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    Subject[PATCH v4 1/8] tracing: Add support for creating hist trigger variables from literal
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    Currently hist trigger expressions don't support the use of numeric
    literals:
    e.g. echo 'hist:keys=common_pid:x=$y-1234'
    --> is not valid expression syntax

    Having the ability to use numeric constants in hist triggers supports
    a wider range of expressions for creating variables.

    Add support for creating trace event histogram variables from numeric
    literals.

    e.g. echo 'hist:keys=common_pid:x=1234,y=size-1024' >> event/trigger

    A negative numeric constant is created, using unary minus operator
    (parentheses are required).

    e.g. echo 'hist:keys=common_pid:z=-(2)' >> event/trigger

    Constants can be used with division/multiplication (added in the
    next patch in this series) to implement granularity filters for frequent
    trace events. For instance we can limit emitting the rss_stat
    trace event to when there is a 512KB cross over in the rss size:

    # Create a synthetic event to monitor instead of the high frequency
    # rss_stat event
    echo 'rss_stat_throttled unsigned int mm_id; unsigned int curr;
    int member; long size' >> tracing/synthetic_events

    # Create a hist trigger that emits the synthetic rss_stat_throttled
    # event only when the rss size crosses a 512KB boundary.
    echo 'hist:keys=keys=mm_id,member:bucket=size/0x80000:onchange($bucket)
    .rss_stat_throttled(mm_id,curr,member,size)'
    >> events/kmem/rss_stat/trigger

    A use case for using constants with addition/subtraction is not yet
    known, but for completeness the use of constants are supported for all
    operators.

    Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
    ---

    Changes in v3:
    - Remove the limit on the number of constants that can be created,
    per Steven Rostedt

    Changes in v2:
    - Add description of use case for constants in arithmetic
    operations in commit message, per Steven Rostedt
    - Add Namhyung's Reviewed-by

    kernel/trace/trace_events_hist.c | 71 +++++++++++++++++++++++++++++++-
    1 file changed, 70 insertions(+), 1 deletion(-)

    diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
    index f01e442716e2..28f711224944 100644
    --- a/kernel/trace/trace_events_hist.c
    +++ b/kernel/trace/trace_events_hist.c
    @@ -66,7 +66,8 @@
    C(EMPTY_SORT_FIELD, "Empty sort field"), \
    C(TOO_MANY_SORT_FIELDS, "Too many sort fields (Max = 2)"), \
    C(INVALID_SORT_FIELD, "Sort field must be a key or a val"), \
    - C(INVALID_STR_OPERAND, "String type can not be an operand in expression"),
    + C(INVALID_STR_OPERAND, "String type can not be an operand in expression"), \
    + C(EXPECT_NUMBER, "Expecting numeric literal"),

    #undef C
    #define C(a, b) HIST_ERR_##a
    @@ -89,6 +90,7 @@ typedef u64 (*hist_field_fn_t) (struct hist_field *field,
    #define HIST_FIELD_OPERANDS_MAX 2
    #define HIST_FIELDS_MAX (TRACING_MAP_FIELDS_MAX + TRACING_MAP_VARS_MAX)
    #define HIST_ACTIONS_MAX 8
    +#define HIST_CONST_DIGITS_MAX 21

    enum field_op_id {
    FIELD_OP_NONE,
    @@ -152,6 +154,9 @@ struct hist_field {
    bool read_once;

    unsigned int var_str_idx;
    +
    + /* Numeric literals are represented as u64 */
    + u64 constant;
    };

    static u64 hist_field_none(struct hist_field *field,
    @@ -163,6 +168,15 @@ static u64 hist_field_none(struct hist_field *field,
    return 0;
    }

    +static u64 hist_field_const(struct hist_field *field,
    + struct tracing_map_elt *elt,
    + struct trace_buffer *buffer,
    + struct ring_buffer_event *rbe,
    + void *event)
    +{
    + return field->constant;
    +}
    +
    static u64 hist_field_counter(struct hist_field *field,
    struct tracing_map_elt *elt,
    struct trace_buffer *buffer,
    @@ -341,6 +355,7 @@ enum hist_field_flags {
    HIST_FIELD_FL_CPU = 1 << 15,
    HIST_FIELD_FL_ALIAS = 1 << 16,
    HIST_FIELD_FL_BUCKET = 1 << 17,
    + HIST_FIELD_FL_CONST = 1 << 18,
    };

    struct var_defs {
    @@ -1516,6 +1531,12 @@ static void expr_field_str(struct hist_field *field, char *expr)
    {
    if (field->flags & HIST_FIELD_FL_VAR_REF)
    strcat(expr, "$");
    + else if (field->flags & HIST_FIELD_FL_CONST) {
    + char str[HIST_CONST_DIGITS_MAX];
    +
    + snprintf(str, HIST_CONST_DIGITS_MAX, "%llu", field->constant);
    + strcat(expr, str);
    + }

    strcat(expr, hist_field_name(field, 0));

