Messages in this thread |  | | | Date | Sun, 22 Mar 2009 18:02:15 +0100 | | From | Ingo Molnar <> | | Subject | Re: [PATCH 0/5] [GIT PULL] updates for tip/tracing/ftrace |
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ok, with Frederic we figured out the problem.
What helped things most was this trace-dump output:
0) + 15.281 us | } 0) | handle_irq() { 1) + 35.871 us | } 1) | timespec_to_ktime() { 0) 4.608 us | } 0) | generic_handle_irq_desc() { 1) 4.097 us | } 1) + 14.171 us | } 0) 4.450 us | _spin_lock(); 1) + 60.127 us | } 1) | ktime_get() { 0) | ack_apic_edge() { 1) | getnstimeofday() { 0) 6.486 us | } 0) 5.619 us | irq_complete_move(); 1) 5.158 us | jiffies_read(); 0) | move_native_irq() { 1) + 15.495 us | } 1) + 26.161 us | } 0) 5.631 us | } 1) 5.549 us | set_normalized_timespec(); 0) + 16.304 us | } 0) | ack_APIC_irq() { 1) + 48.377 us | } 1) | timespec_to_ktime() { 0) 5.762 us | native_apic_mem_write(); 1) 5.751 us | } 0) + 16.162 us | } 1) + 16.413 us | } 0) + 27.185 us | } 1) + 81.519 us | } 0) + 80.245 us | } 1) ! 154.606 us | } 0) | _spin_unlock() { 1) 5.743 us | tick_nohz_update_jiffies(); 0) 5.781 us | } 1) ! 183.912 us | } 0) 5.327 us | preempt_schedule(); 1) ! 202.575 us | } 0) + 25.827 us | } [...] 1) ! 2623.297 us | } i.e. all CPUs spend 2-3 milliseconds to handle a single tick. This is on a Core2 Extreme Edition 2.93 GHz CPU, so this kind of cost was unexpected.
Until i saw this:
CONFIG_TRACE_BRANCH_PROFILING=y CONFIG_PROFILE_ALL_BRANCHES=y
that explains it all. The above sequence is two CPUs 'lock stepped' in a very high overhead series of cacheline ping-pongs. The ping-pongs happen due to every branch in the kernel doing:
______f.miss_hit[______r]++; where the branch info metadata is defined as global variables:
static struct ftrace_branch_data \ __attribute__((__aligned__(4))) \ __attribute__((section("_ftrace_branch"))) \
not only is it global, it's also false cacheline-shared due to a 4 byte alignment.
The proper solution would be to use percpu data and percpu_add() primitives for this.
Anyway ... i turned off the branch tracer for my tests.
Ingo
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