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SubjectRe: [PATCH net-next] trace_events_filter: conditional trace event (tcp_probe full=0)
On Mon, Feb 12, 2018 at 11:29 AM, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> On Mon, 12 Feb 2018 00:08:46 -0500
> "Md. Islam" <mislam4@kent.edu> wrote:
>
>> Recently tcp_probe kernel module has been replaced by trace_event. Old
>> tcp_probe had full=0 option where it only takes a snapshot only when
>> congestion window is changed. However I did not find such
>> functionality in trace_event.
>
> Yes, that seems broken to me. You should filter by using perf script or
> bpf. I'm not so clear about network stack, but it seems that cwnd can be
> set for each tcp connection. This means "current snd_cwnd" must be stored
> for each connection.
>
>> This is why I implemented this
>> "conditional trace_event" where a snapshot is taken only if some field
>> is changed. For instance, following filter will record a snapshot only
>> if snd_cwnd field is changed from the previous snapshot:
>>
>> cd /sys/kernel/debug/tracing
>> echo 1 > events/tcp/tcp_probe/enable
>> echo "(sport == 8554 && snd_cwnd =< 0)" > events/tcp/tcp_probe/filter &
>> cat trace_pipe > /home/tamim/net-next/trace.data
>
> Sounds interesting, but it seems like a hack.
> Filter pred is stored for each trace event (tracepoint) so it is shared
> among several context. At least you need a lock to exclude each thread
> to update it. And I don't recommend to update filter_pred while tracing,
> since it means we have a kind of "hidden state" in the filer. And again,
> that filter_pred is shared, other tcp connection can see it.
> (sport can be same as other connection's sport from other IP)
>
> So IMHO, this kind of hack we don't implement in the kernel. Instead,
> you can use perf-script and program it by python or perl. It gives you
> much safer way to filter it out.
>
> Or, please try to make it as complete feature. For example, maybe we can
> introduce a global 'named' variable (which has correct accessor with spinlock
> or atomic update), and you can refer it with name.
> Then, it becomes safe and more generic, like below;
>
> $ cd /sys/kernel/debug/tracing
> $ touch vars/cwnd
> $ echo '(sport == 12345 && snd_cwnd != $cwnd)' > events/tcp/tcp_probe/filter
> $ echo 'update_var:cwnd:snd_cwnd if traced' > events/tcp/tcp_probe/trigger
>
> Thank you,


Hi Masami

Thanks for the suggestions! It does makes sense. I created a global
atomic variable in predicate. Then I'm updating that based on the
filter. Now it can be done as below:

cd /sys/kernel/debug/tracing &&
echo 1 > events/tcp/tcp_probe/enable &&
echo "(sport == 8554 && snd_cwnd != \$cwnd)" > events/tcp/tcp_probe/filter

The filter identify the global variable by the presence of $. However
needed to pass '\$' to catch $ in kernel. The patch is :



diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 2a6d032..15a83c7 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -1402,6 +1402,7 @@ struct regex {
struct filter_pred {
filter_pred_fn_t fn;
u64 val;
+ atomic64_t global_var;
struct regex regex;
unsigned short *ops;
struct ftrace_event_field *field;
@@ -1427,6 +1428,16 @@ static inline bool is_function_field(struct
ftrace_event_field *field)
return field->filter_type == FILTER_TRACE_FN;
}

+/// The string prerdicate is a global variable if it starts with $.
+/// \param field predicate
+/// \return
+static inline bool is_global_variable(char *field)
+{
+ if(strlen(field) == 0)
+ return false;
+ return field[0] == '$';
+}
+
extern enum regex_type
filter_parse_regex(char *buff, int len, char **search, int *not);
extern void print_event_filter(struct trace_event_file *file,
diff --git a/kernel/trace/trace_events_filter.c
b/kernel/trace/trace_events_filter.c
index 61e7f06..8bf43353 100644
--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -202,6 +202,15 @@ static int filter_pred_##size(struct filter_pred
*pred, void *event) \
return match; \
}

+#define DEFINE_GLOBAL_PRED(size) \
+static int filter_global_pred_##size(struct filter_pred *pred, void
*event) \
+{ \
+ u##size *addr = (u##size *)(event + pred->offset); \
+ u##size old_global_var =
(u##size)atomic64_xchg(&pred->global_var, (u64)*addr); \
+ return (old_global_var == *addr) ^ pred->not; \
+}
+
+
DEFINE_COMPARISON_PRED(s64);
DEFINE_COMPARISON_PRED(u64);
DEFINE_COMPARISON_PRED(s32);
@@ -216,6 +225,11 @@ DEFINE_EQUALITY_PRED(32);
DEFINE_EQUALITY_PRED(16);
DEFINE_EQUALITY_PRED(8);