    @@ -1689,6 +1710,15 @@ static struct hist_field *create_hist_field(struct hist_trigger_data *hist_data,
    goto out;
    }

    + if (flags & HIST_FIELD_FL_CONST) {
    + hist_field->fn = hist_field_const;
    + hist_field->size = sizeof(u64);
    + hist_field->type = kstrdup("u64", GFP_KERNEL);
    + if (!hist_field->type)
    + goto free;
    + goto out;
    + }
    +
    if (flags & HIST_FIELD_FL_STACKTRACE) {
    hist_field->fn = hist_field_none;
    goto out;
    @@ -2090,6 +2120,29 @@ static struct hist_field *create_alias(struct hist_trigger_data *hist_data,
    return alias;
    }

    +static struct hist_field *parse_const(struct hist_trigger_data *hist_data,
    + char *str, char *var_name,
    + unsigned long *flags)
    +{
    + struct trace_array *tr = hist_data->event_file->tr;
    + struct hist_field *field = NULL;
    + u64 constant;
    +
    + if (kstrtoull(str, 0, &constant)) {
    + hist_err(tr, HIST_ERR_EXPECT_NUMBER, errpos(str));
    + return NULL;
    + }
    +
    + *flags |= HIST_FIELD_FL_CONST;
    + field = create_hist_field(hist_data, NULL, *flags, var_name);
    + if (!field)
    + return NULL;
    +
    + field->constant = constant;
    +
    + return field;
    +}
    +
    static struct hist_field *parse_atom(struct hist_trigger_data *hist_data,
    struct trace_event_file *file, char *str,
    unsigned long *flags, char *var_name)
    @@ -2100,6 +2153,15 @@ static struct hist_field *parse_atom(struct hist_trigger_data *hist_data,
    unsigned long buckets = 0;
    int ret = 0;

    + if (isdigit(str[0])) {
    + hist_field = parse_const(hist_data, str, var_name, flags);
    + if (!hist_field) {
    + ret = -EINVAL;
    + goto out;
    + }
    + return hist_field;
    + }
    +
    s = strchr(str, '.');
    if (s) {
    s = strchr(++s, '.');
    @@ -4950,6 +5012,8 @@ static void hist_field_debug_show_flags(struct seq_file *m,

    if (flags & HIST_FIELD_FL_ALIAS)
    seq_puts(m, " HIST_FIELD_FL_ALIAS\n");
    + else if (flags & HIST_FIELD_FL_CONST)
    + seq_puts(m, " HIST_FIELD_FL_CONST\n");
    }

    static int hist_field_debug_show(struct seq_file *m,
    @@ -4971,6 +5035,9 @@ static int hist_field_debug_show(struct seq_file *m,
    field->var.idx);
    }

    + if (field->flags & HIST_FIELD_FL_CONST)
    + seq_printf(m, " constant: %llu\n", field->constant);
    +
    if (field->flags & HIST_FIELD_FL_ALIAS)
    seq_printf(m, " var_ref_idx (into hist_data->var_refs[]): %u\n",
    field->var_ref_idx);
    @@ -5213,6 +5280,8 @@ static void hist_field_print(struct seq_file *m, struct hist_field *hist_field)

    if (hist_field->flags & HIST_FIELD_FL_CPU)
    seq_puts(m, "common_cpu");
    + else if (hist_field->flags & HIST_FIELD_FL_CONST)
    + seq_printf(m, "%llu", hist_field->constant);
    else if (field_name) {
    if (hist_field->flags & HIST_FIELD_FL_VAR_REF ||
    hist_field->flags & HIST_FIELD_FL_ALIAS)
    --
    2.33.0.1079.g6e70778dc9-goog
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