linux-kernel - Re: [PATCH v4 3/3] tracing: kdb: Allow ftdump to skip all but the last few entries

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Date:   Mon, 18 Mar 2019 03:02:38 +0000
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Jason Wessel <jason.wessel@...driver.com>,
        kgdb-bugreport@...ts.sourceforge.net,
        Brian Norris <briannorris@...omium.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/3] tracing: kdb: Allow ftdump to skip all but the
 last few entries

On Fri, Mar 15, 2019 at 04:09:06PM -0700, Douglas Anderson wrote:
> The 'ftdump' command in kdb is currently a bit of a last resort, at
> least if you have lots of traces turned on.  It's going to print a
> whole boatload of data out your serial port which is probably running
> at 115200.  This could easily take many, many minutes.
> 
> Usually you're most interested in what's at the _end_ of the ftrace
> buffer, AKA what happened most recently.  That means you've got to
> wait the full time for the dump.  The 'ftdump' command does attempt to
> help you a little bit by allowing you to skip a fixed number of
> entries.  Unfortunately it provides no way for you to know how many
> entries you should skip.
> 
> Let's do similar to python and allow you to use a negative number to
> indicate that you want to skip all entries except the last few.  This
> allows you to quickly see what you want.
> 
> Note that we also change the printout in ftdump to print the
> (positive) number of entries actually skipped since that could be
> helpful to know when you've specified a negative skip count.
> 
> Signed-off-by: Douglas Anderson <dianders@...omium.org>

There's a small nitpick below but otherwise:
Acked-by: Daniel Thompson <daniel.thompson@...aro.org>


> ---
> 
> Changes in v4:
> - Now uses trace_total_entries() / trace_total_entries_cpu().
> - Based upon new patch that renames "lines" to "entries".
> 
> Changes in v3:
> - Optimize counting as per Steven Rostedt.
> - Down to 1 patch since patch #1 from v2 landed.
> 
>  kernel/trace/trace_kdb.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/trace_kdb.c b/kernel/trace/trace_kdb.c
> index 4b666643d69f..996e1e9cd9a6 100644
> --- a/kernel/trace/trace_kdb.c
> +++ b/kernel/trace/trace_kdb.c
> @@ -39,7 +39,8 @@ static void ftrace_dump_buf(int skip_entries, long cpu_file)
>  	/* don't look at user memory in panic mode */
>  	tr->trace_flags &= ~TRACE_ITER_SYM_USEROBJ;
>  
> -	kdb_printf("Dumping ftrace buffer:\n");
> +	kdb_printf("Dumping ftrace buffer (skipping %d entries):\n",
> +		   skip_entries);

If someone *doesn't* need to skip any entries I'm not a fan of telling
them we are "skipping 0 entries"; it is more unnerving than helpful
("huh? what does it need to tell me that no entried were skipped?
what makes the tracer skip entries?... Doh... I get it").


Daniel.


>  
>  	/* reset all but tr, trace, and overruns */
>  	memset(&iter.seq, 0,
> @@ -109,6 +110,7 @@ static int kdb_ftdump(int argc, const char **argv)
>  	int skip_entries = 0;
>  	long cpu_file;
>  	char *cp;
> +	int cnt;
>  
>  	if (argc > 2)
>  		return KDB_ARGCOUNT;
> @@ -129,6 +131,16 @@ static int kdb_ftdump(int argc, const char **argv)
>  	}
>  
>  	kdb_trap_printk++;
> +
> +	/* A negative skip_entries means skip all but the last entries */
> +	if (skip_entries < 0) {
> +		if (cpu_file == RING_BUFFER_ALL_CPUS)
> +			cnt = trace_total_entries(NULL);
> +		else
> +			cnt = trace_total_entries_cpu(NULL, cpu_file);
> +		skip_entries = max(cnt + skip_entries, 0);
> +	}
> +
>  	ftrace_dump_buf(skip_entries, cpu_file);
>  	kdb_trap_printk--;
>  
> @@ -138,7 +150,8 @@ static int kdb_ftdump(int argc, const char **argv)
>  static __init int kdb_ftrace_register(void)
>  {
>  	kdb_register_flags("ftdump", kdb_ftdump, "[skip_#entries] [cpu]",
> -			    "Dump ftrace log", 0, KDB_ENABLE_ALWAYS_SAFE);
> +			    "Dump ftrace log; -skip dumps last #entries", 0,
> +			    KDB_ENABLE_ALWAYS_SAFE);
>  	return 0;
>  }
>  
> -- 
> 2.21.0.360.g471c308f928-goog
>