From: Michael Ellerman <mpe@ellerman•id.au>
To: Christophe Leroy <christophe.leroy@csgroup•eu>,
kajoljain <kjain@linux•ibm.com>,
linuxppc-dev@lists•ozlabs.org
Cc: Sukadev Bhattiprolu <sukadev@linux•vnet.ibm.com>,
atrajeev@linux•vnet.ibm.com, maddy@linux•ibm.com,
rnsastry@linux•ibm.com
Subject: Re: [PATCH v2 2/2] powerpc/perf: Return regs->nip as instruction pointer value when SIAR is 0
Date: Tue, 17 Aug 2021 22:49:22 +1000 [thread overview]
Message-ID: <871r6sgrsd.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <3a34c79d-b800-1a11-7a4b-1fb3babb9df1@csgroup.eu>
Christophe Leroy <christophe.leroy@csgroup•eu> writes:
> Le 16/08/2021 à 08:44, kajoljain a écrit :
>> On 8/14/21 6:14 PM, Michael Ellerman wrote:
...
>>>
>>> eg.
>>>
>>> if (use_siar && siar_valid(regs) && siar)
>>> return siar + perf_ip_adjust(regs);
>>> else if (use_siar)
>>> return 0; // no valid instruction pointer
>>> else
>>> return regs->nip;
>>>
>>>
>>> I'm also not sure why we have that return 0 case, I can't think of why
>>> we'd ever want to do that rather than using nip. So maybe we should do
>>> another patch to drop that case.
>>
>> Yeah make sense. I will remove return 0 case in my next version.
>
> This was added by commit
> https://github.com/linuxppc/linux/commit/e6878835ac4794f25385522d29c634b7bbb7cca9
>
> Are we sure it was an error to add it and it can be removed ?
I think so.
That commit added siar_valid(), and updated record_and_restart() to only
record if siar_valid() returned true.
- record = 1;
+ record = siar_valid(regs);
It then also changed perf_instruction_pointer():
- if (use_siar)
+ if (use_siar && siar_valid(regs))
return mfspr(SPRN_SIAR) + perf_ip_adjust(regs);
+ else if (use_siar)
+ return 0; // no valid instruction pointer
else
return regs->nip;
The first change means we would never even call
perf_instruction_pointer() if siar_valid() is false, so we could never
hit the use_siar && !siar_valid() case.
But even so it's always preferable to use regs->nip than 0, even if nip
is somewhat skewed due to interrupts being disabled etc.
cheers
next prev parent reply other threads:[~2021-08-17 12:50 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-13 8:24 [PATCH v2 1/2] powerpc/perf: Use stack siar instead of mfspr Kajol Jain
2021-08-13 8:24 ` [PATCH v2 2/2] powerpc/perf: Return regs->nip as instruction pointer value when SIAR is 0 Kajol Jain
2021-08-13 9:29 ` Christophe Leroy
2021-08-14 12:25 ` Michael Ellerman
2021-08-13 9:34 ` Christophe Leroy
2021-08-14 12:44 ` Michael Ellerman
2021-08-16 6:44 ` kajoljain
2021-08-16 6:56 ` Christophe Leroy
2021-08-17 5:37 ` Madhavan Srinivasan
2021-08-17 8:29 ` kajoljain
2021-08-17 12:49 ` Michael Ellerman [this message]
2021-08-16 6:46 ` kajoljain
2021-08-13 8:29 ` [PATCH v2 1/2] powerpc/perf: Use stack siar instead of mfspr kajoljain
2021-08-13 9:23 ` Christophe Leroy
2021-08-14 12:30 ` Michael Ellerman
2021-08-16 5:58 ` kajoljain
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