Messages in this thread Patch in this message | | | Date | Mon, 26 Mar 2018 12:59:13 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH] ftrace: fix stddev calculation in function profiler (again) |
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On Sun, 25 Mar 2018 19:09:39 +0200 Matthias Schiffer <mschiffer@universe-factory.net> wrote:
> > I have two more questions/issues regarding the generic code: > > 1) There is a small window when enabling the profiler where rec->counter > gets incremented, but profile_graph_return() does not record times yet. For > functions that are called a lot, the effect is negligible, but for very > infrequent calls, the shown average time and variance are way off. Moving > the rec->counter increment to profile_graph_return() when > CONFIG_FUNCTION_GRAPH_TRACER is enabled would fix this. Does this sound > reasonable?
Because the counter is done with function_profile_call() which is used by function tracer (when graph tracer is not enabled), I wouldn't move it.
But instead, we could make a function_profile_call() helper that does all that function_profile_call does, and return a value if it does something. For example:
static int calculate_profile_call(unsigned long ip, unsigned long parent_ip, struct ftrace_ops *ops, struct pt_regs *regs) { struct ftrace_profile_stat *stat; struct ftrace_profile *rec; unsigned long flags; int ret = 0;
if (!ftrace_profile_enabled) return 0;
local_irq_save(flags);
stat = this_cpu_ptr(&ftrace_profile_stats); if (!stat->hash || !ftrace_profile_enabled) goto out;
rec = ftrace_find_profiled_func(stat, ip); if (!rec) { rec = ftrace_profile_alloc(stat, ip); if (!rec) goto out; }
rec->counter++; ret = 1; out: local_irq_restore(flags); return ret; }
static void function_profile_call(unsigned long ip, unsigned long parent_ip, struct ftrace_ops *ops, struct pt_regs *regs) { calculate_profile_call(ip, parent_ip, ops, regs); }
#ifdef CONFIG_FUNCTION_GRAPH_TRACER static int profile_graph_entry(struct ftrace_graph_ent *trace) { int index = trace->depth;
if (!calculate_profile_call(trace->func, 0, NULL, NULL)) return 0;
/* If function graph is shutting down, ret_stack can be NULL */ if (!current->ret_stack) return 0;
if (index >= 0 && index < FTRACE_RETFUNC_DEPTH) current->ret_stack[index].subtime = 0;
return 1; }
If we don't calculate the profile call, then we return 0, this will tell the function graph tracer not to perform the return part of the tracing.
> > 2) I noticed that my patch does not handle rec->counter overflows correctly > and will cause a zero division. I can think of two solutions for handling > overflows that keep the avg / stddev fields meaningful: > - Reset time/avg/stddev to 0 on overflows > - Stop updating the record at 0xffffffff, avoiding the overflow
Or add an option that lets the user decide. But I still would like to find a way not to add a that division on every function call. That's really heavy weight.
-- Steve
> > Matthias > > > > > > > > > >> --- > >> kernel/trace/ftrace.c | 33 +++++++++++++++++++-------------- > >> 1 file changed, 19 insertions(+), 14 deletions(-) > >> > >> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c > >> index eac9ce2c57a2..16dce67b855a 100644 > >> --- a/kernel/trace/ftrace.c > >> +++ b/kernel/trace/ftrace.c > >> @@ -460,7 +460,8 @@ struct ftrace_profile { > >> unsigned long counter; > >> #ifdef CONFIG_FUNCTION_GRAPH_TRACER > >> unsigned long long time; > >> - unsigned long long time_squared; > >> + unsigned long long avg; > >> + unsigned long long stddev; > >> #endif > >> }; > >> > >> @@ -580,7 +581,6 @@ static int function_stat_show(struct seq_file *m, void *v) > >> int ret = 0; > >> #ifdef CONFIG_FUNCTION_GRAPH_TRACER > >> static struct trace_seq s; > >> - unsigned long long avg; > >> unsigned long long stddev; > >> #endif > >> mutex_lock(&ftrace_profile_lock); > >> @@ -592,9 +592,7 @@ static int function_stat_show(struct seq_file *m, void *v) > >> } > >> > >> #ifdef CONFIG_FUNCTION_GRAPH_TRACER > >> - avg = rec->time; > >> - do_div(avg, rec->counter); > >> - if (tracing_thresh && (avg < tracing_thresh)) > >> + if (tracing_thresh && (rec->avg < tracing_thresh)) > >> goto out; > >> #endif > >> > >> @@ -608,24 +606,19 @@ static int function_stat_show(struct seq_file *m, void *v) > >> if (rec->counter <= 1) > >> stddev = 0; > >> else { > >> - /* > >> - * Apply Welford's method: > >> - * s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2) > >> - */ > >> - stddev = rec->counter * rec->time_squared - > >> - rec->time * rec->time; > >> + stddev = rec->stddev; > >> > >> /* > >> * Divide only 1000 for ns^2 -> us^2 conversion. > >> * trace_print_graph_duration will divide 1000 again. > >> */ > >> - do_div(stddev, rec->counter * (rec->counter - 1) * 1000); > >> + do_div(stddev, 1000 * (rec->counter - 1)); > >> } > >> > >> trace_seq_init(&s); > >> trace_print_graph_duration(rec->time, &s); > >> trace_seq_puts(&s, " "); > >> - trace_print_graph_duration(avg, &s); > >> + trace_print_graph_duration(rec->avg, &s); > >> trace_seq_puts(&s, " "); > >> trace_print_graph_duration(stddev, &s); > >> trace_print_seq(m, &s); > >> @@ -905,8 +898,20 @@ static void profile_graph_return(struct ftrace_graph_ret *trace) > >> > >> rec = ftrace_find_profiled_func(stat, trace->func); > >> if (rec) { > >> + unsigned long long avg, delta1, delta2; > >> + > >> rec->time += calltime; > >> - rec->time_squared += calltime * calltime; > >> + > >> + /* Apply Welford's method */ > >> + delta1 = calltime - rec->avg; > >> + > >> + avg = rec->time; > >> + do_div(avg, rec->counter); > >> + rec->avg = avg; > >> + > >> + delta2 = calltime - rec->avg; > >> + > >> + rec->stddev += delta1 * delta2; > >> } > >> > >> out: > >> > > > > > >
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