Re: [PATCHV5 1/3] tracing: add a possibility of exporting function trace to other places instead of ring buffer only

[Chunyan Zhang <zhang.chunyan@linaro.org>]

On 30 August 2016 at 22:30, Steven Rostedt <rostedt@goodmis.org> wrote:
> On Tue, 30 Aug 2016 16:07:28 +0800
> Chunyan Zhang <zhang.chunyan@linaro.org> wrote:
>
>> Currently Function traces can be only exported to ring buffer, this
>> patch added trace_export concept which can process traces and export
>> them to a registered destination as an addition to the current only
>> output of Ftrace - i.e. ring buffer.
>>
>> In this way, if we want Function traces to be sent to other destination
>> rather than ring buffer only, we just need to register a new
>> trace_export and implement its own .commit() callback or just use
>> 'trace_generic_commit()' which this patch also added and hooks up its
>> own .write() function for writing traces to the storage.
>>
>> With this patch, only Function trace (trace type is TRACE_FN)
>> is supported.
>>
>> Signed-off-by: Chunyan Zhang <zhang.chunyan@linaro.org>
>> ---
>>  include/linux/trace.h |  35 ++++++++++++
>>  kernel/trace/trace.c  | 155 +++++++++++++++++++++++++++++++++++++++++++++++++-
>>  kernel/trace/trace.h  |   1 +
>>  3 files changed, 190 insertions(+), 1 deletion(-)
>>  create mode 100644 include/linux/trace.h
>>
>> diff --git a/include/linux/trace.h b/include/linux/trace.h
>> new file mode 100644
>> index 0000000..30ded92
>> --- /dev/null
>> +++ b/include/linux/trace.h
>> @@ -0,0 +1,35 @@
>> +#ifndef _LINUX_TRACE_H
>> +#define _LINUX_TRACE_H
>> +
>> +#include <linux/ring_buffer.h>
>> +struct trace_array;
>> +
>> +#ifdef CONFIG_TRACING
>> +/*
>> + * The trace export - an export of Ftrace. The trace_export can process
>> + * traces and export them to a registered destination as an addition to
>> + * the current only output of Ftrace - i.e. ring buffer.
>> + *
>> + * If you want traces to be sent to some other place rather than
>> + * ring buffer only, just need to register a new trace_export and
>> + * implement its own .commit() callback or just directly use
>> + * 'trace_generic_commit()' and hooks up its own .write() function
>> + * for writing traces to the storage.
>> + *
>> + * next              - pointer to the next trace_export
>> + * commit    - commit the traces to the destination
>> + * write     - copy traces which have been delt with ->commit() to
>> + *             the destination
>> + */
>> +struct trace_export {
>> +     struct trace_export __rcu       *next;
>> +     void (*commit)(struct trace_array *, struct ring_buffer_event *);
>> +     void (*write)(const char *, unsigned int);
>> +};
>> +
>> +int register_ftrace_export(struct trace_export *export);
>> +int unregister_ftrace_export(struct trace_export *export);
>> +
>> +#endif       /* CONFIG_TRACING */
>> +
>> +#endif       /* _LINUX_TRACE_H */
>> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>> index dade4c9..3163fa6 100644
>> --- a/kernel/trace/trace.c
>> +++ b/kernel/trace/trace.c
>> @@ -40,6 +40,7 @@
>>  #include <linux/poll.h>
>>  #include <linux/nmi.h>
>>  #include <linux/fs.h>
>> +#include <linux/trace.h>
>>  #include <linux/sched/rt.h>
>>
>>  #include "trace.h"
>> @@ -2128,6 +2129,155 @@ void trace_buffer_unlock_commit_regs(struct trace_array *tr,
>>       ftrace_trace_userstack(buffer, flags, pc);
>>  }
>>
>> +static DEFINE_STATIC_KEY_FALSE(ftrace_exports_enabled);
>> +
>> +static void ftrace_exports_enable(void)
>> +{
>> +     static_branch_enable(&ftrace_exports_enabled);
>> +}
>> +
>> +static void ftrace_exports_disable(void)
>> +{
>> +     static_branch_disable(&ftrace_exports_enabled);
>> +}
>> +
>> +static size_t trace_size[] = {
>> +     [TRACE_FN]              = sizeof(struct ftrace_entry),
>> +     [TRACE_CTX]             = sizeof(struct ctx_switch_entry),
>> +     [TRACE_WAKE]            = sizeof(struct ctx_switch_entry),
>> +     [TRACE_STACK]           = sizeof(struct stack_entry),
>> +     [TRACE_PRINT]           = sizeof(struct print_entry),
>> +     [TRACE_BPRINT]          = sizeof(struct bprint_entry),
>> +     [TRACE_MMIO_RW]         = sizeof(struct trace_mmiotrace_rw),
>> +     [TRACE_MMIO_MAP]        = sizeof(struct trace_mmiotrace_map),
>> +     [TRACE_BRANCH]          = sizeof(struct trace_branch),
>> +     [TRACE_GRAPH_RET]       = sizeof(struct ftrace_graph_ret_entry),
>> +     [TRACE_GRAPH_ENT]       = sizeof(struct ftrace_graph_ent_entry),
>> +     [TRACE_USER_STACK]      = sizeof(struct userstack_entry),
>> +     [TRACE_BPUTS]           = sizeof(struct bputs_entry),
>> +};
>> +
>> +static void
>> +trace_generic_commit(struct trace_array *tr,
>> +            struct ring_buffer_event *event)
>> +{
>> +     struct trace_entry *entry;
>> +     struct trace_export *export = tr->export;
>
> Why not just pass in the export directly?
>
>> +     unsigned int size = 0;
>> +
>> +     entry = ring_buffer_event_data(event);
>> +
>> +     size = trace_size[entry->type];
>
> BTW, it would make much more sense if you just did:
>
>         size = ring_buffer_event_length(event);
>
> and get rid of that silly array, as that is prone to bugs if you add a
> new type.
>
>> +     if (!size)
>> +             return;
>> +
>> +     if (export && export->write)
>> +             export->write((char *)entry, size);
>> +}
>> +
>> +static DEFINE_MUTEX(ftrace_export_lock);
>> +
>> +static struct trace_export __rcu *ftrace_exports_list __read_mostly;
>> +
>> +static inline void
>> +ftrace_exports(struct trace_array *tr, struct ring_buffer_event *event)
>> +{
>> +     struct trace_export *export;
>> +
>> +     preempt_disable_notrace();
>> +
>> +     for (export = rcu_dereference_raw_notrace(ftrace_exports_list);
>> +          export && export->commit;
>> +          export = rcu_dereference_raw_notrace(export->next)) {
>> +             tr->export = export;
>> +             export->commit(tr, event);
>
> Um, what's the commit for?
>
> Why not just call the trace_generic_commit() above. Nothing has

