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

From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Oskolkov <posk@posk.io>, Ingo Molnar <mingo@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	linux-api@vger.kernel.org, Paul Turner <pjt@google.com>,
	Ben Segall <bsegall@google.com>, Peter Oskolkov <posk@google.com>,
	Andrei Vagin <avagin@google.com>, Jann Horn <jannh@google.com>,
	Thierry Delisle <tdelisle@uwaterloo.ca>
Subject: Re: [PATCH v0.9.1 3/6] sched/umcg: implement UMCG syscalls
Date: Mon, 29 Nov 2021 23:22:43 +0100	[thread overview]
Message-ID: <20211129222243.GR721624@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <87r1ayd3as.ffs@tglx>

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.