Re: [PATCH] ARC: make sure instruction_pointer() returns unsigned value

From: Vineet Gupta
Date: Mon Jul 13 2015 - 04:06:27 EST


On Monday 13 July 2015 12:55 PM, Alexey Brodkin wrote:
> Currently instruction_pointer() returns pt_regs->ret and so return value
> is of type "long", which implicitly stands for "signed long".
>
> While that's perfectly fine when dealing with 32-bit values if return
> value of instruction_pointer() gets assigned to 64-bit variable sign
> extension may happen.
>
> And at least in one real use-case it happens already.
> In perf_prepare_sample() return value of perf_instruction_pointer()
> (which is an alias to instruction_pointer() in case of ARC) is assigned
> to (struct perf_sample_data)->ip (which type is "u64").
>
> And what we see if instuction pointer points to user-space application
> that in case of ARC lays below 0x8000_0000 "ip" gets set properly with
> leading 32 zeros. But if instruction pointer points to kernel address
> space that starts from 0x8000_0000 then "ip" is set with 32 leadig
> "f"-s. I.e. id instruction_pointer() returns 0x8100_0000, "ip" will be
> assigned with 0xffff_ffff__8100_0000. Which is obviously wrong.
>
> In particular that issuse broke output of perf, because perf was unable
> to associate addresses like 0xffff_ffff__8100_0000 with anything from
> /proc/kallsyms.
>
> That's what we used to see:
> ----------->8----------
> 6.27% ls [unknown] [k] 0xffffffff8046c5cc
> 2.96% ls libuClibc-0.9.34-git.so [.] memcpy
> 2.25% ls libuClibc-0.9.34-git.so [.] memset
> 1.66% ls [unknown] [k] 0xffffffff80666536
> 1.54% ls libuClibc-0.9.34-git.so [.] 0x000224d6
> 1.18% ls libuClibc-0.9.34-git.so [.] 0x00022472
> ----------->8----------
>
> With that change perf output looks much better now:
> ----------->8----------
> 8.21% ls [kernel.kallsyms] [k] memset
> 3.52% ls libuClibc-0.9.34-git.so [.] memcpy
> 2.11% ls libuClibc-0.9.34-git.so [.] malloc
> 1.88% ls libuClibc-0.9.34-git.so [.] memset
> 1.64% ls [kernel.kallsyms] [k] _raw_spin_unlock_irqrestore
> 1.41% ls [kernel.kallsyms] [k] __d_lookup_rcu
> ----------->8----------
>
> Signed-off-by: Alexey Brodkin <abrodkin@xxxxxxxxxxxx>

Thx Alexey - this solves a long standing mystery with some weird perf profiles.

Applied !

Thx,
-Vineet
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