linux-kernel - Re: [patch 0/4] Port KVM-trace to tracepoints

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Date:	Tue, 22 Jul 2008 22:31:51 +0300
From:	Avi Kivity <avi@...ranet.com>
To:	"Frank Ch. Eigler" <fche@...hat.com>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>, kvm@...r.kernel.org
Subject: Re: [patch 0/4] Port KVM-trace to tracepoints

Frank Ch. Eigler wrote:
> Avi Kivity <avi@...ranet.com> writes:
>
>   
>> [...]
>> kvm tracepoints are heavily tied into the implementation; and making
>> them harder to write means we will have less information.  In fact, I
>> am contemplating moving in another direction (when looking at the
>> pgprintk()s scattered around arch/x86/kvm/mmu.c:
>>
>>   kvm_trace("pfentry", "page_fault entry addr %lx error code %x\n",
>> cr2, error_code);
>>
>> Unlike printk()s, no actual formatting would occur during runtime.
>>     
>
> Have you considered using trace_mark() directly - eliminating the
> KVM_TRACEN() middlemen?
>
>   

Eliminating KVM_TRACEN -- yes.  There are too many of them, they aren't 
type-aware, and they're in uppercase.

Using trace_mark() directly -- looking at it, seems to fit the 
requirements exactly.  Should have looked at it earlier.  Is there a way 
to get a list of all markers?

Perhaps the kvmtrace marker->relay integration should be made a marker 
feature, since there is nothing specific to kvm in it.

>> Instead, at initialization time all the strings would be parsed into
>> a data structure that describes the data types, and the runtime
>> would simply consult this structure and copy the arguments into
>> trace records.  User space would also be able to pull this structure
>> and so recreate the formatted string.
>>     
>
> If one really wanted to, one could build such a mechanism on top of
> marker-based callbacks.
>
>   

One does want to.

>>  - no need to have a formats file in userspace (which is tied to the
>> kernel version)
>>     
>
> OTOH, you'd have the kernel collecting compact binary records
> containing just the parameters, which are at least as tied to kernel
> version.
>
>   

Yes, but the userspace side would collect the format strings as well 
(just once) and could put them in the same file.  The aggregation is 
portable across kernel versions.

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

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