The current scheme of using the timer tick was fine for per-thread events. However, it was causing bias issues in system-wide mode (including for uncore PMUs). Event groups would not get their fair share of runtime on the PMU. With tickless kernels, if a core is idle there is no timer tick, and thus no event rotation (multiplexing). However, there are events (especially uncore events) which do count even though cores are asleep.
This patch changes the timer source for multiplexing. It introduces a per-cpu hrtimer. The advantage is that even when the core goes idle, it will come back to service the hrtimer, thus multiplexing on system-wide events works much better. In order to minimize the impact of the hrtimer, it is turned on and off on demand. When the PMU on a CPU is overcommited, the hrtimer is activated. It is stopped when the PMU is not overcommitted. In order for this to work properly with HOTPLUG_CPU, we had to change the order of initialization in start_kernel() such that hrtimer_init() is run before perf_event_init(). The second patch provide a sysctl control to adjust the multiplexing interval. Unit is milliseconds. Here is a simple before/after example with two event groups which do require multiplexing. This is done in system-wide mode on an idle system. What matters here is the scaling factor in [] in not the total counts. Before: # perf stat -a -e ref-cycles,ref-cycles sleep 10 Performance counter stats for 'sleep 10': 34,319,545 ref-cycles [56.51%] 31,917,229 ref-cycles [43.50%] 10.000827569 seconds time elapsed After: # perf stat -a -e ref-cycles,ref-cycles sleep 10 Performance counter stats for 'sleep 10': 11,144,822,193 ref-cycles [50.00%] 11,103,760,513 ref-cycles [50.00%] 10.000672946 seconds time elapsed Signed-off-by: Stephane Eranian <eran...@google.com> --- Stephane Eranian (3): perf: use hrtimer for event multiplexing perf: add sysctl control to adjust multiplexing interval perf: remove jiffies_interval include/linux/perf_event.h | 6 ++- init/main.c | 2 +- kernel/events/core.c | 155 +++++++++++++++++++++++++++++++++++++++++--- kernel/sysctl.c | 8 ++ 4 files changed, 160 insertions(+), 11 deletions(-) -- 1.7.5.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
