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

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Date:   Thu, 12 Oct 2023 08:03:04 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-perf-users@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Stephane Eranian <eranian@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        linux-toolchains@...r.kernel.org,
        linux-trace-devel@...r.kernel.org,
        Ben Woodard <woodard@...hat.com>,
        Joe Mario <jmario@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        David Blaikie <blaikie@...gle.com>,
        Xu Liu <xliuprof@...gle.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Ravi Bangoria <ravi.bangoria@....com>
Subject: Re: [RFC 00/48] perf tools: Introduce data type profiling (v1)


* Namhyung Kim <namhyung@...nel.org> 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
> 
>     # Similar for the AMD (but it requires 6.3+ kernel for BPF filters)
>     $ sudo perf mem record -a --filter 'mem_op == load, ip > 0x8000000000000000' -- sleep 1

BTW., it would be nice for 'perf mem record' to just do the right thing on 
whatever machine it is running on.

Also, why are BPF filters required - due to the IP filtering of mem-load 
events?

Could we perhaps add an IP filter to perf events to get this built-in? 
Perhaps attr->exclude_user would achieve something similar?

> 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)
>         ...

This looks really useful!

Thanks,

	Ingo