Re: [PATCH 07/15] ftrace: fix event alignment: kvm:kvm_hv_hypercall

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From: David Sharp

Subject: Re: [PATCH 07/15] ftrace: fix event alignment: kvm:kvm_hv_hypercall

Date: Tuesday, December 7, 2010 - 2:16 pm

On Tue, Dec 7, 2010 at 1:22 AM, Avi Kivity <avi@redhat.com> wrote:
quoted text> On 12/06/2010 10:38 PM, David Sharp wrote:
>> On Sat, Dec 4, 2010 at 12:11 AM, Avi Kivity<avi@redhat.com>  wrote:
>> >  On 12/04/2010 02:13 AM, David Sharp wrote:
>> >>
>> >>  Signed-off-by: David Sharp<dhsharp@google.com>
>> >>  ---
>> >>    arch/x86/kvm/trace.h |    8 ++++----
>> >>    1 files changed, 4 insertions(+), 4 deletions(-)
>> >>
>> >>  diff --git a/arch/x86/kvm/trace.h b/arch/x86/kvm/trace.h
>> >>  index a6544b8..ab41fb0 100644
>> >>  --- a/arch/x86/kvm/trace.h
>> >>  +++ b/arch/x86/kvm/trace.h
>> >>  @@ -62,21 +62,21 @@ TRACE_EVENT(kvm_hv_hypercall,
>> >>          TP_ARGS(code, fast, rep_cnt, rep_idx, ingpa, outgpa),
>> >>
>> >>          TP_STRUCT__entry(
>> >>  -               __field(        __u16,          code            )
>> >>  -               __field(        bool,           fast            )
>> >>                  __field(        __u16,          rep_cnt         )
>> >>                  __field(        __u16,          rep_idx         )
>> >>                  __field(        __u64,          ingpa           )
>> >>                  __field(        __u64,          outgpa          )
>> >>  +               __field(        __u16,          code            )
>> >>  +               __field(        bool,           fast            )
>> >>          ),
>> >>
>> >
>> >  Looks like a pessimisation.
>> >
>> >  Before: 24 bytes
>> >  After: 32 bytes
>> >
>> >  (on a 64-bit machine, assuming no packing)
>>
>> This patch is predicated on packing the event structures. And since
>> the ring buffer is 32-bit addressable, I don't attempt to improve
>> alignment beyond 32-bit boundaries.
>
> I don't understand this.  Can you elaborate?  What does "32-bit addressable"
> mean?

The ring buffer gives you space that is a multiple of 4 bytes in
length, and 32-bit aligned. Therefore it is useless to attempt to
align the structure beyond 32-bit boundaries, eg, a 64-bit boundary,
because it is unpredictable if the memory the structure will be
written to is at a 64-bit boundary (addr % 8 could be 0 or 4).

quoted text> And "predicated on packing the event structures"?  Is the structure
> __attribute__((packed)), or is it not?

It is not packed in Linus' tree, but one of the patches before this
patch in this patch series adds __attribute__((packed)). This patch
assumes that the event packing patch has been applied. This patch
should not be applied if the packing patch is not (hence,
"predicated").

quoted text>
> --
> error compiling committee.c: too many arguments to function
>
>
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Messages in current thread:

[Patch 00/15] Reduce tracing payload size., David Sharp, (Fri Dec 3, 5:13 pm)

[PATCH 01/15] tracing: Add a 'buffer_overwrite' debugfs file, David Sharp, (Fri Dec 3, 5:13 pm)

[PATCH 02/15] ring_buffer.c: Remove unused #include <linux ..., David Sharp, (Fri Dec 3, 5:13 pm)

[PATCH 03/15] ring_buffer: Align buffer_page struct alloca ..., David Sharp, (Fri Dec 3, 5:13 pm)

[PATCH 04/15] ftrace: pack event structures., David Sharp, (Fri Dec 3, 5:13 pm)

[PATCH 05/15] ftrace: fix event alignment: ftrace:context_ ..., David Sharp, (Fri Dec 3, 5:13 pm)

[PATCH 06/15] ftrace: fix event alignment: module:module_r ..., David Sharp, (Fri Dec 3, 5:13 pm)

[PATCH 07/15] ftrace: fix event alignment: kvm:kvm_hv_hype ..., David Sharp, (Fri Dec 3, 5:13 pm)

[PATCH 08/15] ftrace: fix event alignment: mce:mce_record, David Sharp, (Fri Dec 3, 5:13 pm)

[PATCH 09/15] ftrace: fix event alignment: skb:kfree_skb, David Sharp, (Fri Dec 3, 5:13 pm)

[PATCH 10/15] ftrace: fix event alignment: jbd2:*, David Sharp, (Fri Dec 3, 5:13 pm)

[PATCH 11/15] ftrace: fix event alignment: ext4:*, David Sharp, (Fri Dec 3, 5:13 pm)

[PATCH 12/15] trace_output.c: adjust conditional expressio ..., David Sharp, (Fri Dec 3, 5:13 pm)

[PATCH 13/15] small_traces: Add config option to shrink tr ..., David Sharp, (Fri Dec 3, 5:13 pm)

