[PATCH V4 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
Wed Aug 17 05:48:07 PDT 2016
Hello Steve,
Please correct me if I'm missing something in my following reply.
On 15 August 2016 at 22:28, Steven Rostedt <rostedt at goodmis.org> wrote:
> On Mon, 15 Aug 2016 19:50:01 +0800
> Chunyan Zhang <zhang.chunyan at linaro.org> wrote:
>
>> +int register_trace_export(struct trace_export *export);
>> +int unregister_trace_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..0247ac2 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 +2129,127 @@ void trace_buffer_unlock_commit_regs(struct trace_array *tr,
>> ftrace_trace_userstack(buffer, flags, pc);
>> }
>>
>> +static inline void
>> +trace_generic_commit(struct trace_array *tr,
>> + struct ring_buffer_event *event)
>
> Why is this marked inline? It is never called directly here.
>
>> +{
>> + struct trace_entry *entry;
>> + struct trace_export *export = tr->export;
>> + unsigned int size = 0;
>> +
>> + entry = ring_buffer_event_data(event);
>> +
>> + trace_entry_size(size, entry->type);
>> + if (!size)
>> + return;
>> +
>> + if (export->write)
>> + export->write((char *)entry, size);
>> +}
>> +
>> +static inline void
>
> Same here.
>
>> +trace_rb_commit(struct trace_array *tr,
>> + struct ring_buffer_event *event)
>> +{
>> + __buffer_unlock_commit(tr->trace_buffer.buffer, event);
>> +}
>> +
>> +static DEFINE_MUTEX(trace_export_lock);
>> +
>> +static struct trace_export trace_export_rb __read_mostly = {
>> + .name = "rb",
>> + .commit = trace_rb_commit,
>
> Make sure equal signs are lined up.
>
>> + .next = NULL,
>> +};
>> +static struct trace_export *trace_exports_list __read_mostly = &trace_export_rb;
>
> trace_exports_list needs to be annotated as rcu, if you are going to
> dereference it via rcu_dereference. That was the kbuild warning you
> received.
Ok, will do.
But I guess 'ftrace_ops_list' in ftrace.c [1] also should be annotated as rcu?
[1] http://lxr.free-electrons.com/source/kernel/trace/ftrace.c#L460
>
>> +
>> +inline void
>> +trace_exports(struct trace_array *tr, struct ring_buffer_event *event)
>> +{
>> + struct trace_export *export;
>> +
>> + preempt_disable_notrace();
>> +
>> + for (export = rcu_dereference_raw_notrace(trace_exports_list);
>> + export && export->commit;
>> + export = rcu_dereference_raw_notrace(export->next)) {
>> + tr->export = export;
>> + export->commit(tr, event);
>> + }
>> +
>> + preempt_enable_notrace();
>> +}
>
> I'm not too happy about the added overhead to normal function tracing
> (it will be noticeable), but I can fix that later.
I think I will try to revise here in the next revision.
>
>> +
>> +static void
>> +add_trace_export(struct trace_export **list, struct trace_export *export)
>> +{
>> + export->next = *list;
>
> As export->next will most likely need to be marked __rcu as well, this
> may need an rcu_assign_pointer() too.
>
>> + /*
>> + * 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 int
>> +rm_trace_export(struct trace_export **list, struct trace_export *export)
>> +{
>> + struct trace_export **p;
>> +
>> + for (p = list; *p != &trace_export_rb; p = &(*p)->next)
>> + if (*p == export)
>> + break;
>> +
>> + if (*p != export)
>> + return -1;
>> +
>> + *p = (*p)->next;
>
> I believe this requires an rcu_assign_pointer() as well.
>
>> +
>> + return 0;
>> +}
>> +
>> +int register_trace_export(struct trace_export *export)
>> +{
>> + if (!export->write) {
>> + pr_warn("trace_export must have the write() call back.\n");
>
> Probably should be a "WARN_ON_ONCE()".
Yes, will revise.
>
>> + return -1;
>> + }
>> +
>> + if (!export->name) {
>> + pr_warn("trace_export must have a name.\n");
>> + return -1;
>> + }
>
> If name isn't used don't add it till it is. That is, this patch should
> not have "name" in the structure. It's confusing, because I don't see a
> purpose for it.
