Re: [PATCH 5/5] ftrace: Update dynamic ftrace calls only if necessary

[Namhyung Kim <namhyung@kernel.org>]

On Wed, Mar 16, 2016 at 03:34:33PM +0100, Jiri Olsa wrote:
> Currently dynamic ftrace calls are updated any time
> the ftrace_ops is un/registered. If we do  this update
> only when it's needed, we save lot of time for perf
> system wide ftrace function sampling/counting.
> 
> The reason is that for system wide sampling/counting,
> perf creates event for each cpu in the system.
> 
> Each event then registers separate copy of ftrace_ops,
> which ends up in FTRACE_UPDATE_CALLS updates. On servers
> with many cpus that means serious stall (240 cpus server):
> 
> Counting:
>   # time ./perf stat -e ftrace:function -a sleep 1
> 
>    Performance counter stats for 'system wide':
> 
>               370,663      ftrace:function
> 
>           1.401427505 seconds time elapsed
> 
>   real    3m51.743s
>   user    0m0.023s
>   sys     3m48.569s
> 
> Sampling:
>   # time ./perf record -e ftrace:function -a sleep 1
>   [ perf record: Woken up 0 times to write data ]
>   Warning:
>   Processed 141200 events and lost 5 chunks!
> 
>   [ perf record: Captured and wrote 10.703 MB perf.data (135950 samples) ]
> 
>   real    2m31.429s
>   user    0m0.213s
>   sys     2m29.494s
> 
> There's no reason to do the FTRACE_UPDATE_CALLS update
> for each event in perf case, because all the ftrace_ops
> always share the same filter, so the updated calls are
> always the same.
> 
> It's required that only first ftrace_ops registration
> does the FTRACE_UPDATE_CALLS update (also sometimes
> the second if the first one used the trampoline), but
> the rest can be only cheaply linked into the ftrace_ops
> list.
> 
> Counting:
>   # time ./perf stat -e ftrace:function -a sleep 1
> 
>    Performance counter stats for 'system wide':
> 
>              398,571      ftrace:function
> 
>          1.377503733 seconds time elapsed
> 
>   real    0m2.787s
>   user    0m0.005s
>   sys     0m1.883s
> 
> Sampling:
>   # time ./perf record -e ftrace:function -a sleep 1
>   [ perf record: Woken up 0 times to write data ]
>   Warning:
>   Processed 261730 events and lost 9 chunks!
> 
>   [ perf record: Captured and wrote 19.907 MB perf.data (256293 samples) ]
> 
>   real    1m31.948s
>   user    0m0.309s
>   sys     1m32.051s
> 
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>

Acked-by: Namhyung Kim <namhyung@kernel.org>

Thanks,
Namhyung


> ---
>  kernel/trace/ftrace.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 3a9a12215b50..22ef6fc597a2 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -2651,7 +2651,6 @@ static int ftrace_startup(struct ftrace_ops *ops, int command)
>  		return ret;
>  
>  	ftrace_start_up++;
> -	command |= FTRACE_UPDATE_CALLS;
>  
>  	/*
>  	 * Note that ftrace probes uses this to start up
> @@ -2672,7 +2671,8 @@ static int ftrace_startup(struct ftrace_ops *ops, int command)
>  		return ret;
>  	}
>  
> -	ftrace_hash_rec_enable(ops, 1);
> +	if (ftrace_hash_rec_enable(ops, 1))
> +		command |= FTRACE_UPDATE_CALLS;
>  
>  	ftrace_startup_enable(command);
>  
> @@ -2702,11 +2702,11 @@ static int ftrace_shutdown(struct ftrace_ops *ops, int command)
>  
>  	/* Disabling ipmodify never fails */
>  	ftrace_hash_ipmodify_disable(ops);
> -	ftrace_hash_rec_disable(ops, 1);
>  
> -	ops->flags &= ~FTRACE_OPS_FL_ENABLED;
> +	if (ftrace_hash_rec_disable(ops, 1))
> +		command |= FTRACE_UPDATE_CALLS;
>  
> -	command |= FTRACE_UPDATE_CALLS;
> +	ops->flags &= ~FTRACE_OPS_FL_ENABLED;
>  
>  	if (saved_ftrace_func != ftrace_trace_function) {
>  		saved_ftrace_func = ftrace_trace_function;
> -- 
> 2.4.3
>