Messages in this thread | | | Date | Fri, 28 Aug 2020 15:53:06 +0800 | From | Wen Gong <> | Subject | Re: [for-next][PATCH 2/2] tracing: Use temp buffer when filtering events |
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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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