Ok will remove that.
>
>> +
>> + mutex_lock(&trace_export_lock);
>> +
>> + export->tr = trace_exports_list->tr;
>
> I don't see where tr is ever assigned.
>
>> + export->commit = trace_generic_commit;
>
> Shouldn't the caller pass in the commit function too?
The trace_export::write() callback is for caller, commit function
mainly deal with traces, is it better to keep 'trace_generic_commit'
in trace.c, i.e don't expose it to callers?
>
>> +
>> + add_trace_export(&trace_exports_list, export);
>> +
>> + mutex_unlock(&trace_export_lock);
>> +
>> + return 0;
>> +}
>> +EXPORT_SYMBOL_GPL(register_trace_export);
>> +
>> +int unregister_trace_export(struct trace_export *export)
>> +{
>> + int ret;
>> +
>> + mutex_lock(&trace_export_lock);
>> +
>> + ret = rm_trace_export(&trace_exports_list, export);
>> +
>> + mutex_unlock(&trace_export_lock);
>> +
>> + return ret;
>> +}
>> +EXPORT_SYMBOL_GPL(unregister_trace_export);
>> +
>> void
>> trace_function(struct trace_array *tr,
>> unsigned long ip, unsigned long parent_ip, unsigned long flags,
>> @@ -2147,7 +2269,7 @@ trace_function(struct trace_array *tr,
>> entry->parent_ip = parent_ip;
>>
>> if (!call_filter_check_discard(call, entry, buffer, event))
>
> How do you handle the discard? If the entry doesn't match the filter,
> it will try to discard the event. I don't see how the "trace_exports"
> handle that.
I directly used the entries which had already been filtered.
Humm.. sorry, actually you lost me here.
>
>
>> - __buffer_unlock_commit(buffer, event);
>> + trace_exports(tr, event);
>> }
>>
>> #ifdef CONFIG_STACKTRACE
>> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
>> index f783df4..a40f07c 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 {
>> @@ -301,6 +302,13 @@ static inline struct trace_array *top_trace_array(void)
>> break; \
>> }
>>
>> +#undef IF_SIZE
>> +#define IF_SIZE(size, var, etype, id) \
>> + if (var == id) { \
>> + size = (sizeof(etype)); \
>> + break; \
>> + }
>> +
>> /* Will cause compile errors if type is not found. */
>> extern void __ftrace_bad_type(void);
>>
>> @@ -339,6 +347,29 @@ extern void __ftrace_bad_type(void);
>> } while (0)
>>
>> /*
>> + * The trace_entry_size return the size of specific trace type
>> + *
>> + * IF_SIZE(size, var);
>> + *
>> + * Where "var" is just the given trace type.
>> + */
>> +#define trace_entry_size(size, var) \
>> + do { \
>> + IF_SIZE(size, var, struct ftrace_entry, TRACE_FN); \
>> + IF_SIZE(size, var, struct stack_entry, TRACE_STACK); \
>> + IF_SIZE(size, var, struct userstack_entry, \
>> + TRACE_USER_STACK); \
>> + IF_SIZE(size, var, struct print_entry, TRACE_PRINT); \
>> + IF_SIZE(size, var, struct bprint_entry, TRACE_BPRINT); \
>> + IF_SIZE(size, var, struct bputs_entry, TRACE_BPUTS); \
>> + IF_SIZE(size, var, struct trace_branch, TRACE_BRANCH); \
>> + IF_SIZE(size, var, struct ftrace_graph_ent_entry, \
>> + TRACE_GRAPH_ENT); \
>> + IF_SIZE(size, var, struct ftrace_graph_ret_entry, \
>> + TRACE_GRAPH_RET); \
>
> I really really dislike this. This is a big if statement that all
> needs to go down one by one. Very inefficient!
Ok, will change to other implementation.
Thanks for your comments,
Chunyan
>
> -- Steve
>
>> + } while (0)
>> +
>> +/*
>> * An option specific to a tracer. This is a boolean value.
>> * The bit is the bit index that sets its value on the
>> * flags value in struct tracer_flags.
>
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