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SubjectRe: [PATCH 5/6] tracing: reduce latency and remove percpu trace_seq
Frederic Weisbecker wrote:
> On Tue, Jan 19, 2010 at 03:34:22PM +0800, Lai Jiangshan wrote:
>> __print_flags() and __print_symbolic() use percpu trace_seq:
>>
>> 1) Its memory is preallocated, it wastes memory when we don't use tracing.
>> 2) It wastes memory for multi-cpus system.
>> 3) It disables preemption when it executes its core routine
>> "trace_seq_printf(s, "%s: ", #call);" and introduce latency
>> for more important process.
>>
>> So we move this trace_seq to struct trace_iterator.
>>
>> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
>> ---
>> diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
>> index be9ece5..348500d 100644
>> --- a/include/linux/ftrace_event.h
>> +++ b/include/linux/ftrace_event.h
>> @@ -12,9 +12,6 @@ struct dentry;
>>
>> #define FTRACE_SEQ_BUFSIZE PAGE_SIZE
>>
>> -DECLARE_PER_CPU(struct trace_seq, ftrace_event_seq);
>> -DECLARE_PER_CPU(unsigned char[FTRACE_SEQ_BUFSIZE], ftrace_event_buffer);
>> -
>> struct trace_print_flags {
>> unsigned long mask;
>> const char *name;
>> @@ -60,6 +57,10 @@ struct trace_iterator {
>> struct trace_seq seq;
>> unsigned char buffer[FTRACE_SEQ_BUFSIZE];
>>
>> + /* trace_seq for __print_flags() and __print_symbolic() */
>> + struct trace_seq tmp_seq;
>> + unsigned char tmp_buffer[FTRACE_SEQ_BUFSIZE];
>
>
>
>
> Well, I don't like much that because it's a temporary buffer
> in trace iter only used by few events.
> But the problem is indeed tricky.
>
> May be should we use a kmalloc in raw_output?
>

But we have to preallocate it before raw_output().
a kmalloc in raw_output make ftrace_dump() unhappy.

At real system, tracepoints are used more frequently,
So it is not "only used by few events."

But maybe FTRACE_SEQ_BUFSIZE is too large, 128 is enough.


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