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ftrace: Allow enabling of filters via index of available_filter_functions

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Enabling of large number of functions by echoing in a large subset of the
functions in available_filter_functions can take a very long time. The
process requires testing all functions registered by the function tracer
(which is in the 10s of thousands), and doing a kallsyms lookup to convert
the ip address into a name, then comparing that name with the string passed
in.

When a function causes the function tracer to crash the system, a binary
bisect of the available_filter_functions can be done to find the culprit.
But this requires passing in half of the functions in
available_filter_functions over and over again, which makes it basically a
O(n^2) operation. With 40,000 functions, that ends up bing 1,600,000,000
opertions! And enabling this can take over 20 minutes.

As a quick speed up, if a number is passed into one of the filter files,
instead of doing a search, it just enables the function at the corresponding
line of the available_filter_functions file. That is:

 # echo 50 > set_ftrace_filter
 # cat set_ftrace_filter
 x86_pmu_commit_txn

 # head -50 available_filter_functions | tail -1
 x86_pmu_commit_txn

This allows setting of half the available_filter_functions to take place in
less than a second!

 # time seq 20000 > set_ftrace_filter
 real    0m0.042s
 user    0m0.005s
 sys     0m0.015s

 # wc -l set_ftrace_filter
 20000 set_ftrace_filter

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
This commit is contained in:
Steven Rostedt (VMware)
2019-02-11 15:00:48 -05:00
parent 85acbb21b9
commit f79b3f3385
4 changed files with 74 additions and 0 deletions
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30
kernel/trace/ftrace.c
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@@ -3701,6 +3701,31 @@ enter_record(struct ftrace_hash *hash, struct dyn_ftrace *rec, int clear_filter)
return ret;
}
static int
add_rec_by_index(struct ftrace_hash *hash, struct ftrace_glob *func_g,
int clear_filter)
{
long index = simple_strtoul(func_g->search, NULL, 0);
struct ftrace_page *pg;
struct dyn_ftrace *rec;
/* The index starts at 1 */
if (--index < 0)
return 0;
do_for_each_ftrace_rec(pg, rec) {
if (pg->index <= index) {
index -= pg->index;
/* this is a double loop, break goes to the next page */
break;
}
rec = &pg->records[index];
enter_record(hash, rec, clear_filter);
return 1;
} while_for_each_ftrace_rec();
return 0;
}
static int
ftrace_match_record(struct dyn_ftrace *rec, struct ftrace_glob *func_g,
struct ftrace_glob *mod_g, int exclude_mod)
@@ -3769,6 +3794,11 @@ match_records(struct ftrace_hash *hash, char *func, int len, char *mod)
if (unlikely(ftrace_disabled))
goto out_unlock;
if (func_g.type == MATCH_INDEX) {
found = add_rec_by_index(hash, &func_g, clear_filter);
goto out_unlock;
}
do_for_each_ftrace_rec(pg, rec) {
if (rec->flags & FTRACE_FL_DISABLED)