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SubjectRe: [for-next][PATCH 2/2] tracing: Use temp buffer when filtering events
On 2016-05-04 21:52, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>
>
> Filtering of events requires the data to be written to the ring buffer
> before it can be decided to filter or not. This is because the
> parameters of
> the filter are based on the result that is written to the ring buffer
> and
> not on the parameters that are passed into the trace functions.
>
> The ftrace ring buffer is optimized for writing into the ring buffer
> and
> committing. The discard procedure used when filtering decides the event
> should be discarded is much more heavy weight. Thus, using a temporary
> filter when filtering events can speed things up drastically.
>
> Without a temp buffer we have:
>
> # trace-cmd start -p nop
> # perf stat -r 10 hackbench 50
> 0.790706626 seconds time elapsed ( +- 0.71% )
>
> # trace-cmd start -e all
> # perf stat -r 10 hackbench 50
> 1.566904059 seconds time elapsed ( +- 0.27% )
>
> # trace-cmd start -e all -f 'common_preempt_count==20'
> # perf stat -r 10 hackbench 50
> 1.690598511 seconds time elapsed ( +- 0.19% )
>
> # trace-cmd start -e all -f 'common_preempt_count!=20'
> # perf stat -r 10 hackbench 50
> 1.707486364 seconds time elapsed ( +- 0.30% )
>
> The first run above is without any tracing, just to get a based figure.
> hackbench takes ~0.79 seconds to run on the system.
>
> The second run enables tracing all events where nothing is filtered.
> This
> increases the time by 100% and hackbench takes 1.57 seconds to run.
>
> The third run filters all events where the preempt count will equal
> "20"
> (this should never happen) thus all events are discarded. This takes
> 1.69
> seconds to run. This is 10% slower than just committing the events!
>
> The last run enables all events and filters where the filter will
> commit all
> events, and this takes 1.70 seconds to run. The filtering overhead is
> approximately 10%. Thus, the discard and commit of an event from the
> ring
> buffer may be about the same time.
>
> With this patch, the numbers change:
>
> # trace-cmd start -p nop
> # perf stat -r 10 hackbench 50
> 0.778233033 seconds time elapsed ( +- 0.38% )
>
> # trace-cmd start -e all
> # perf stat -r 10 hackbench 50
> 1.582102692 seconds time elapsed ( +- 0.28% )
>
> # trace-cmd start -e all -f 'common_preempt_count==20'
> # perf stat -r 10 hackbench 50
> 1.309230710 seconds time elapsed ( +- 0.22% )
>
> # trace-cmd start -e all -f 'common_preempt_count!=20'
> # perf stat -r 10 hackbench 50
> 1.786001924 seconds time elapsed ( +- 0.20% )
>
> The first run is again the base with no tracing.
>
> The second run is all tracing with no filtering. It is a little slower,
> but
> that may be well within the noise.
>
> The third run shows that discarding all events only took 1.3 seconds.
> This
> is a speed up of 23%! The discard is much faster than even the commit.
>
> The one downside is shown in the last run. Events that are not
> discarded by
> the filter will take longer to add, this is due to the extra copy of
> the
> event.
>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> ---
> kernel/trace/trace.c | 154
> +++++++++++++++++++++++++++++++++++--
> kernel/trace/trace.h | 19 ++++-
> kernel/trace/trace_events.c | 10 +++
> kernel/trace/trace_events_filter.c | 10 +++
> 4 files changed, 185 insertions(+), 8 deletions(-)
>
...
> @@ -1695,8 +1820,23 @@ trace_event_buffer_lock_reserve(struct
> ring_buffer **current_rb,
> unsigned long flags, int pc)
> {
> struct ring_buffer_event *entry;
> + int val;
>
> *current_rb = trace_file->tr->trace_buffer.buffer;
> +
> + if ((trace_file->flags &
> + (EVENT_FILE_FL_SOFT_DISABLED | EVENT_FILE_FL_FILTERED)) &&
> + (entry = this_cpu_read(trace_buffered_event))) {
> + /* Try to use the per cpu buffer first */
> + val = this_cpu_inc_return(trace_buffered_event_cnt);
> + if (val == 1) {
> + trace_event_setup(entry, type, flags, pc);
> + entry->array[0] = len;
> + return entry;
> + }
> + this_cpu_dec(trace_buffered_event_cnt);
> + }
> +
> entry = trace_buffer_lock_reserve(*current_rb,
> type, len, flags, pc);
> /*
this patch commit id is : 0fc1b09ff1ff404ddf753f5ffa5cd0adc8fdcdc9 which
has upstream.

how much size is the per cpu buffer?
seems it is initilized in trace_buffered_event_enable,
it is only 1 page size as below:
void trace_buffered_event_enable(void)
{
...
for_each_tracing_cpu(cpu) {
page = alloc_pages_node(cpu_to_node(cpu),
GFP_KERNEL | __GFP_NORETRY, 0);
If the size of buffer to trace is more than 1 page, such as 46680, then
it trigger kernel crash/panic in my case while run trace-cmd.
After debugging, the trace_file->flags in
trace_event_buffer_lock_reserve is 0x40b while run trace-cmd, and it is
0x403 while collecting ftrace log.

Is it have any operation to disable this patch dynamically?

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