linux-kernel - Re: [tip:timers/core] [timers] 7ee9887703: netperf.Throughput

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Date: Tue, 5 Mar 2024 12:35:30 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Oliver Sang <oliver.sang@...el.com>
Cc: Anna-Maria Behnsen <anna-maria@...utronix.de>, oe-lkp@...ts.linux.dev,
	lkp@...el.com, linux-kernel@...r.kernel.org, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>, ying.huang@...el.com,
	feng.tang@...el.com, fengwei.yin@...el.com
Subject: Re: [tip:timers/core] [timers]  7ee9887703:  netperf.Throughput_Mbps
 -1.2% regression

On Tue, Mar 05, 2024 at 10:17:43AM +0800, Oliver Sang wrote:
>      20.97 ±  3%      -9.7       11.23 ± 16%      -9.6       11.40 ±  4%  perf-profile.calltrace.cycles-pp.__napi_poll.net_rx_action.__do_softirq.irq_exit_rcu.sysvec_apic_timer_interrupt

And yes, fewer time spent in IRQ-tail softirq processing.

>       1.70 ± 13%      +5.6        7.28 ± 14%      +5.8        7.54 ± 15%  perf-profile.calltrace.cycles-pp.__napi_poll.net_rx_action.__do_softirq.run_ksoftirqd.smpboot_thread_fn

And more time spent in ksoftirqd softirq processing.

This can match the increase in involuntary context switches: IRQ-tail softirq
processing takes too much time, possibly due to long lasting remote timer
expiring, ksoftirqd is then scheduled, preempting netperf (through
might_resched()/cond_resched()) as this is a voluntary preemption kernel).

Thanks.