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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Message-ID: <4C1F5452.3000107@redhat.com>
Date:	Mon, 21 Jun 2010 15:00:18 +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 06/21/2010 12:31 PM, Zhang, Yanmin wrote:
> The 2nd patch is to change the definition of perf_event to facilitate
> perf attr copy when a hypercall happens.
>
> Signed-off-by: Zhang Yanmin<yanmin_zhang@...ux.intel.com>
>
> ---
>
> --- linux-2.6_tip0620/include/linux/perf_event.h	2010-06-21 15:19:52.821999849 +0800
> +++ linux-2.6_tip0620perfkvm/include/linux/perf_event.h	2010-06-21 16:53:49.283999849 +0800
> @@ -188,7 +188,10 @@ struct perf_event_attr {
>   	__u64			sample_type;
>   	__u64			read_format;
>
>    

Assuming these flags are available to the guest?

> -	__u64			disabled       :  1, /* off by default        */
> +	union {
> +		__u64		flags;
> +		struct {
> +			__u64	disabled       :  1, /* off by default        */
>   				inherit	       :  1, /* children inherit it   */
>    

inherit is meaningless for a guest.

>   				pinned	       :  1, /* must always be on PMU */
>    

We cannot allow a guest to pin a counter.

The other flags are also problematic.  I'd like to see virt-specific 
flags (probably we'll only need kernel/user and nested_hv for nested 
virtualization).

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?

> +	/*
> +	 * 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.

> @@ -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.  This would reduce the number of exits.  Also adjust the 
frequency less frequently.

> +
> +	if (adjust_freq) {
> +		perf_ctx_adjust_freq(&cpuctx->ctx);
> +		if (ctx)
> +			perf_ctx_adjust_freq(ctx);
> +	}
>
>    

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

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