linux-kernel - Re: [PATCH v0.9.1 3/6] sched/umcg: implement UMCG syscalls

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Message-ID: <20211129222243.GR721624@worktop.programming.kicks-ass.net>
Date:   Mon, 29 Nov 2021 23:22:43 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Peter Oskolkov <posk@...k.io>, Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>,
        Peter Oskolkov <posk@...gle.com>,
        Andrei Vagin <avagin@...gle.com>, Jann Horn <jannh@...gle.com>,
        Thierry Delisle <tdelisle@...terloo.ca>
Subject: Re: [PATCH v0.9.1 3/6] sched/umcg: implement UMCG syscalls

On Mon, Nov 29, 2021 at 11:07:07PM +0100, Thomas Gleixner wrote:
> On Fri, Nov 26 2021 at 22:52, Peter Zijlstra wrote:

> The size is the same, i.e. 1 bit per nanosecond :)

:-)

> > The only time that's relevant though is when you're going to mix these
> > timestamps with CLOCK_THREAD_CPUTIME_ID, which might just be
> > interesting.
> 
> Uuurg. If you want to go towards CLOCK_THREAD_CPUTIME_ID, that's going
> to be really nasty. Actually you can sleep on that clock, but that's a
> completely different universe. If anything like that is desired then we
> need to rewrite that posix CPU timer muck completely with all the bells
> and whistels and race conditions attached to it. *Shudder*

Oh, I wasn't thinking anything as terrible as that. Sleeping on that
clock is fundamentally daft since it doesn't run when thats is
sleeping, consider trying to sleep on your own runtime :-)

I was only considering combining THREAD_CPUTIME timestamps with the
UMCG timestamps to compute how much unmanaged time there was, or other
such things.

Anyway, lets forget I bought this up and assume that for practical
purposes all [ns] are of equal length.