Messages in this thread Patch in this message | | | Date | Wed, 14 Mar 2018 12:54:01 -0400 | From | Steven Rostedt <> | Subject | [PATCH 0/3 v2] tracing: Rewrite the function filter code |
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Al Viro reviewed the ftrace event filter logic and was horrified. He wrote Tom and I a lengthy private email with a formal proof on how to do it simpler as well as more efficient.
Currently the filter logic creates a binary search tree (with some optimizations), and walks it to get a result.
Say we had the following filter: a && !(!b || c) || d && !e Where each letter is a predicate. A tree would be made to look something like:
|| / \ && && / | | \ a !|| d !e / \ !b c
If a == 1, b == 1, c == 1, d == 1, e == 0, then all nodes would be walked.
|| -> && -> a -> && -> !|| -> !b -> !|| -> c -> !|| -> && -> || -> && -> d -> && -> !e -> && -> || -> return TRUE!
That's 16 steps across vertices.
Al's method would create a program using an array that denotes the predicate function, when to branch, and a target to branch to if the value is the same as when to branch. Storing the array as:
prog[0] = { a, 0, 2}; // predicate, when_to_branch, target prog[1] = { b, 0, 2}; prog[2] = { c, 0, 4}; prog[3] = { d, 0, 5}; prog[4] = { e, 1, 5}; prog[5] = { NULL, 0, 1}; // TRUE prog[6] = { NULL, 0, 0}; // FALSE
Where the code to execute the above looks like:
for (i = 0; prog[i].pred; i++) { struct filter_pred *pred = prog[i].pred; int match = pred->fn(pred, rec); if (match == prog[i].when_to_branch) i = prog[i].target; } return prog[i].target;
Which translates the above program array into:
n0: if (!a) goto n3; n1: if (!b) goto n3; n2: if (!c) goto T; n3: if (!d) goto F; n4: if (e) goto F; T: return TRUE; F: return FALSE;
And the above example only takes 6 steps to return TRUE.
Special thanks goes out to Al for his patience and his time that he spent in educating us in a proper logical parser.
Two patches were added to do some initial clean up. The last patch implements Al's suggestions. I wrote up a very lengthy comment describing what Al taught us in my own words (hopefully I got it right), and the implementation is similar to what Al showed with a few changes (again, hopefully I got that right too). I tested this with lots of different filters and it looks to be holding up.
git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git ftrace/core
Head SHA1: 80765597bc587feae8dbc8ce97a0f32e12a6e625
Steven Rostedt (VMware) (3): tracing: Combine enum and arrays into single macro in filter code tracing: Clean up and document pred_funcs_##type creation and use tracing: Rewrite filter logic to be simpler and faster
---- kernel/trace/trace.h | 15 +- kernel/trace/trace_events_filter.c | 2337 ++++++++++++++++-------------------- 2 files changed, 1068 insertions(+), 1284 deletions(-)
Changes from v1: - Fixed echoing nothing into the filter (the tests echoed '0') (I fixed my tests) - Added better RCU annotations. (Thanks to zero day bot for pointing this out)
Diff from v1:
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h index e6f82f74ad65..6fb46a06c9dc 100644 --- a/kernel/trace/trace.h +++ b/kernel/trace/trace.h @@ -1219,7 +1219,7 @@ struct ftrace_event_field { struct prog_entry; struct event_filter { - struct prog_entry *prog; + struct prog_entry __rcu *prog; char *filter_string; }; diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c index 8d7da0ded43b..703a416aa5c2 100644 --- a/kernel/trace/trace_events_filter.c +++ b/kernel/trace/trace_events_filter.c @@ -982,14 +982,16 @@ void print_subsystem_event_filter(struct event_subsystem *system, static void free_prog(struct event_filter *filter) { - struct prog_entry *prog = filter->prog; + struct prog_entry *prog; int i; + prog = rcu_access_pointer(filter->prog); if (!prog) return; for (i = 0; prog[i].pred; i++) kfree(prog[i].pred); + kfree(prog); } static void filter_disable(struct trace_event_file *file) @@ -1497,12 +1499,15 @@ static int process_preds(struct trace_event_call *call, return ret; } - prog = predicate_parse(filter_string, nr_parens, nr_preds, + if (!nr_preds) { + prog = NULL; + } else { + prog = predicate_parse(filter_string, nr_parens, nr_preds, parse_pred, call, pe); - if (IS_ERR(prog)) - return PTR_ERR(prog); - - filter->prog = prog; + if (IS_ERR(prog)) + return PTR_ERR(prog); + } + rcu_assign_pointer(filter->prog, prog); return 0; }
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