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

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Date:	Tue, 03 Aug 2010 09:59:16 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	vatsa@...ux.vnet.ibm.com, Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...hat.com>, linux-kernel@...r.kernel.org,
	npiggin@...e.de, kvm@...r.kernel.org, bharata@...ibm.com,
	Balbir Singh <balbir@...ibm.com>,
	Jan Beulich <JBeulich@...ell.com>
Subject: Re: [PATCH RFC 3/4] Paravirtualized spinlock implementation for KVM
 guests

  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.

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