Re: [PATCH RFC 3/4] Paravirtualized spinlock implementation for KVM guests

Jeremy Fitzhardinge
2010-08-03T17:47:19Z

On 08/02/2010 11:59 PM, Avi Kivity wrote:
> On 08/02/2010 06:20 PM, Jeremy Fitzhardinge wrote:
>> On 08/02/2010 01:48 AM, Avi Kivity wrote:
>>> On 07/26/2010 09:15 AM, Srivatsa Vaddagiri wrote:
>>>> Paravirtual spinlock implementation for KVM guests, based heavily
>>>> on Xen guest's
>>>> spinlock implementation.
>>>>
>>>>
>>>> +
>>>> +static struct spinlock_stats
>>>> +{
>>>> + u64 taken;
>>>> + u32 taken_slow;
>>>> +
>>>> + u64 released;
>>>> +
>>>> +#define HISTO_BUCKETS 30
>>>> + u32 histo_spin_total[HISTO_BUCKETS+1];
>>>> + u32 histo_spin_spinning[HISTO_BUCKETS+1];
>>>> + u32 histo_spin_blocked[HISTO_BUCKETS+1];
>>>> +
>>>> + u64 time_total;
>>>> + u64 time_spinning;
>>>> + u64 time_blocked;
>>>> +} spinlock_stats;
>>>
>>> Could these be replaced by tracepoints when starting to
>>> spin/stopping spinning etc? Then userspace can reconstruct the
>>> histogram as well as see which locks are involved and what call paths.
>>
>> Unfortunately not; the tracing code uses spinlocks.
>>
>> (TBH I haven't actually tried, but I did give the code an eyeball to
>> this end.)
>
> Hm. The tracing code already uses a specialized lock
> (arch_spinlock_t), perhaps we can make this lock avoid the tracing?

That's not really a specialized lock; that's just the naked
architecture-provided spinlock implementation, without all the lockdep,
etc, etc stuff layered on top. All these changes are at a lower level,
so giving tracing its own type of spinlock amounts to making the
architectures provide two complete spinlock implementations. We could
make tracing use, for example, an rwlock so long as we promise not to
put tracing in the rwlock implementation - but that's hardly elegant.

> It's really sad, btw, there's all those nice lockless ring buffers and
> then a spinlock for ftrace_vbprintk(), instead of a per-cpu buffer.

Sad indeed.

J