+DEFINE_GLOBAL_PRED(64);
+DEFINE_GLOBAL_PRED(32);
+DEFINE_GLOBAL_PRED(16);
+DEFINE_GLOBAL_PRED(8);
+
/* Filter predicate for fixed sized arrays of characters */
static int filter_pred_string(struct filter_pred *pred, void *event)
{
@@ -988,6 +1002,29 @@ static bool is_legal_op(struct
ftrace_event_field *field, enum filter_op_ids op)
return true;
}

+static filter_pred_fn_t select_global_fn(enum filter_op_ids op,
+ int field_size, int field_is_signed)
+{
+ filter_pred_fn_t fn = NULL;
+
+ switch (field_size) {
+ case 8:
+ fn = filter_global_pred_64;
+ break;
+ case 4:
+ fn = filter_global_pred_32;
+ break;
+ case 2:
+ fn = filter_global_pred_16;
+ break;
+ case 1:
+ fn = filter_global_pred_8;
+ break;
+ }
+
+ return fn;
+}
+
static filter_pred_fn_t select_comparison_fn(enum filter_op_ids op,
int field_size, int field_is_signed)
{
@@ -1066,7 +1103,15 @@ static int init_pred(struct filter_parse_state *ps,
parse_error(ps, FILT_ERR_IP_FIELD_ONLY, 0);
return -EINVAL;
}
- } else {
+ } else if(is_global_variable(pred->regex.pattern)){
+ atomic64_set(&pred->global_var, 0);
+ fn = select_global_fn(pred->op, field->size,
+ field->is_signed);
+ if (!fn) {
+ parse_error(ps, FILT_ERR_INVALID_OP, 0);
+ return -EINVAL;
+ }
+ }else {
if (field->is_signed)
ret = kstrtoll(pred->regex.pattern, 0, &val);
else

I didn't however updating it in predicate yet. I'm updating the global
variable using atomic64_xchg() in the filter. Do you still prefer to
implement it in trigger? Please let me know any suggestion regrading
the patch.


