Messages in this thread | | | Date | Mon, 6 Feb 2012 13:26:19 -0800 | Subject | Re: [RFC] Extend mwait idle to optimize away IPIs when possible | From | Venki Pallipadi <> |
| |
On Mon, Feb 6, 2012 at 1:02 PM, Peter Zijlstra <peterz@infradead.org> wrote: > On Mon, 2012-02-06 at 12:42 -0800, Venkatesh Pallipadi wrote: >> smp_call_function_single and ttwu_queue_remote sends unconditional IPI >> to target CPU. However, if the target CPU is in mwait based idle, we can >> do IPI-less wakeups using the magical powers of monitor-mwait. >> Doing this has certain advantages: >> * Lower overhead on Async IPI send path. Measurements on Westmere based >> systems show savings on "no wait" smp_call_function_single with idle >> target CPU (as measured on the sender side). >> local socket smp_call_func cost goes from ~1600 to ~1200 cycles >> remote socket smp_call_func cost goes from ~2000 to ~1800 cycles >> * Avoiding actual interrupts shows a measurable reduction (10%) in system >> non-idle cycles and cache-references with micro-benchmark sending IPI from >> one CPU to all the other mostly idle CPUs in the system. >> * On a mostly idle system, turbostat shows a tiny decrease in C0(active) time >> and a corresponding increase in C6 state (Each row being 10min avg) >> %c0 %c1 %c6 >> Before >> Run 1 1.51 2.93 95.55 >> Run 2 1.48 2.86 95.65 >> Run 3 1.46 2.78 95.74 >> After >> Run 1 1.35 2.63 96.00 >> Run 2 1.46 2.78 95.74 >> Run 3 1.37 2.63 95.98 >> >> * As a bonus, we can avoid sched/call IPI overhead altogether in a special case. >> When CPU Y has woken up CPU X (which can take 50-100us to actually wakeup >> from a deep idle state) and CPU Z wants to send IPI to CPU X in this period. >> It can get it for free. >> >> We started looking at this with one of our workloads where system is partially >> busy and we noticed some kernel hotspots in find_next_bit and >> default_send_IPI_mask_sequence_phys coming from sched wakeup (futex wakeups) >> and networking call functions. So, this change addresses those two specific >> IPI types. This could be extended to nohz_kick, etc. >> >> Note: >> * This only helps when target CPU is idle. When it is busy we will still send >> IPI as before. >> * Only for X86_64 and mwait_idle_with_hints for now, with limited testing. >> * Will need some accounting for these wakeups exported for powertop and friends. >> >> Comments? > > Curiously you avoided the existing tsk_is_polling() magic, which IIRC is > doing something similar for waking from the idle loop. >
Yes. That needs remote CPU's current task, which extends onto rq lock, which I was trying to avoid. So, I went with conditional waiting on idle exit for the small window of WAKING to WOKEN state change, as we know we are always polling in the mwait loop. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |