linux-kernel - Re: [PATCH V2 2/5] ara virt interface of perf to support kvm guest os statistics collection in guest os

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Date:	Tue, 22 Jun 2010 12:12:03 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
CC:	LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Lin Ming <ming.m.lin@...el.com>,
	Sheng Yang <sheng@...ux.intel.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	oerg Roedel <joro@...tes.org>,
	Jes Sorensen <Jes.Sorensen@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>, zhiteng.huang@...el.com,
	tim.c.chen@...el.com
Subject: Re: [PATCH V2 2/5] ara virt interface of perf to support kvm guest
 os statistics collection in guest os

On 06/22/2010 05:08 AM, Zhang, Yanmin wrote:
>
>> Something that is worrying is that we don't expose group information.
>> perf will multiplex the events for us, but there will be a loss in accuracy.
>>
>>      
>>>    #ifdef CONFIG_HAVE_HW_BREAKPOINT
>>>    #include<asm/hw_breakpoint.h>
>>>    #endif
>>> @@ -753,6 +752,20 @@ struct perf_event {
>>>
>>>    	perf_overflow_handler_t		overflow_handler;
>>>
>>> +	/*
>>> +	 * pointers used by kvm perf paravirt interface.
>>> +	 *
>>> +	 * 1) Used in host kernel and points to host_perf_shadow which
>>> +	 * has information about guest perf_event
>>> +	 */
>>> +	void				*host_perf_shadow;
>>>
>>>        
>> Can we have real types instead of void pointers?
>>      
> I just want perf generic codes have less dependency on KVM codes.
>    

One way to do that and retain type safety is to have

     struct perf_client {
           struct perf_client_ops *ops;
           ...
     }

The client (kvm) can do

    struct kvm_perf_client {
          struct perf_client pc;
          // kvm specific stuff
    };

the callbacks receive struct perf_client and use container_of to reach 
the kvm_perf_client that contains it.

>>> +	/*
>>> +	 * 2) Used in guest kernel and points to guest_perf_shadow which
>>> +	 * is used as a communication area with host kernel. Host kernel
>>> +	 * copies overflow data to it when an event overflows.
>>> +	 */
>>> +	void				*guest_perf_shadow;
>>>
>>>        
>> It's strange to see both guest and host parts in the same patch.
>> Splitting to separate patches will really help review.
>>      
> It's a little hard to split the patches if they change the same file. Perhaps
> I could add more statements before the patch when I send it out.
>    

With git, it's easy (once you're used to it):

   # go back one commit:
   git reset HEAD^
   # selectively add bits:
   git add -p
   # commit first patch
   git commit -s
   # selectively add bits:
   git add -p
   # commit second patch
   git commit -s


>>> @@ -1626,9 +1629,22 @@ void perf_event_task_tick(struct task_st
>>>    	if (ctx&&   ctx->nr_events&&   ctx->nr_events != ctx->nr_active)
>>>    		rotate = 1;
>>>
>>> -	perf_ctx_adjust_freq(&cpuctx->ctx);
>>> -	if (ctx)
>>> -		perf_ctx_adjust_freq(ctx);
>>> +#ifdef CONFIG_KVM_PERF
>>> +	if (kvm_para_available()) {
>>> +		/*
>>> +		 * perf_ctx_adjust_freq causes lots of pmu->read which would
>>> +		 * trigger too many vmexit to host kernel. We disable it
>>> +		 * under para virt situation
>>> +		 */
>>> +		adjust_freq = 0;
>>> +	}
>>> +#endif
>>>
>>>        
>> Perhaps we can have a batch read interface which will read many counters
>> at once.
>>      
> It's a good idea. But that will touch many perf generic codes which causes it's hard
> to maintain or follow future changes.
>    

I'm talking about the guest/host interface.  So you have one vmexit and 
many host perf calls.

-- 
error compiling committee.c: too many arguments to function

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