_to_nsecs() directly instead of deriving the result according to jiffies_to_usecs().
As the return type of jiffies_to_usecs() is 'unsigned int', when the return value is more than the size of 'unsigned int', the leading 32 bits would be discarded.
Suppose USEC_PER_SEC=1000000L and HZ=1000, below are the expected and actual incorrect result of jiffies_to_usecs(0x7770ef70):
- expected : jiffies_to_usecs(0x7770ef70) = 0x000001d291274d80 - incorrect : jiffies_to_usecs(0x7770ef70) = 0x0000000091274d80
The leading 0x000001d200000000 is discarded.
After xen vcpu hotplug and when the new vcpu steal clock is calculated for the first time, the result of this_rq()->prev_steal_time in steal_account_process_tick() would be far smaller than the expected value, due to that jiffies_to_usecs() discards the leading 32 bits.
As a result, the diff between current steal and this_rq()->prev_steal_time is always very large. Steal usage would become 100% when the initial steal clock obtained from xen hypervisor is very large during xen vcpu hotplug, that is, when the guest is already up for a long time.
The bug can be detected by doing the following:
* Boot xen guest with vcpus=2 and maxvcpus=4 * Leave the guest running for a month so that the initial steal clock for the new vcpu would be very large * Hotplug 2 extra vcpus * The steal time of new vcpus in /proc/stat would increase abnormally and sometimes steal usage in top can become 100%
This was incidentally fixed in the patch set starting by commit 93825f2ec736 ("jiffies: Reuse TICK_NSEC instead of NSEC_PER_JIFFY") and ended with commit b672592f0221 ("sched/cputime: Remove generic asm headers").
This version applies to the v4.9 series.
Link: https://lkml.org/lkml/2019/2/28/1373 Suggested-by: Juergen Gross jgross@suse.com Signed-off-by: Dongli Zhang dongli.zhang@oracle.com --- include/linux/jiffies.h | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h index 734377a..94aff43 100644 --- a/include/linux/jiffies.h +++ b/include/linux/jiffies.h @@ -287,13 +287,13 @@ extern unsigned long preset_lpj; extern unsigned int jiffies_to_msecs(const unsigned long j); extern unsigned int jiffies_to_usecs(const unsigned long j);
+extern u64 jiffies64_to_nsecs(u64 j); + static inline u64 jiffies_to_nsecs(const unsigned long j) { - return (u64)jiffies_to_usecs(j) * NSEC_PER_USEC; + return jiffies64_to_nsecs(j); }
-extern u64 jiffies64_to_nsecs(u64 j); - extern unsigned long __msecs_to_jiffies(const unsigned int m); #if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ) /*