Re: [PATCH v3 1/8] tracing: Add support for creating hist trigger variables from literal

[Kalesh Singh <kaleshsingh@google.com>]

On Mon, Oct 25, 2021 at 12:23 PM Kalesh Singh <kaleshsingh@google.com> wrote:
>
> 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>
> Change-Id: I142121d28dc3475dfbc3a882e7b2368d833474eb

Ahh. Sorry, I forgot to remove these Change-Id tags, I'll resend
another version.

> ---
>
> 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
>