Re: [Patch] mm tracepoints update - use case.

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> Larry Woodman wrote:
> 
> >> - Please don't display mm and/or another kernel raw pointer.
> >>   if we assume non stop system, we can't use kernel-dump. Thus kernel pointer
> >>   logging is not so useful.
> > 
> > OK, I just dont know how valuable the trace output is with out some raw
> > data like the mm_struct.
> 
> I believe that we do want something like the mm_struct in
> the trace info, so we can figure out which process was
> allocating pages, etc...

Yes.
I think we need to print tgid, it is needed to imporove CONFIG_MM_OWNER.
current CONFIG_MM_OWNER back-pointer point to semi-random task_struct.


> >> - Please consider how do this feature works on mem-cgroup.
> >>   (IOW, please don't ignore many "if (scanning_global_lru())")
> 
> Good point, we want to trace cgroup vs non-cgroup reclaims,
> too.

thank you.

> 
> >> - tracepoint caller shouldn't have any assumption of displaying representation.
> >>   e.g.
> >>     wrong)  trace_mm_pagereclaim_pgout(mapping, page->index<<PAGE_SHIFT, PageAnon(page));
> >>     good)   trace_mm_pagereclaim_pgout(mapping, page)
> > 
> > OK.
> > 
> >>   that's general and good callback and/or hook manner.
> 
> How do we figure those out from the page pointer at the time
> the tracepoint triggers?
> 
> I believe that it would be useful to export that info in the
> trace point, since we cannot expect the userspace trace tool
> to figure out these things from the struct page address.
> 
> Or did I overlook something here?

current, TRACE_EVENT have two step information trasformation.

 - step1 - TP_fast_assign()
   it is called from tracepoint directly. it makes ring-buffer representaion.
 - step2 - TP_printk
   it is called when reading debug/tracing/trace file. it makes printable
   representation from ring-buffer data.

example:

trace_sched_switch() has three argument, rq, prev, next.

--------------------------------------------------
static inline void
context_switch(struct rq *rq, struct task_struct *prev,
               struct task_struct *next)
{
(snip)
        trace_sched_switch(rq, prev, next);

-------------------------------------------------

TP_fast_assing extract data from argument pointer.
-----------------------------------------------------
        TP_fast_assign(
                memcpy(__entry->next_comm, next->comm, TASK_COMM_LEN);
                __entry->prev_pid       = prev->pid;
                __entry->prev_prio      = prev->prio;
                __entry->prev_state     = prev->state;
                memcpy(__entry->prev_comm, prev->comm, TASK_COMM_LEN);
                __entry->next_pid       = next->pid;
                __entry->next_prio      = next->prio;
        ),
-----------------------------------------------------


I think mm tracepoint can do the same way.




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