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Message-ID: <CAG2=9p_SKF4TAbbqF6L4u=y9-_m73vZJ3tOAbRKHCT0MOsLwGQ@mail.gmail.com>
Date: Fri, 21 Oct 2016 20:13:13 +0800
From: Chunyan Zhang <zhang.chunyan@...aro.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Poirier <mathieu.poirier@...aro.org>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
mingo@...hat.com, Mike Leach <mike.leach@....com>,
Tor Jeremiassen <tor@...com>, philippe.langlais@...com,
Nicolas GUION <nicolas.guion@...com>,
Felipe Balbi <felipe.balbi@...ux.intel.com>,
Lyra Zhang <zhang.lyra@...il.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH V7 1/3] tracing: add a possibility of exporting function
trace to other places instead of ring buffer only
On 18 October 2016 at 23:44, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Tue, 18 Oct 2016 16:08:58 +0800
> Chunyan Zhang <zhang.chunyan@...aro.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@...aro.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.
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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