linux-kernel - Re: [GIT PULL] cputime: Convert core use of cputime

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Date:   Mon, 30 Jan 2017 15:56:49 +0100
From:   Frederic Weisbecker <fweisbec@...il.com>
To:     Stanislaw Gruszka <sgruszka@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tony Luck <tony.luck@...el.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Mackerras <paulus@...ba.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Rik van Riel <riel@...hat.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [GIT PULL] cputime: Convert core use of cputime_t to nsecs

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.