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authorSteven Rostedt <rostedt@goodmis.org>2008-07-09 00:15:33 -0400
committerIngo Molnar <mingo@elte.hu>2008-07-11 15:53:28 +0200
commitc300ba252829e9325e08f0af60687add94445b25 (patch)
tree93c583012a3f26617e95aea516c02716ce254f5d /kernel/rcuclassic.c
parenta83bc47c33ab182f1e48977fd5a04024d713c75e (diff)
sched_clock: and multiplier for TSC to gtod drift
The sched_clock code currently tries to keep all CPU clocks of all CPUS somewhat in sync. At every clock tick it records the gtod clock and uses that and jiffies and the TSC to calculate a CPU clock that tries to stay in sync with all the other CPUs. ftrace depends heavily on this timer and it detects when this timer "jumps". One problem is that the TSC and the gtod also drift. When the TSC is 0.1% faster or slower than the gtod it is very noticeable in ftrace. To help compensate for this, I've added a multiplier that tries to keep the CPU clock updating at the same rate as the gtod. I've tried various ways to get it to be in sync and this ended up being the most reliable. At every scheduler tick we calculate the new multiplier: multi = delta_gtod / delta_TSC This means we perform a 64 bit divide at the tick (once a HZ). A shift is used to handle the accuracy. Other methods that failed due to dynamic HZ are: (not used) multi += (gtod - tsc) / delta_gtod (not used) multi += (gtod - (last_tsc + delta_tsc)) / delta_gtod as well as other variants. This code still allows for a slight drift between TSC and gtod, but it keeps the damage down to a minimum. Signed-off-by: Steven Rostedt <srostedt@redhat.com> Cc: Steven Rostedt <srostedt@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: john stultz <johnstul@us.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'kernel/rcuclassic.c')
0 files changed, 0 insertions, 0 deletions