[Xen-devel] Re: [PATCH] CPUIDLE: shorten hpet spin_lock holding time

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 20/04/2010 17:05, "Wei, Gang" <gang.wei@xxxxxxxxx> wrote:

> Resend.
> 
> CPUIDLE: shorten hpet spin_lock holding time
> 
> Try to reduce spin_lock overhead for deep C state entry/exit. This will
> benefit systems with a lot of cpus which need the hpet broadcast to wakeup
> from deep C state.
> 
> Signed-off-by: Wei Gang <gang.wei@xxxxxxxxx>

It fixes the unsafe accesses to timer_deadline_{start,end} but I still think
this optimisation is misguided and also unsafe. There is nothing to stop new
CPUs being added to ch->cpumask after you start scanning ch->cpumask. For
example, some new CPU which has a timer_deadline_end greater than
ch->next_event, so it does not reprogram the HPET. But handle_hpet_broadcast
is already mid-scan and misses this new CPU, so it does not reprogram the
HPET either. Hence no timer fires for the new CPU and it misses its
deadline.

Really I think a better approach than something like this patch would be to
better advertise the timer_slop=xxx Xen boot parameter for power-saving
scenarios. I wonder what your numbers look like if you re-run your benchmark
with (say) timer_slop=10000000 (i.e., 10ms slop) on the Xen command line?

 -- Keir




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