On Thu, 2017-03-23 at 16:55 -0400, Luiz Capitulino wrote: > 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 > > 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. Jiffies should be advanced by the timer interrupt, on the housekeeping CPU, which is not doing context tracking. Why is the isolated/nohz_full CPU receiving timer interrupts at all? I thought it would not, but obviously I am wrong. What is going on here? -- 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
