Re: [tip:tracing/ftrace] ftrace: remove struct list_head from struct dyn_ftrace

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Mar 16, 2009 at 01:36:09PM +0100, Frederic Weisbecker wrote:
> On Fri, Mar 13, 2009 at 10:39:36AM +0000, Lai Jiangshan wrote:
> > Commit-ID:  e94142a67f8bad494c593f0a07c9fc2fbec98c0e
> > Gitweb:     http://git.kernel.org/tip/e94142a67f8bad494c593f0a07c9fc2fbec98c0e
> > Author:     Lai Jiangshan <laijs@cn.fujitsu.com>
> > AuthorDate: Fri, 13 Mar 2009 17:51:27 +0800
> > Commit:     Ingo Molnar <mingo@elte.hu>
> > CommitDate: Fri, 13 Mar 2009 11:36:20 +0100
> > 
> > ftrace: remove struct list_head from struct dyn_ftrace
> > 
> > Impact: save memory
> > 
> > The struct dyn_ftrace table is very large, this patch will save
> > about 50%.
> > 
> > Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> > Cc: Steven Rostedt <srostedt@redhat.com>
> > LKML-Reference: <49BA2C9F.8020009@cn.fujitsu.com>
> > Signed-off-by: Ingo Molnar <mingo@elte.hu>
> > 
> > 
> > ---
> >  include/linux/ftrace.h |    1 -
> >  kernel/trace/ftrace.c  |   14 ++++++++------
> >  2 files changed, 8 insertions(+), 7 deletions(-)
> > 
> > diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> > index c146c10..9d598bb 100644
> > --- a/include/linux/ftrace.h
> > +++ b/include/linux/ftrace.h
> > @@ -145,7 +145,6 @@ enum {
> >  };
> >  
> >  struct dyn_ftrace {
> > -	struct list_head	list;
> >  	unsigned long		ip; /* address of mcount call-site */
> >  	unsigned long		flags;
> >  	struct dyn_arch_ftrace	arch;
> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > index bf78a4c..90d5729 100644
> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -272,7 +272,7 @@ enum {
> >  
> >  static int ftrace_filtered;
> >  
> > -static LIST_HEAD(ftrace_new_addrs);
> > +static struct dyn_ftrace *ftrace_new_addrs;
> >  
> >  static DEFINE_MUTEX(ftrace_regex_lock);
> >  
> > @@ -409,8 +409,8 @@ ftrace_record_ip(unsigned long ip)
> >  		return NULL;
> >  
> >  	rec->ip = ip;
> > -
> > -	list_add(&rec->list, &ftrace_new_addrs);
> > +	rec->flags = (unsigned long)ftrace_new_addrs;
> > +	ftrace_new_addrs = rec;
> 
> 
> But... you are overwritting all the ftrace flags infrastructure by
> using it as a list pointer.
> 
> This is breaking dynamic tracing.


May be I'm wrong. May be I misunderstood this patch.
But I don't understand how flags can be used at the same time as a pointer
and as a flag value.

 
> >  
> >  	return rec;
> >  }
> > @@ -716,19 +716,21 @@ unsigned long		ftrace_update_tot_cnt;
> >  
> >  static int ftrace_update_code(struct module *mod)
> >  {
> > -	struct dyn_ftrace *p, *t;
> > +	struct dyn_ftrace *p;
> >  	cycle_t start, stop;
> >  
> >  	start = ftrace_now(raw_smp_processor_id());
> >  	ftrace_update_cnt = 0;
> >  
> > -	list_for_each_entry_safe(p, t, &ftrace_new_addrs, list) {
> > +	while (ftrace_new_addrs) {
> >  
> >  		/* If something went wrong, bail without enabling anything */
> >  		if (unlikely(ftrace_disabled))
> >  			return -1;
> >  
> > -		list_del_init(&p->list);
> > +		p = ftrace_new_addrs;
> > +		ftrace_new_addrs = (struct dyn_ftrace *)p->flags;
> > +		p->flags = 0L;
> >  
> >  		/* convert record (i.e, patch mcount-call with NOP) */
> >  		if (ftrace_code_disable(mod, p)) {
> > --
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