The original thought was that allow other export (maybe added in the
future) to implement its own .commit().
But I accept your suggestion since there's no other trace_export so
far rather than stm_ftrace only, the modification should be kept as
simple as it can be.

Thanks for your review,
Chunyan

> anything different. But change that name. Maybe call it
> "process_export()" or something. And pass the export directly and get
> rid of the tr->export.
>
>> +     }
>> +
>> +     preempt_enable_notrace();
>> +}
>> +
>> +static inline void
>> +add_trace_export(struct trace_export **list, struct trace_export *export)
>> +{
>> +     rcu_assign_pointer(export->next, *list);
>> +     /*
>> +      * We are entering export into the list but another
>> +      * CPU might be walking that list. We need to make sure
>> +      * the export->next pointer is valid before another CPU sees
>> +      * the export pointer included into the list.
>> +      */
>> +     rcu_assign_pointer(*list, export);
>> +}
>> +
>> +static inline int
>> +rm_trace_export(struct trace_export **list, struct trace_export *export)
>> +{
>> +     struct trace_export **p;
>> +
>> +     for (p = list; *p != NULL; p = &(*p)->next)
>> +             if (*p == export)
>> +                     break;
>> +
>> +     if (*p != export)
>> +             return -1;
>> +
>> +     rcu_assign_pointer(*p, (*p)->next);
>> +
>> +     return 0;
>> +}
>> +
>> +static inline void
>> +add_ftrace_export(struct trace_export **list, struct trace_export *export)
>> +{
>> +     if (*list == NULL)
>> +             ftrace_exports_enable();
>> +
>> +     add_trace_export(list, export);
>> +}
>> +
>> +static inline int
>> +rm_ftrace_export(struct trace_export **list, struct trace_export *export)
>> +{
>> +     int ret;
>> +
>> +     ret = rm_trace_export(list, export);
>> +     if (*list == NULL)
>> +             ftrace_exports_disable();
>> +
>> +     return ret;
>> +}
>> +
>> +int register_ftrace_export(struct trace_export *export)
>> +{
>> +     if (WARN_ON_ONCE(!export->write))
>> +             return -1;
>> +
>> +     mutex_lock(&ftrace_export_lock);
>> +
>> +     export->commit = trace_generic_commit;
>> +
>> +     add_ftrace_export(&ftrace_exports_list, export);
>> +
>> +     mutex_unlock(&ftrace_export_lock);
>> +
>> +     return 0;
>> +}
>> +EXPORT_SYMBOL_GPL(register_ftrace_export);
>> +
>> +int unregister_ftrace_export(struct trace_export *export)
>> +{
>> +     int ret;
>> +
>> +     mutex_lock(&ftrace_export_lock);
>> +
>> +     ret = rm_ftrace_export(&ftrace_exports_list, export);
>> +
>> +     mutex_unlock(&ftrace_export_lock);
>> +
>> +     return ret;
>> +}
>> +EXPORT_SYMBOL_GPL(unregister_ftrace_export);
>> +
>>  void
>>  trace_function(struct trace_array *tr,
>>              unsigned long ip, unsigned long parent_ip, unsigned long flags,
>> @@ -2146,8 +2296,11 @@ trace_function(struct trace_array *tr,
>>       entry->ip                       = ip;
>>       entry->parent_ip                = parent_ip;
>>
>> -     if (!call_filter_check_discard(call, entry, buffer, event))
>> +     if (!call_filter_check_discard(call, entry, buffer, event)) {
>> +             if (static_branch_unlikely(&ftrace_exports_enabled))
>> +                     ftrace_exports(tr, event);
>>               __buffer_unlock_commit(buffer, event);
>
> We do the generic commit here.
>
> -- Steve
>
>> +     }
>>  }
>>
>>  #ifdef CONFIG_STACKTRACE
>> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
>> index f783df4..26a3088 100644
>> --- a/kernel/trace/trace.h
>> +++ b/kernel/trace/trace.h
>> @@ -260,6 +260,7 @@ struct trace_array {
>>       /* function tracing enabled */
>>       int                     function_enabled;
>>  #endif
>> +     struct trace_export     *export;
>>  };
>>
>>  enum {
>