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Date: Mon, 30 Jan 2017 15:56:49 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>, 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>, Fenghua Yu <fenghua.yu@intel.com>,
        Rik van Riel <riel@redhat.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>
Subject: Re: [GIT PULL] cputime: Convert core use of cputime_t to nsecs
Message-ID: <20170130145648.GA16945@lerouge>
References: <1485751603-15913-1-git-send-email-fweisbec@gmail.com>
 <20170130143223.GC2669@redhat.com>
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On Mon, Jan 30, 2017 at 03:32:24PM +0100, Stanislaw Gruszka wrote:
> On Mon, Jan 30, 2017 at 05:46:43AM +0100, Frederic Weisbecker wrote:
> > Now lets admit one drawback: s390 and powerpc with
> > CONFIG_VIRT_CPU_ACCOUNTING_NATIVE have new cputime_t to nsecs conversion
> > on cputime accounting path. But this should be leveraged by the recent
> > changes which delay the cputime accounting to tick and context switch. 
> 
> I think it would be worth to mention that there are other drawbacks on
> 32bit architectures that use cputime-jiffies currently, like:
> - cache utilization will be worse

Due to utime and stime becoming 64 bits? Yeah indeed.

> - conversion cputime_to_jiffies() and cputime_to_clock_t() (with HZ == USER_HS)
>   will no longer be an no-op

Yes probably some cputime_to_jiffies() have been replaced with nsecs_to_jiffies().
In turn many cputime_to_nsecs() are now no-op.

Now concerning cputime_to_clock_t(), it seems that USER_HZ=100 most of the time. This
value seems to be seldom used for HZ on distros in general.

> - to keep values consistent will need to add protection of u64 store/load,
>   which will create additional performance costs

Yes but note these issues were already there before this patchset.
Especially for kcpustat. And utime/stime also have these issues under CONFIG_NO_HZ_FULL.

Now this patchset extends the problem for utime/stime to all configs. That's certainly
one more reason to fix it. But that's outside the scope of this patchset.