Re: [PATCH RFC nohz_full v2 2/7] nohz_full: Add rcu_dyntick data forscalable detection of all-idle state
From: Paul E. McKenney
Date: Mon Jul 01 2013 - 14:23:38 EST
On Mon, Jul 01, 2013 at 11:16:01AM -0700, Josh Triplett wrote:
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On Mon, Jul 01, 2013 at 08:52:20AM -0700, Paul E. McKenney wrote:
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> On Mon, Jul 01, 2013 at 08:31:50AM -0700, Josh Triplett wrote:
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> > On Fri, Jun 28, 2013 at 01:10:17PM -0700, Paul E. McKenney wrote:
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> > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
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> > >
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> > > This commit adds fields to the rcu_dyntick structure that are used to
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> > > detect idle CPUs. These new fields differ from the existing ones in
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> > > that the existing ones consider a CPU executing in user mode to be idle,
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> > > where the new ones consider CPUs executing in user mode to be busy.
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> >
>
> > Can you explain, both in the commit messages and in the comments added
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> > by the next commit, *why* this code doesn't consider userspace a
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> > quiescent state?
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>
>
> Good point! Does the following explain it?
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>
>
> Although one of RCU's quiescent states is usermode execution,
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> it is not a full-system idle state. This is because the purpose
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> of the full-system idle state is not RCU, but rather determining
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> when accurate timekeeping can safely be disabled. Whenever
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> accurate timekeeping is required in a CONFIG_NO_HZ_FULL kernel,
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> at least one CPU must keep the scheduling-clock tick going.
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> If even one CPU is executing in user mode, accurate timekeeping
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> is requires, particularly for architectures where gettimeofday()
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> and friends do not enter the kernel. Only when all CPUs are
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> really and truly idle can accurate timekeeping be disabled,
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> allowing all CPUs to turn off the scheduling clock interrupt,
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> thus greatly improving energy efficiency.
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>
>
> This naturally raises the question "Why is this code in RCU rather
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> than in timekeeping?", and the answer is that RCU has the data
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> and infrastructure to efficiently make this determination.
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>
Good explanation, thanks.
>
>
This also naturally raises the question "How can we let userspace get
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accurate time without forcing a timer tick?".
We don't. ;-)
Without CONFIG_NO_HZ_FULL, if a CPU is running in user mode, that CPU
takes scheduling-clock interrupts. User-mode code will therefore always
see accurate time. For some definition of "accurate", anyway.
With CONFIG_NO_HZ_FULL and without CONFIG_NO_HZ_FULL_SYSIDLE, a single
designated CPU will always be taking scheduling-clock interrupts, which
again ensures that user-mode code will always see accurate time.
With both CONFIG_NO_HZ_FULL and CONFIG_NO_HZ_FULL_SYSIDLE, if
any CPU other than the timekeeping CPU is nonidle (where "nonidle"
includes usermode execution), then the timekeeping CPU will be taking
scheduling-clock interrupts, yet again ensuring that user-mode code will
always see accurate time. If all CPUs are idle (in other words, we are
in RCU_SYSIDLE_FULL_NOTED state and the timekeeping CPU is also idle),
scheduling-clock interrupts will be globally disabled. Or will be,
once I fix the bug noted by Frederic.
I am guessing that you would like this added to the explanation? ;-)
Thanx, Paul
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