Hello! This series accelerates RCU Tasks Trace updates, reducing the average grace-period latencies from about a second to about 20 milliseconds on my x86 laptop. These are benchmark numbers, based on a previously posted patch to rcuscale.c [1] running on my x86 laptop. The patches in this series are as follows: 1. Mark variables static, noted during this effort but otherwise unconnected. This change has no effect, so that the average grace-period latency remains at 980 milliseconds. 2. Use more aggressive polling for RCU Tasks Trace. This polling starts at five-millisecond intervals instead of the prior 100-millisecond intervals. As before, the polling interval increases in duration as the grace period ages, and again as before is capped at one second. This change reduces the average grace-period latency to about 620 milliseconds. 3. Selectively enable more RCU Tasks Trace IPIs. This retains the old behavior of suppressing IPIs for grace periods that are younger than 500 milliseconds for CONFIG_TASKS_TRACE_RCU_READ_MB=y kernels, including CONFIG_PREEMPT_RT=y kernels, but allows IPIs immediately on other kernels. It is quite possible that a more sophisticated decision procedure will be required, and changes to RCU's dyntick-idle code might also be needed. This change (along with the earlier ones) reduces the average grace-period latency to about 120 milliseconds. 4. Shorten per-grace-period sleep for RCU Tasks Trace. The current code sleeps for 100 milliseconds after the end of each grace period, which by itself prevents a back-to-back string of grace-period waits from completing faster than ten per second. This patch also retains this old behavior for CONFIG_TASKS_TRACE_RCU_READ_MB=y (and again thus also for CONFIG_PREEMPT_RT=y kernels). For other kernels, this post-grace-period sleep is reduced to five milliseconds. This change (along with the earlier ones) reduced the average grace-period latency to about 18 milliseconds, for an overall factor-of-50 reduction in latency. Alexei Starovoitov benchmarked an earlier patch [2], producing results that are roughly consistent with the above reduction in latency [3]. Thanx, Paul [1] https://lore.kernel.org/bpf/20200909193900.GK29330@paulmck-ThinkPad-P72/ [2] https://lore.kernel.org/bpf/20200910052727.GA4351@paulmck-ThinkPad-P72/ [3] https://lore.kernel.org/bpf/619554b2-4746-635e-22f3-7f0f09d97760@xxxxxx/ ------------------------------------------------------------------------ tasks.h | 37 +++++++++++++++++++++++++++++-------- 1 file changed, 29 insertions(+), 8 deletions(-)
