Re: [PATCH V5 6/7] arm64/perf: Add BRBE driver

[James Clark]

On 30/11/2022 04:49, Anshuman Khandual wrote:
> 
> 
> On 11/29/22 21:23, James Clark wrote:
>>
>>
>> On 07/11/2022 06:25, Anshuman Khandual wrote:
>>> This adds a BRBE driver which implements all the required helper functions
>>> for struct arm_pmu. Following functions are defined by this driver which
>>> will configure, enable, capture, reset and disable BRBE buffer HW as and
>>> when requested via perf branch stack sampling framework.
>>
>> Hi Anshuman,
>>
>> I've got a rough version of an updated test for branch stacks here [1].
>> A couple of interesting things that I've noticed running it:
>>
>> First one is that sometimes I get (null) for the branch type. Debugging
>> in GDB shows me that the type is actually type == PERF_BR_EXTEND_ABI &&
>> new_type == 11. I can't see how this is possible looking at the driver
> 
> Hmm, that is strange.
> 
> brbe_fetch_perf_type() evaluates captured brbinf and extracts BRBE branch
> type and later maps into perf branch types. All new perf branch types are
> contained inside [PERF_BR_NEW_FAULT_ALGN = 0 .. PERF_BR_NEW_ARCH_5 = 7].
> Hence wondering how '11' can be a new_type value after PERF_BR_EXTEND_ABI
> switch.

I got to the bottom of the issue and posted the fix here [2]. A new
entry was added to the branch records somewhere around the time new_type
was added and it wasn't added to Perf so the records weren't being
interpreted properly.

> 
>> code. I think I saw this on a previous version of the patchset too but
>> didn't mention it because I thought it wasn't significant, but now I see
>> that something strange is going on. An interesting pattern is that they
>> are always after ERET samples and go from userspace to kernel:
> 
> Unless it can be ascertained that wrong values are getting passed into the
> perf ring buffer via cpuc->branches->brbe_entries[idx].[type | new_type],
> the problem might be with perf report parsing the branch records ?
> 
> There are valid new branch types such as ARM64_DEBUG_DATA reported after
> ERET records as well. I guess the only way to figure out the problem here
> is to track the errant branch record from cpuc->branches->brbe_entries to
> all the way upto perf report processing.
> 
>>
>> 41992866945460 0x6e8 [0x360]: PERF_RECORD_SAMPLE(IP, 0x1): 501/501:
>> 0xffff800008010118 period: 1229 addr: 0
>> ... branch stack: nr:34
>> .. 007a9988 -> 00000000 0 cycles  P   9fbfbfbf IRQ
>> .. 00000000 -> 007a9988 0 cycles  P   9fbfbfbf ERET
>> .. 007a9988 -> 00000000 0 cycles  P   9fbfbfbf (null)
>> .. 00747668 -> 007a9988 0 cycles  P   9fbfbfbf CALL
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00747664 -> 00747660 0 cycles  P   9fbfbfbf COND
>> .. 00000000 -> 00747658 0 cycles  P   9fbfbfbf ERET
>> .. 00747658 -> 00000000 0 cycles  P   9fbfbfbf ARM64_DEBUG_DATA
>> .. 00000000 -> 00747650 0 cycles  P   9fbfbfbf ERET
>> .. 00747650 -> 00000000 0 cycles  P   9fbfbfbf ARM64_DEBUG_DATA
>> .. 00747624 -> 00747634 0 cycles  P   9fbfbfbf COND
>> .. 00000000 -> 007475f4 0 cycles  P   9fbfbfbf ERET
>> .. 007475f4 -> 00000000 0 cycles  P   9fbfbfbf ARM64_DEBUG_DATA
>> .. 00000000 -> 007475e8 0 cycles  P   9fbfbfbf ERET
>> .. 007475e8 -> 00000000 0 cycles  P   9fbfbfbf (null)
>> .. 004005ac -> 007475e8 0 cycles  P   9fbfbfbf CALL
>> .. 00000000 -> 00400564 0 cycles  P   9fbfbfbf ERET
>> .. 00400564 -> 00000000 0 cycles  P   9fbfbfbf (null)
>> .. 00000000 -> 00400564 0 cycles  P   9fbfbfbf ERET
>>  .. thread: perf:501
>>  ...... dso: [kernel.kallsyms]
>>
>> The second one is that sometimes I get kernel addresses and RET branches
>> even if the option is any_call,u. The pattern here is that it's the last
>> non empty branch stack of a run, so maybe there is some disable path
>> where the filters aren't configured properly:
> 
> The latest code (not posted), disables TRBE completely while reading the
> branch records during PMU interrupt. Could you please apply those changes
> as well, or rather just use the branch instead.
> 
> https://gitlab.arm.com/linux-arm/linux-anshuman/-/commit/ab17879711f0e61c280ed52400ccde172b67e04a
> 

I don't think I've seen it on that version, but I need to run it a bit
more to be sure.

> 
>>
>>
>> [1]:
>> https://gitlab.arm.com/linux-arm/linux-jc/-/commit/7260b7bef06ac161eac88d05266e8c5c303d9881

[2]:
https://lore.kernel.org/linux-perf-users/20221130165158.517385-1-james.clark@xxxxxxx/T/#u