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

From: KOSAKI Motohiro
Date: Sun Jun 21 2009 - 23:38:20 EST

> 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...

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.


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)
trace_sched_switch(rq, prev, next);


TP_fast_assing extract data from argument pointer.
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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