[PATCH V7 1/3] tracing: add a possibility of exporting function trace to other places instead of ring buffer only
Chunyan Zhang
zhang.chunyan at linaro.org
Sun Nov 13 18:06:42 PST 2016
Hi Steve,
Resend this since the subject was lost in the prior one due to unknown reason.
On 21 October 2016 at 20:13, Chunyan Zhang <zhang.chunyan at linaro.org> wrote:
> On 18 October 2016 at 23:44, Steven Rostedt <rostedt at goodmis.org> wrote:
>> On Tue, 18 Oct 2016 16:08:58 +0800
>> Chunyan Zhang <zhang.chunyan at 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
>>> one 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 .write() function for writing traces to storage.
>>>
>>> With this patch, only Function trace (trace type is TRACE_FN)
>>> is supported.
>>
>> This is getting better, but I still have some nits.
>>
>
> Thanks.
>
>>>
>>> Signed-off-by: Chunyan Zhang <zhang.chunyan at linaro.org>
>>> ---
>>> include/linux/trace.h | 28 +++++++++++
>>> kernel/trace/trace.c | 132 +++++++++++++++++++++++++++++++++++++++++++++++++-
>>> 2 files changed, 159 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..eb1c5b8
>>> --- /dev/null
>>> +++ b/include/linux/trace.h
>>> @@ -0,0 +1,28 @@
>>> +#ifndef _LINUX_TRACE_H
>>> +#define _LINUX_TRACE_H
>>> +
>>> +#ifdef CONFIG_TRACING
>>> +/*
>>> + * The trace export - an export of Ftrace output. 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 .write() function for writing traces to the storage.
>>> + *
>>> + * next - pointer to the next trace_export
>>> + * write - copy traces which have been delt with ->commit() to
>>> + * the destination
>>> + */
>>> +struct trace_export {
>>> + struct trace_export __rcu *next;
>>> + void (*write)(const char *, unsigned int);
>>
>> Why const char*? Why not const void *? This will never be a string.
>>
>
> Will revise this.
>
>>
>>> +};
>>> +
>>> +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 8696ce6..db94ec1 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 +2129,132 @@ void trace_buffer_unlock_commit_regs(struct trace_array *tr,
>>> ftrace_trace_userstack(buffer, flags, pc);
>>> }
>>>
>>> +static void
>>> +trace_process_export(struct trace_export *export,
>>> + struct ring_buffer_event *event)
>>> +{
>>> + struct trace_entry *entry;
>>> + unsigned int size = 0;
>>> +
>>> + entry = ring_buffer_event_data(event);
>>> +
>>> + size = ring_buffer_event_length(event);
>>> +
>>> + if (export->write)
>>> + export->write((char *)entry, size);
>>
>> Is there ever going to be a time where export->write wont be set?
>
> There hasn't been since only one trace_export (i.e. stm_ftrace) was
> added in this patch-set , I just wanted to make sure the write() has
> been set before registering trace_export like what I added in 2/3 of
> this series.
>
>>
>> And if there is, this can be racy. As in
>>
>>
>> CPU 0: CPU 1:
>> ------ ------
>> if (export->write)
>>
>> export->write = NULL;
>
> Is there going to be this kind of use case? Why some one needs to
> change export->write() rather than register a new trace_export?
>
> I probably haven't understood your point thoroughly, please correct me
> if my guess was wrong.
>
Any further comments? :)
Thanks,
Chunyan
>
> Thanks for the review,
> Chunyan
>
>>
>> export->write(entry, size);
>>
>> BOOM!
>>
>>
>> -- Steve
>>
>>> +}
>>> +
>>> +static DEFINE_MUTEX(ftrace_export_lock);
>>> +
>>> +static struct trace_export __rcu *ftrace_exports_list __read_mostly;
>>> +
>>> +static DEFINE_STATIC_KEY_FALSE(ftrace_exports_enabled);
>>> +
>>> +static inline void ftrace_exports_enable(void)
>>> +{
>>> + static_branch_enable(&ftrace_exports_enabled);
>>> +}
>>> +
>>> +static inline void ftrace_exports_disable(void)
>>> +{
>>> + static_branch_disable(&ftrace_exports_enabled);
>>> +}
>>> +
>>> +void ftrace_exports(struct ring_buffer_event *event)
>>> +{
>>> + struct trace_export *export;
>>> +
>>> + preempt_disable_notrace();
>>> +
>>> + export = rcu_dereference_raw_notrace(ftrace_exports_list);
>>> + while (export) {
>>> + trace_process_export(export, event);
>>> + export = rcu_dereference_raw_notrace(export->next);
>>> + }
>>> +
>>> + 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);
>>> +
>>> + 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 +2273,11 @@ 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(event);
>>> __buffer_unlock_commit(buffer, event);
>>> + }
>>> }
>>>
>>> #ifdef CONFIG_STACKTRACE
>>
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