linux-kernel - Re: [BUG] msr-trace.h:42 suspicious rcu_dereference

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Date:   Mon, 28 Nov 2016 13:48:25 -0800
From:   Andi Kleen <andi@...stfloor.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Andi Kleen <andi@...stfloor.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Olsa <jolsa@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Jan Stancek <jstancek@...hat.com>
Subject: Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage!

> I took a look at this and forced some more functions to be inlined. I
> did a little tweaking here and there. Could you pull my tree and see if
> things are better?  I don't currently have the hardware to run this
> myself.
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git
> 
>   branch: ftrace/core

Didn't help much unfortunately. I think you need a real fast path here.

http://halobates.de/tracepoint-trace2

$ wc -l tracepoint-trace2
595 tracepoint-trace2

Opcode Histogram: 

    201 mov
     51 push
     51 pop
     34 test
     32 cmp
     26 jz
     24 jnz
     24 and
     16 ret
     16 lea
     15 call
     12 add
     10 jmp

Functions with # of instructions:

    25 trace_event_raw_event_sched_switch
     19 trace_event_buffer_reserve
     21 trace_event_buffer_lock_reserve
     57 ring_buffer_lock_reserve
      3 trace_clock_local
      3 sched_clock
     24 native_sched_clock
      3 sched_clock
      2 trace_clock_local
     11 ring_buffer_lock_reserve
     81 __rb_reserve_next
     11 ring_buffer_lock_reserve
      6 trace_event_buffer_lock_reserve
     18 ring_buffer_event_data
      5 trace_event_buffer_lock_reserve
     38 tracing_generic_entry_update
     11 trace_event_buffer_lock_reserve
      5 trace_event_buffer_reserve
     18 ring_buffer_event_data
      4 trace_event_buffer_reserve
     28 trace_event_raw_event_sched_switch
     30 trace_event_buffer_commit
     19 trace_buffer_unlock_commit_regs
     11 ring_buffer_unlock_commit
     85 rb_commit
     18 ring_buffer_unlock_commit
     10 trace_buffer_unlock_commit_regs
      3 ftrace_trace_userstack
      7 trace_buffer_unlock_commit_regs
     11 trace_event_buffer_commit
      8 trace_event_raw_event_sched_switch

-Andi