linux-kernel - Re: [PATCH 2/2] Add a thread cpu time implementation to vDSO

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Date:	Mon, 12 Dec 2011 15:32:52 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Arun Sharma <asharma@...com>
Cc:	linux-kernel@...r.kernel.org, Kumar Sundararajan <kumar@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andy Lutomirski <luto@....EDU>
Subject: Re: [PATCH 2/2] Add a thread cpu time implementation to vDSO

On Mon, 2011-12-12 at 15:20 -0800, Arun Sharma wrote:
> On 12/12/11 3:09 PM, john stultz wrote:
> > On Mon, 2011-12-12 at 11:36 -0800, Arun Sharma wrote:
> >> From: Kumar Sundararajan<kumar@...com>
> >>
> >> This primarily speeds up clock_gettime(CLOCK_THREAD_CPUTIME_ID, ..)
> >> via a new vsyscall. We also add a direct vsyscall that returns
> >> time in ns (RFC: the direct vsyscall doesn't have a corresponding
> >> regular syscall, although clock_gettime() is pretty close).
> >
> > I'm still not super psyched about providing a vdso-only API.
> >
> > If a nanosecond interface like thread_cpu_time() is actually a big win
> > over clock_gettime(CLOCK_THREAD_CPUTIME,...) it seems it should have its
> > own syscall as well, no?
> 
> The win is relatively small when we're dealing with syscalls. But with 
> vsyscalls, it starts showing up in micro benchmarks.
> 
> Happy to post patches for regular syscalls (assuming I can get them 
> allocated :).
> 
> >
> > Possibly something like clock_gettime_ns(), which would return the same
> > values as clock_gettime() but in nanoseconds rather then a timespec?
> >
> 
> If we're doing non-POSIXy things there, how about allocating one syscall 
> per clock instead of multiplexing them through a single syscall?
> 
> This would be a nice to have (clock_gettime_ns() should get us most of 
> the perf benefit).

Well, it makes it a little easier to extend if we get a new clockid,
rather then having to add a whole new syscall. Keeps parity between the
timespec and ns interfaces.

Is it just that you're concerned about the clockid switch costs being
too high?

thanks
-john


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