On 8/16/21 12:26 PM, Christophe Leroy wrote:
Le 16/08/2021 à 08:44, kajoljain a écrit :
On 8/14/21 6:14 PM, Michael Ellerman wrote:
Christophe Leroy <christophe.le...@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?
Hi Michael,
Sure I will update these details in my next patch-set.
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
