Re: [RFC 00/48] perf tools: Introduce data type profiling (v1)

[Peter Zijlstra]

W00t!! Finally! :-)

On Wed, Oct 11, 2023 at 08:50:23PM -0700, Namhyung Kim wrote:

> * How to use it
> 
> To get precise memory access samples, users can use `perf mem record`
> command to utilize those events supported by their architecture.  Intel
> machines would work best as they have dedicated memory access events but
> they would have a filter to ignore low latency loads like less than 30
> cycles (use --ldlat option to change the default value).
> 
>     # To get memory access samples in kernel for 1 second (on Intel)
>     $ sudo perf mem record -a -K --ldlat=4 -- sleep 1

Fundamentally this should work with anything PEBS from MEM_ as
well, no? No real reason to rely on perf mem for this.

> In perf report, it's just a matter of selecting new sort keys: 'type'
> and 'typeoff'.  The 'type' shows name of the data type as a whole while
> 'typeoff' shows name of the field in the data type.  I found it useful
> to use it with --hierarchy option to group relevant entries in the same
> level.
> 
>     $ sudo perf report -s type,typeoff --hierarchy --stdio
>     ...
>     #
>     #    Overhead  Data Type / Data Type Offset
>     # ...........  ............................
>     #
>         23.95%     (stack operation)
>            23.95%     (stack operation) +0 (no field)
>         23.43%     (unknown)
>            23.43%     (unknown) +0 (no field)
>         10.30%     struct pcpu_hot
>             4.80%     struct pcpu_hot +0 (current_task)
>             3.53%     struct pcpu_hot +8 (preempt_count)
>             1.88%     struct pcpu_hot +12 (cpu_number)
>             0.07%     struct pcpu_hot +24 (top_of_stack)
>             0.01%     struct pcpu_hot +40 (softirq_pending)
>          4.25%     struct task_struct
>             1.48%     struct task_struct +2036 (rcu_read_lock_nesting)
>             0.53%     struct task_struct +2040 (rcu_read_unlock_special.b.blocked)
>             0.49%     struct task_struct +2936 (cred)
>             0.35%     struct task_struct +3144 (audit_context)
>             0.19%     struct task_struct +46 (flags)
>             0.17%     struct task_struct +972 (policy)
>             0.15%     struct task_struct +32 (stack)
>             0.15%     struct task_struct +8 (thread_info.syscall_work)
>             0.10%     struct task_struct +976 (nr_cpus_allowed)
>             0.09%     struct task_struct +2272 (mm)
>         ...
> 
> The (stack operation) and (unknown) have no type and field info.  FYI,
> the stack operations are samples in PUSH, POP or RET instructions which
> save or restore registers from/to the stack.  They are usually parts of
> function prologue and epilogue and have no type info.  The next is the
> struct pcpu_hot and you can see the first field (current_task) at offset
> 0 was accessed mostly.  It's listed in order of access frequency (not in
> offset) as you can see it in the task_struct.
> 
> In perf annotate, new --data-type option was added to enable data
> field level annotation.  Now it only shows number of samples for each
> field but we can improve it.
> 
>     $ sudo perf annotate --data-type
>     Annotate type: 'struct pcpu_hot' in [kernel.kallsyms] (223 samples):
>     ============================================================================
>         samples     offset       size  field
>             223          0         64  struct pcpu_hot       {
>             223          0         64      union     {
>             223          0         48          struct        {
>              78          0          8              struct task_struct*      current_task;
>              98          8          4              int      preempt_count;
>              45         12          4              int      cpu_number;
>               0         16          8              u64      call_depth;
>               1         24          8              long unsigned int        top_of_stack;
>               0         32          8              void*    hardirq_stack_ptr;
>               1         40          2              u16      softirq_pending;
>               0         42          1              bool     hardirq_stack_inuse;
>                                                };
>             223          0         64          u8*  pad;
>                                            };
>                                        };
>     ...
> 
> This shows each struct one by one and field-level access info in C-like
> style.  The number of samples for the outer struct is a sum of number of
> samples in every field in the struct.  In unions, each field is placed
> in the same offset so they will have the same number of samples.

This is excellent -- and pretty much what I've been asking for forever.

Would it be possible to have multiple sample columns, for eg.
MEM_LOADS_UOPS_RETIRED.L1_HIT and MEM_LOADS_UOPS_RETIRED.L1_MISS
or even more (adding LLC hit and miss as well etc.) ?

(for bonus points: --data-type=typename, would be awesome)

Additionally, annotating the regular perf-annotate output with data-type
information (where we have it) might also be very useful. That way, even
when profiling with PEBS-cycles, an expensive memop immediately gives a
clue as to what data-type to look at.

> No TUI support yet.

Yeah, nobody needs that anyway :-)

> This can generate instructions like below.
> 
>     ...
>     0x123456:  mov    0x18(%rdi), %rcx
>     0x12345a:  mov    0x10(%rcx), %rax     <=== sample
>     0x12345e:  test   %rax, %rax
>     0x123461:  je     <...>
>     ...
> 
> And imagine we have a sample at 0x12345a.  Then it cannot find a
> variable for %rcx since DWARF didn't generate one (it only knows about
> 'bar').  Without compiler support, all it can do is to track the code
> execution in each instruction and propagate the type info in each
> register and stack location by following the memory access.

Right, this has more or less been the 'excuse' for why doing this has
been 'difficult' for the past 10+ years :/

> Actually I found a discussion in the DWARF mailing list to support
> "inverted location lists" and it seems a perfect fit for this project.
> It'd be great if new DWARF would provide a way to lookup variable and
> type info using a concrete location info (like a register number).
> 
>   https://lists.dwarfstd.org/pipermail/dwarf-discuss/2023-June/002278.html 

Stephane was going to talk to tools people about this over 10 years ago
:-)

Thanks for *finally* getting this started!!