linux-kernel - Re: [PATCH for v4.9-stable] sched: fix softirq time accounting

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Date:   Wed, 17 Jan 2018 14:54:22 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Rabin Vincent <rabin.vincent@...s.com>
Cc:     fweisbec@...il.com, riel@...hat.com, mingo@...hat.com,
        peterz@...radead.org, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org, Rabin Vincent <rabinv@...s.com>
Subject: Re: [PATCH for v4.9-stable] sched: fix softirq time accounting

On Wed, Dec 13, 2017 at 11:54:56AM +0100, Greg KH wrote:
> On Wed, Dec 13, 2017 at 11:11:16AM +0100, Rabin Vincent wrote:
> > From: Rabin Vincent <rabinv@...s.com>
> > 
> > softirq time accounting is broken on v4.9.x if ksoftirqd runs.
> > 
> > With
> > 	CONFIG_IRQ_TIME_ACCOUNTING=y
> > 	# CONFIG_VIRT_CPU_ACCOUNTING_GEN is not set
> > 
> > this test code:
> > 
> > 	struct tasklet_struct tasklet;
> > 
> > 	static void delay_tasklet(unsigned long data)
> > 	{
> > 		udelay(10);
> > 		tasklet_schedule(&tasklet);
> > 	}
> > 
> > 	tasklet_init(&tasklet, delay_tasklet, 0);
> > 	tasklet_schedule(&tasklet);
> > 
> > results in:
> > 
> > $ while :; do grep cpu0 /proc/stat; done
> > cpu0 5 0 80 25 16 107 1 0 0 0
> > cpu0 5 0 80 25 16 107 0 0 0 0
> > cpu0 5 0 80 25 16 107 0 0 0 0
> > cpu0 5 0 80 25 16 107 0 0 0 0
> > cpu0 5 0 81 25 16 107 0 0 0 0
> > cpu0 5 0 81 25 16 107 1 0 0 0
> > cpu0 5 0 81 25 16 108 18446744073709551615 0 0 0
> > cpu0 5 0 81 25 16 108 18446744073709551615 0 0 0
> > cpu0 5 0 81 25 16 108 18446744073709551615 0 0 0
> > cpu0 5 0 81 25 16 108 0 0 0 0
> > cpu0 6 0 81 25 16 108 0 0 0 0
> > cpu0 6 0 81 25 16 108 0 0 0 0
> > 
> > As can be seen, the softirq numbers are totally bogus.
> > 
> > When ksoftirq is running, irqtime_account_process_tick() increments
> > cpustat[CPUSTAT_SOFTIRQ].  This causes the "nsecs_to_cputime64(irqtime)
> > - cpustat[CPUSTAT_SOFTIRQ]" calculation in irqtime_account_update() to
> > underflow the next time a softirq is handled leading to the above
> > values.
> > 
> > The underflow bug was added by 57430218317e5b280 ("sched/cputime: Count
> > actually elapsed irq & softirq time").
> > 
> > But ksoftirqd accounting was wrong even in earlier kernels.  In earlier
> > kernels, after a ksoftirq run, the kernel would simply stop accounting
> > softirq time spent outside of ksoftirqd until that (accumulated) time
> > exceeded the time for which ksofirqd previously had run.
> > 
> > Fix both the underflow and the wrong accounting by using a counter
> > specifically for the non-ksoftirqd softirq case.
> > 
> > This code has been fixed in current mainline by a499a5a14db
> > ("sched/cputime: Increment kcpustat directly on irqtime account") [note
> > also the followup 25e2d8c1b9e327e ("sched/cputime: Fix ksoftirqd cputime
> > accounting regression")], but that patch is a part of the many changes
> > for eliminating of cputime_t so it does not seem suitable for backport.
> 
> I _really_ only want to take the exact upstream patches, as every time
> we do something like what you are proposing to do here, we get it wrong.
> Seriously, our track record is horrible.  Like 90% wrong.
> 
> Can you try just those two patches instead?

Dropping from my mbox due to a lack of response.  If you do test this
out, please resend.

thanks,

greg k-h