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

Avi Kivity
2010-08-03T06:59:16Z

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?

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.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.