Re: [PATCH 1/3] dynticks - implement no idle hz for x86

[Srivatsa Vaddagiri]

On Sun, Sep 04, 2005 at 09:26:16PM +0100, Russell King wrote:
> I'd be really surprised if any architecture couldn't use what ARM has
> today - in other words, this is the only kernel-side interface:

Russel,
	I went thr' the ARM implementation and have some remarks (mostly
from a SMP perspective):

1. On a SMP platform, we want to let individual CPUs "sleep" independent of 
   each other. What this mean is there has to be some way of tracking which
   CPU's are sleeping currently, so that code like RCU ignores sleeping CPUs.
   This was the reason nohz_cpu_mask bitmap was added. I don't see that
   bitmap being updated at all in ARM implementation.

2. On architectures like x86 there is a separate jiffy interrupt source 
   (PIT) which is used to update time-of-day. This is different from the
   HZ timer interrupts used on each CPU (local apic timer). When all 
   CPUs are idle and sleeping, we want to shut off this PIT timer as well.
   That's why I added 'arch_all_cpus_idle' interface. One could argue that
   this can be done as part of the dyn_tick->reprogram interface as well,
   but I felt that having a separate arch_all_cpus_idle is cleaner and
   makes it clear what its purpose is.

3. The fact that we want to manipulate the bitmap (set a bit when CPU is going
   idle and unset it when it is waking up) _and_ the fact that want to take
   some action when all CPUs are idle or when the first CPU is waking up, 
   requires the use of a spinlock, which is again not present in the ARM 
   implementation.

4. Again the fact that CPUs could be sleeping independent of each other
   requires do_IRQ to check out whether the current CPU was sleeping as 
   its first step. If the CPU was sleeping, it needs to unset itself
   from the bitmap _and_ if we are coming out of "all-cpu-asleep" state,
   the PIT timer needs to be restarted as well as time recovered. Note
   that these two steps need not be undertaken if we were not in 
   "all-cpus-asleep" state.

I don't see provisions for all these in the current ARM implementation.
In fact the x86 patch that Tony/Con posted didnt take into account most of these
as well, which is the reason I jumped in to fix the above issues.

5. Don't see how DYN_TICK_SKIPPING is being used. In SMP scenario,
   it doesnt make sense since it will have to be per-cpu. The bitmap
   that I talked of exactly tells that (whether a CPU is skipping
   ticks or not).

6. S390 makes use of notifier mechanism to notify when CPUs are coming
   in and out of idle state. Don't know how it will be used in other
   arches. But obviously, if we are talking of unifying, we have to
   provide one.

I hope this makes clear why some of the rework happened, which
in a way is extending the interface that ARM already has. Having
said all these, I do agree that having a consistent interface 
is good (for example: x86 has dyn_tick_state structure whereas
ARM uses dyn_tick_timer strucuture itself to store the state etc).
   

-- 


Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017
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