linux-kernel - Re: [PATCH] ARC: make sure instruction

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Date:	Mon, 13 Jul 2015 08:05:59 +0000
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>
CC:	"arc-linux-dev@...opsys.com" <arc-linux-dev@...opsys.com>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARC: make sure instruction_pointer() returns unsigned
 value

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@...opsys.com>

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

Applied !

Thx,
-Vineet
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