[PATCH 14/15] small_traces: Remove trace output of large f ..., David Sharp, (Fri Dec 3, 5:13 pm)

[PATCH 15/15] small_traces: Remove 8 bytes from trace_entry., David Sharp, (Fri Dec 3, 5:13 pm)

Re: [PATCH 03/15] ring_buffer: Align buffer_page struct al ..., Steven Rostedt, (Fri Dec 3, 6:43 pm)

Re: [PATCH 06/15] ftrace: fix event alignment: module:modu ..., Steven Rostedt, (Fri Dec 3, 6:47 pm)

Re: [PATCH 07/15] ftrace: fix event alignment: kvm:kvm_hv_ ..., Steven Rostedt, (Fri Dec 3, 6:49 pm)

Re: [PATCH 08/15] ftrace: fix event alignment: mce:mce_record, Steven Rostedt, (Fri Dec 3, 6:50 pm)

Re: [PATCH 09/15] ftrace: fix event alignment: skb:kfree_skb, Steven Rostedt, (Fri Dec 3, 6:52 pm)

Re: [PATCH 10/15] ftrace: fix event alignment: jbd2:*, Steven Rostedt, (Fri Dec 3, 6:52 pm)

Re: [PATCH 11/15] ftrace: fix event alignment: ext4:*, Steven Rostedt, (Fri Dec 3, 6:53 pm)

Re: [PATCH 13/15] small_traces: Add config option to shrin ..., Steven Rostedt, (Fri Dec 3, 6:56 pm)

Re: [PATCH 01/15] tracing: Add a 'buffer_overwrite' debugf ..., Steven Rostedt, (Fri Dec 3, 6:57 pm)

Re: [PATCH 13/15] small_traces: Add config option to shrin ..., David Sharp, (Fri Dec 3, 7:33 pm)

Re: [PATCH 13/15] small_traces: Add config option to shrin ..., Steven Rostedt, (Fri Dec 3, 7:54 pm)

Re: [PATCH 07/15] ftrace: fix event alignment: kvm:kvm_hv_ ..., Avi Kivity, (Sat Dec 4, 1:11 am)

Re: [PATCH 09/15] ftrace: fix event alignment: skb:kfree_skb, Neil Horman, (Sat Dec 4, 6:38 am)

Re: [PATCH 06/15] ftrace: fix event alignment: module:modu ..., Li Zefan, (Sun Dec 5, 6:28 pm)

Re: [Patch 00/15] Reduce tracing payload size., Andi Kleen, (Mon Dec 6, 6:22 am)

Re: [Patch 00/15] Reduce tracing payload size., Ted Ts'o, (Mon Dec 6, 6:56 am)

Re: [Patch 00/15] Reduce tracing payload size., Andi Kleen, (Mon Dec 6, 7:58 am)

Re: [Patch 00/15] Reduce tracing payload size., Steven Rostedt, (Mon Dec 6, 9:17 am)

Re: [Patch 00/15] Reduce tracing payload size., Miguel Ojeda, (Mon Dec 6, 9:31 am)

Re: [Patch 00/15] Reduce tracing payload size., Andi Kleen, (Mon Dec 6, 9:41 am)

Re: [PATCH 07/15] ftrace: fix event alignment: kvm:kvm_hv_ ..., David Sharp, (Mon Dec 6, 1:38 pm)

Re: [PATCH 07/15] ftrace: fix event alignment: kvm:kvm_hv_ ..., Avi Kivity, (Tue Dec 7, 2:22 am)

Re: [PATCH 07/15] ftrace: fix event alignment: kvm:kvm_hv_ ..., David Sharp, (Tue Dec 7, 2:16 pm)

Re: [PATCH 03/15] ring_buffer: Align buffer_page struct al ..., David Sharp, (Tue Dec 7, 3:44 pm)

Re: [PATCH 07/15] ftrace: fix event alignment: kvm:kvm_hv_ ..., Avi Kivity, (Wed Dec 8, 2:18 am)

Re: [PATCH 01/15] tracing: Add a 'buffer_overwrite' debugf ..., David Sharp, (Wed Dec 8, 1:15 pm)

[PATCH] tracing: Add an 'overwrite' trace_option., David Sharp, (Wed Dec 8, 2:46 pm)

Re: [PATCH 08/15] ftrace: fix event alignment: mce:mce_record, Frederic Weisbecker, (Thu Dec 9, 6:33 am)

Re: [PATCH 13/15] small_traces: Add config option to shrin ..., Frederic Weisbecker, (Thu Dec 9, 7:55 am)

Re: [PATCH 13/15] small_traces: Add config option to shrin ..., Steven Rostedt, (Thu Dec 9, 8:08 am)

Re: [PATCH 13/15] small_traces: Add config option to shrin ..., Frederic Weisbecker, (Thu Dec 9, 8:28 am)

Re: [PATCH 13/15] small_traces: Add config option to shrin ..., Mathieu Desnoyers, (Thu Dec 9, 9:16 am)

Re: [PATCH] tracing: Add an 'overwrite' trace_option., David Sharp, (Mon Dec 13, 5:39 pm)