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Message-ID: <4A0ADF34.2040001@cosmosbay.com> Date: Wed, 13 May 2009 16:54:44 +0200 From: Eric Dumazet <dada1@...mosbay.com> To: Andi Kleen <andi@...stfloor.org> CC: Thomas Gleixner <tglx@...utronix.de>, Chris Friesen <cfriesen@...tel.com>, Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Steven Rostedt <rostedt@...dmis.org>, David Miller <davem@...emloft.net>, linuxppc-dev@...abs.org, paulus@...ba.org, netdev@...r.kernel.org Subject: Re: question about softirqs Andi Kleen a écrit : > Thomas Gleixner <tglx@...utronix.de> writes: > > >> Err, no. Chris is completely correct: >> >> if (!in_interrupt()) >> wakeup_softirqd(); > > Yes you have to wake it up just in case, but it doesn't normally > process the data because a normal softirq comes in faster. It's > just a safety policy. > > You can check this by checking the accumulated CPU time on your > ksoftirqs. Mine are all 0 even on long running systems. > Then its a bug Andi. Its quite easy to trigger ksoftirqd with a Gb ethernet link. commit f5f293a4e3d0a0c52cec31de6762c95050156516 corrected something (making mpstat and top correctly display softirq on cpu stats), but apparently we still have a problem to report correct time on processes, particularly on ksoftirq/x I have one machine SMP flooded by network frames, CPU0 handling all the work, inside ksoftirq/0 (napi processing : almost no more hard interrupts delivered) Still, top or ps reports no more than 30% of cpu time used by ksoftirqd, while this cpu only runs ksoftirqd/0 (100% in sirq), and has no idle time. $ps -fp 4 ; mpstat -P 0 1 10 ; ps -fp 4 UID PID PPID C STIME TTY TIME CMD root 4 2 1 15:35 ? 00:00:46 [ksoftirqd/0] Linux 2.6.30-rc5-tip-01595-g6f75dad-dirty (svivoipvnx001) 05/13/2009 _i686_ 04:45:01 PM CPU %usr %nice %sys %iowait %irq %soft %steal %guest %idle 04:45:02 PM 0 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 04:45:03 PM 0 0.00 0.00 0.00 0.00 0.00 99.01 0.00 0.00 0.99 04:45:04 PM 0 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 04:45:05 PM 0 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 04:45:06 PM 0 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 04:45:07 PM 0 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 04:45:08 PM 0 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 04:45:09 PM 0 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 04:45:10 PM 0 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 04:45:11 PM 0 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 Average: 0 0.00 0.00 0.00 0.00 0.00 99.90 0.00 0.00 0.10 UID PID PPID C STIME TTY TIME CMD root 4 2 1 15:35 ? 00:00:49 [ksoftirqd/0] You can see here time consumed by ksoftirqd/0 suring this 10 seconds time frame is *only* 3 seconds. Therefore, we cannot trust ps, not with current kernel. # cat /proc/4/stat ; sleep 10 ; cat /proc/4/stat 4 (ksoftirqd/0) R 2 0 0 0 -1 2216730688 0 0 0 0 0 15347 0 0 15 -5 1 0 6 0 0 4294967295 0 0 0 0 0 0 0 2147483647 0 0 0 0 17 0 0 0 0 0 0 4 (ksoftirqd/0) R 2 0 0 0 -1 2216730688 0 0 0 0 0 15670 0 0 15 -5 1 0 6 0 0 4294967295 0 0 0 0 0 0 0 2147483647 0 0 0 0 17 0 0 0 0 0 0 > The reason Andrea originally added the softirqds was just that > if you have very softirq intensive workloads they would tie > up too much CPU time or not make enough process with the default > "don't loop too often" heuristics. > >> We can not rely on irqs coming in when the softirq is raised from > > You can't rely on it, but it happens in near all cases. > > -Andi -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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