From: Michael Ellerman <mpe@ellerman•id.au>
To: Christophe Leroy <christophe.leroy@csgroup•eu>,
Kajol Jain <kjain@linux•ibm.com>,
linuxppc-dev@lists•ozlabs.org
Cc: 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: Sat, 14 Aug 2021 22:44:05 +1000 [thread overview]
Message-ID: <871r6wmc16.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <c6110aa1-90e2-77aa-1ab5-355975037227@csgroup.eu>
Christophe Leroy <christophe.leroy@csgroup•eu> writes:
> Le 13/08/2021 à 10:24, Kajol Jain a écrit :
>> Incase of random sampling, there can be scenarios where SIAR is not
>> latching sample address and results in 0 value. Since current code
>> directly returning the siar value, we could see multiple instruction
>> pointer values as 0 in perf report.
Can you please give more detail on that? What scenarios? On what CPUs?
>> Patch resolves this issue by adding a ternary condition to return
>> regs->nip incase SIAR is 0.
>
> Your description seems rather similar to
> https://github.com/linuxppc/linux/commit/2ca13a4cc56c920a6c9fc8ee45d02bccacd7f46c
>
> Does it mean that the problem occurs on more than the power10 DD1 ?
>
> In that case, can the solution be common instead of doing something for power10 DD1 and something
> for others ?
Agreed.
This change would seem to make that P10 DD1 logic superfluous.
Also we already have a fallback to regs->nip in the else case of the if,
so we should just use that rather than adding a ternary condition.
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.
cheers
next prev parent reply other threads:[~2021-08-14 12:44 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 [this message]
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
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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