2017-03-24 4:55 GMT+08:00 Luiz Capitulino <lcapitulino@xxxxxxxxxx>: > > When there are two or more tasks executing in user-space and > taking 100% of a nohz_full CPU, top reports 70% system time > and 30% user time utilization. Sometimes I'm even able to get > 100% system time and 0% user time. > > This was reproduced with latest Linus tree (093b995), but I > don't believe it's a regression (at least not a recent one) > as I can reproduce it with older kernels. Also, I have > CONFIG_IRQ_TIME_ACCOUNTING=y and haven't tried to reproduce > without it yet. > > Below you'll find the steps to reproduce and some initial > analysis. > > Steps to reproduce > ------------------ > > 1. Set up a CPU for nohz_full with isolcpus= nohz_full= > > 2. Pin two tasks that hog the CPU 100% of the time to that CPU > > 3. Run top -d1 and check system time > > NOTE: When there's only one task hogging a nohz_full CPU, top > shows 100% user-time, as expected I just saw at most 12% system time instead of 30% or 100%. Could you grep HZ /boot/config-`uname -r` and post here? Regards, Wanpeng Li > > Initial analysis > ---------------- > > When tracing vtime accounting functions and the user-space/kernel > transitions when the issue is taking place, I see several of the > following: > > hog-10552 [015] 1132.711104: function: enter_from_user_mode <-- apic_timer_interrupt > hog-10552 [015] 1132.711105: function: __context_tracking_exit <-- enter_from_user_mode > hog-10552 [015] 1132.711105: bprint: __context_tracking_exit.part.4: new state=1 cur state=1 active=1 > hog-10552 [015] 1132.711105: function: vtime_account_user <-- __context_tracking_exit.part.4 > hog-10552 [015] 1132.711105: function: smp_apic_timer_interrupt <-- apic_timer_interrupt > hog-10552 [015] 1132.711106: function: irq_enter <-- smp_apic_timer_interrupt > hog-10552 [015] 1132.711106: function: tick_sched_timer <-- __hrtimer_run_queues > hog-10552 [015] 1132.711108: function: irq_exit <-- smp_apic_timer_interrupt > hog-10552 [015] 1132.711108: function: __context_tracking_enter <-- prepare_exit_to_usermode > hog-10552 [015] 1132.711108: bprint: __context_tracking_enter.part.2: new state=1 cur state=0 active=1 > hog-10552 [015] 1132.711109: function: vtime_user_enter <-- __context_tracking_enter.part.2 > hog-10552 [015] 1132.711109: function: __vtime_account_system <-- vtime_user_enter > hog-10552 [015] 1132.711109: function: account_system_time <-- __vtime_account_system > > On entering the kernel due to a timer interrupt, vtime_account_user() > skips user-time accounting. Then later on when returning to user-space, > vtime_user_enter() is probably accounting the whole time (ie. user-space > plus kernel-space) to system time. > > Now, when does vtime_account_user() skips accounting? Well, when the > time delta is less then one jiffie. This would imply that vtime_account_user() > is being called less than one jiffie since the last accounting, but I haven't > confirmed any of this yet. -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
