Re: [PATCH] kvm: Drop obsolete cpu_get/put in make_all_cpus_request

[Jan Kiszka <jan.kiszka@web.de>]

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Marcelo Tosatti wrote:
> On Tue, Jul 21, 2009 at 10:24:08AM +0200, Jan Kiszka wrote:
>> Marcelo Tosatti wrote:
>>> Jan,
>>>
>>> This was suggested but we thought it might be safer to keep the
>>> get_cpu/put_cpu pair in case -rt kernels require it (which might be
>>> bullshit, but nobody verified).
>> -rt stumbles over both patterns (that's why I stumbled over it in the
>> first place: get_cpu disables preemption, but spin_lock is a sleeping
>> lock under -rt) and actually requires requests_lock to become
>> raw_spinlock_t. Reordering get_cpu and spin_lock would be another
>> option, but not really a gain for both scenarios.
> 
> I see.
> 
>> So unless there is a way to make the whole critical section preemptible
>> (thus migration-agnostic), I think we can micro-optimize it like this.
> 
> Can't you switch requests_lock to be raw_spinlock_t then? (or whatever
> is necessary to make it -rt compatible).
> 

raw_spinlock_t over -rt is not comparable to raw_spinlock_t over
mainline. So I'm currently carrying a local patch with

#ifdef CONFIG_PREEMPT_RT
	raw_spinlock_t some_lock;
#else
	spinlock_t some_lock;
#endif

for all locks that need it (there are three ATM).

That said, I'm suspecting there are more problems with kvm over -rt
right now. I'm seeing significant latency peeks on the host. Still
investigating, though.

However I don't think we should bother too much about -rt compliance in
mainline unless the diff is trivial and basically irrelevant for the
common non-rt cases.

Jan


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