lkml.org 
[lkml]   [2011]   [Aug]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH -tip v3 2/5] perf: change perf_event_header.misc to PERF_RECORD_MISC_USER for BTS
(2011/08/11 21:18), Peter Zijlstra wrote:
> On Thu, 2011-08-11 at 21:06 +0900, Akihiro Nagai wrote:
>> Change perf_event_headder.misc to PERF_RECORD_MISC_USER for
>> BTS records, because BTS traces both kernel and user spaces
>> nevertheless perf specifies to trace only kernel or user space.
>
> Now I'm confused..
>
> If BTS traces both kernel and user, the MISC bit should reflect the
> right state per-sample, on x86 that's easy enough to do by the address.
Yes.
However, PERF_RECORD_MISC_KERNEL can be specified only when
both from_addr and to_addr are kernel-space. Since current perf always enables
IA32_DEBUGCTL_MSR.BTS_OFF_OS flag when it uses BTS, such BTS records are not
output. So, it's enough to specify only PERF_RECORD_MISC_USER.

>
>
>> ---
>>
>> arch/x86/kernel/cpu/perf_event_intel_ds.c | 8 ++++++++
>> 1 files changed, 8 insertions(+), 0 deletions(-)
>>
>> diff --git a/arch/x86/kernel/cpu/perf_event_intel_ds.c b/arch/x86/kernel/cpu/perf_event_intel_ds.c
>> index 1b1ef3a..323f3f0 100644
>> --- a/arch/x86/kernel/cpu/perf_event_intel_ds.c
>> +++ b/arch/x86/kernel/cpu/perf_event_intel_ds.c
>> @@ -340,6 +340,14 @@ static int intel_pmu_drain_bts_buffer(void)
>> */
>> perf_prepare_sample(&header,&data, event,&regs);
>>
>> + /*
>> + * Since BTS can not trace kernel and user space separately, set
>
> Uhm, IA32_DEBUGCTL_MSR.BTS_OFF_{OS,USR} seem to suggest it can?!
Current perf always enables IA32_DEBUGCTL_MSR.BTS_OFF_OS when traces with BTS.
However, BTS records branches which jump from kernel(irq_return) to user, because
the msr is to stop tracing in Ring0. It's different with kernel-space in the
strict sense.

>
>> + * PERF_RECORD_MISC_USER in header.misc to resolve both kernel and
>> + * user DSOs and symbols.
>> + */
>> + header.misc&= ~PERF_RECORD_MISC_CPUMODE_MASK;
>> + header.misc |= PERF_RECORD_MISC_USER;
>
>
> So what's wrong with something like:
>
> header.misc |= is_kernel_address(at->from) ?
> PERF_RECORD_MISC_KERNEL :
> PERF_RECORD_MISC_USER;
>
It looks good.
However, current perf doesn't output "kernel to kernel" BTS records.
So, it is unnecessary yet.

Thank you.

>> if (perf_output_begin(&handle, event, header.size * (top - at)))
>> return 1;
>>
>>
>
>



\
 
 \ /
  Last update: 2011-08-17 04:23    [W:0.276 / U:0.144 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site