Re: [PATCH v2 2/2] powerpc/perf: Return regs->nip as instruction pointer value when SIAR is 0

On 8/16/21 12:26 PM, Christophe Leroy wrote:

Le 16/08/2021 à 08:44, kajoljain a écrit :

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On 8/14/21 6:14 PM, Michael Ellerman wrote:

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Christophe Leroy [off-list ref] writes:

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Le 13/08/2021 à 10:24, Kajol Jain a écrit :

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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?

Hi Michael,
     Sure I will update these details in my next patch-set.

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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.

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 ?

pc having 0 is wrong (kernel does not execute at 0x0 or userspace).
yeah we should drop it.

Maddy

Christophe