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Date: Wed, 25 Jan 2017 16:25:20 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Tony Luck <tony.luck@intel.com>,
        Wanpeng Li <wanpeng.li@hotmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul Mackerras <paulus@samba.org>, Ingo Molnar <mingo@kernel.org>,
        Fenghua Yu <fenghua.yu@intel.com>, Rik van Riel <riel@redhat.com>,
        Stanislaw Gruszka <sgruszka@redhat.com>
Subject: Re: [PATCH 37/37] s390: Prevent from cputime leaks
Message-ID: <20170125152518.GA3302@lerouge>
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On Mon, Jan 23, 2017 at 10:44:56AM +0100, Martin Schwidefsky wrote:
> On Sun, 22 Jan 2017 19:20:13 +0100
> Frederic Weisbecker <fweisbec@gmail.com> wrote:
> 
> > The s390 clock has a higher granularity than nanoseconds. 1 nanosec
> > equals 4.096 in s390 cputime_t. Therefore we leak a remainder while
> > flushing the cputime through cputime_to_nsecs().
> > 
> > For more precision, make sure we keep that remainder on cputime
> > accumulators for later accounting.
> > 
> > Reported-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
> > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > Cc: Paul Mackerras <paulus@samba.org>
> > Cc: Michael Ellerman <mpe@ellerman.id.au>
> > Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> > Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> > Cc: Tony Luck <tony.luck@intel.com>
> > Cc: Fenghua Yu <fenghua.yu@intel.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Rik van Riel <riel@redhat.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@kernel.org>
> > Cc: Stanislaw Gruszka <sgruszka@redhat.com>
> > Cc: Wanpeng Li <wanpeng.li@hotmail.com>
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> 
> NAK. Good intention but the patch is just broken. with 36 of the 37
> patches applied all looks good but the last one completely breaks the
> accounting for s390. This is from an idle system:
> 
> top - 10:39:33 up 0 min,  1 user,  load average: 0,00, 0,00, 0,00
> Tasks: 106 total,   1 running, 105 sleeping,   0 stopped,   0 zombie
> %Cpu0  :  8,9 us, 21,6 sy,  0,0 ni,  0,0 id,  0,0 wa, 10,8 hi,  4,3 si, 54,4 st
> %Cpu1  :  0,0 us, 23,5 sy,  0,0 ni,  0,0 id,  0,0 wa, 19,0 hi, 13,1 si, 44,3 st
> %Cpu2  :  0,0 us, 30,3 sy,  0,0 ni,  0,0 id,  0,0 wa, 14,7 hi, 14,8 si, 40,2 st
> KiB Mem :  1009304 total,   818808 free,    57284 used,   133212 buff/cache
> KiB Swap:  1048556 total,  1048556 free,        0 used.   917356 avail Mem

Oh ok. I must have done something wrong.

> 
> There is another issue that affects precision, there is no s390 specific
> version of cputime_to_nsecs. The generic version uses cputime_to_usecs
> and mulitplies by 1000 to get nano-seconds. That already looses precision.

That's right. And that's the point of this patch. I'm not sure we can have a
more precise version of cputime_to_nsecs() if 1 nsec == 4.096 cputime_t

> 
> For now just drop that last patch please.

Ok, I'm leaving it apart.

Thanks.