Many thanks
Tamim

>
>>
>> This will record a snapshot where source port is 8554 and snd_cwnd is
>> not same as the snd_cwnd in previous snapshot. For lack of better
>> operator, I used "=<" to represent the field on which the filter will
>> be applied.
>>
>> The way it works is, it updates the predicate with new value every
>> time. So the next time, if the field is same as in the predicate, the
>> snapshot is ignored. Initially it checks if the field is 0 or not.
>>
>> diff --git a/kernel/trace/trace_events_filter.c
>> b/kernel/trace/trace_events_filter.c
>> index 61e7f06..8d733fa 100644
>> --- a/kernel/trace/trace_events_filter.c
>> +++ b/kernel/trace/trace_events_filter.c
>> @@ -39,6 +39,7 @@ enum filter_op_ids
>> OP_AND,
>> OP_GLOB,
>> OP_NE,
>> + OP_NE_PREV,
>> OP_EQ,
>> OP_LT,
>> OP_LE,
>> @@ -62,6 +63,7 @@ static struct filter_op filter_ops[] = {
>> { OP_AND, "&&", 2 },
>> { OP_GLOB, "~", 4 },
>> { OP_NE, "!=", 4 },
>> + { OP_NE_PREV, "=<", 4 },
>> { OP_EQ, "==", 4 },
>> { OP_LT, "<", 5 },
>> { OP_LE, "<=", 5 },
>> @@ -202,6 +204,21 @@ static int filter_pred_##size(struct filter_pred
>> *pred, void *event) \
>> return match; \
>> }
>>
>> +#define DEFINE_NE_PREV_PRED(size) \
>> +static int filter_ne_prev_pred_##size(struct filter_pred *pred, void
>> *event) \
>> +{ \
>> + u##size *addr = (u##size *)(event + pred->offset); \
>> + u##size val = (u##size)pred->val; \
>> + int match; \
>> + \
>> + match = (val == *addr) ^ pred->not; \
>> + \
>> + if(match == 1) \
>> + pred->val = *addr; \
>> + return match; \
>> +}
>> +
>> +
>> DEFINE_COMPARISON_PRED(s64);
>> DEFINE_COMPARISON_PRED(u64);
>> DEFINE_COMPARISON_PRED(s32);
>> @@ -216,6 +233,11 @@ DEFINE_EQUALITY_PRED(32);
>> DEFINE_EQUALITY_PRED(16);
>> DEFINE_EQUALITY_PRED(8);
>>
>> +DEFINE_NE_PREV_PRED(64);
>> +DEFINE_NE_PREV_PRED(32);
>> +DEFINE_NE_PREV_PRED(16);
>> +DEFINE_NE_PREV_PRED(8);
>> +
>> /* Filter predicate for fixed sized arrays of characters */
>> static int filter_pred_string(struct filter_pred *pred, void *event)
>> {
>> @@ -980,7 +1002,7 @@ int filter_assign_type(const char *type)
>> static bool is_legal_op(struct ftrace_event_field *field, enum
>> filter_op_ids op)
>> {
>> if (is_string_field(field) &&
>> - (op != OP_EQ && op != OP_NE && op != OP_GLOB))
>> + (op != OP_EQ && op != OP_NE && op != OP_GLOB && op != OP_NE_PREV))
>> return false;
>> if (!is_string_field(field) && op == OP_GLOB)
>> return false;
>> @@ -997,6 +1019,8 @@ static filter_pred_fn_t select_comparison_fn(enum
>> filter_op_ids op,
>> case 8:
>> if (op == OP_EQ || op == OP_NE)
>> fn = filter_pred_64;
>> + else if (op == OP_NE_PREV)
>> + fn = filter_ne_prev_pred_64;
>> else if (field_is_signed)
>> fn = pred_funcs_s64[op - PRED_FUNC_START];
>> else
>> @@ -1005,6 +1029,8 @@ static filter_pred_fn_t
>> select_comparison_fn(enum filter_op_ids op,
>> case 4:
>> if (op == OP_EQ || op == OP_NE)
>> fn = filter_pred_32;
>> + else if (op == OP_NE_PREV)
>> + fn = filter_ne_prev_pred_32;
>> else if (field_is_signed)
>> fn = pred_funcs_s32[op - PRED_FUNC_START];
>> else
>> @@ -1013,6 +1039,8 @@ static filter_pred_fn_t
>> select_comparison_fn(enum filter_op_ids op,
>> case 2:
>> if (op == OP_EQ || op == OP_NE)
>> fn = filter_pred_16;
>> + else if (op == OP_NE_PREV)
>> + fn = filter_ne_prev_pred_16;
>> else if (field_is_signed)
>> fn = pred_funcs_s16[op - PRED_FUNC_START];
>> else
>> @@ -1021,6 +1049,8 @@ static filter_pred_fn_t
>> select_comparison_fn(enum filter_op_ids op,
>> case 1:
>> if (op == OP_EQ || op == OP_NE)
>> fn = filter_pred_8;
>> + else if (op == OP_NE_PREV)
>> + fn = filter_ne_prev_pred_8;
>> else if (field_is_signed)
>> fn = pred_funcs_s8[op - PRED_FUNC_START];
>> else
>> @@ -1088,7 +1118,7 @@ static int init_pred(struct filter_parse_state *ps,
>> }
>> }
>>
>> - if (pred->op == OP_NE)
>> + if (pred->op == OP_NE || pred->op == OP_NE_PREV)
>> pred->not ^= 1;
>>
>> pred->fn = fn;
>> @@ -2197,7 +2227,7 @@ static int ftrace_function_check_pred(struct
>> filter_pred *pred, int leaf)
>> * - only '==' and '!=' is used
>> * - the 'ip' field is used
>> */
>> - if ((pred->op != OP_EQ) && (pred->op != OP_NE))
>> + if ((pred->op != OP_EQ) && (pred->op != OP_NE) && (pred->op
>> != OP_NE_PREV))
>> return -EINVAL;
>>
>> if (strcmp(field->name, "ip"))
>>
>>
>> I'm new to upstream kernel development. Please let me any suggestion.
>>
>> Many thanks
>> Tamim
>> PhD Candidate,
>> Kent State University
>
>
> --
> Masami Hiramatsu <mhiramat@kernel.org>



--
Tamim
PhD Candidate,
Kent State University
http://web.cs.kent.edu/~mislam4/

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