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Message-ID: <20161018114418.3445c390@gandalf.local.home>
Date: Tue, 18 Oct 2016 11:44:18 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Chunyan Zhang <zhang.chunyan@...aro.org>
Cc: mathieu.poirier@...aro.org, alexander.shishkin@...ux.intel.com,
mingo@...hat.com, mike.leach@....com, tor@...com,
philippe.langlais@...com, nicolas.guion@...com,
felipe.balbi@...ux.intel.com, zhang.lyra@...il.com,
linux-kernel@...r.kernel.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 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.
>
> 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.
> +};
> +
> +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?
And if there is, this can be racy. As in
CPU 0: CPU 1:
------ ------
if (export->write)
export->write = NULL;
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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