Re: [PATCH v4 1/5] mm: cma: add trace events to debug physically-contiguous memory allocations

[Stefan Strogin <s.strogin@partner.samsung.com>]

On 17/03/15 10:40, Ingo Molnar wrote:
> 
> * Stefan Strogin <s.strogin@partner.samsung.com> wrote:
> 
>>> +TRACE_EVENT(cma_alloc,
>>> +
>>> +	TP_PROTO(struct cma *cma, struct page *page, int count),
>>> +
>>> +	TP_ARGS(cma, page, count),
>>> +
>>> +	TP_STRUCT__entry(
>>> +		__field(struct page *, page)
>>> +		__field(unsigned long, count)
>>> +	),
>>> +
>>> +	TP_fast_assign(
>>> +		__entry->page = page;
>>> +		__entry->count = count;
>>> +	),
>>> +
>>> +	TP_printk("page=%p pfn=%lu count=%lu",
>>> +		  __entry->page,
>>> +		  __entry->page ? page_to_pfn(__entry->page) : 0,
>>> +		  __entry->count)
> 
> So I'm wondering, the fast-assign side is not equivalent to the 
> TP_printk() side:
> 
>>> +		__entry->page = page;
>>> +		  __entry->page ? page_to_pfn(__entry->page) : 0,
> 
> to me it seems it would be useful if MM tracing standardized on pfn 
> printing. Just like you did for trace_cma_release().
> 

Hello Ingo, thank you for the reply.
I afraid there is no special sense in printing both struct page * and
pfn. But cma_alloc() returns struct page *, cma_release receives struct
page *, and pr_debugs in these functions print struct page *. Maybe it
would be better to print the same here too?

> Also:
> 
>>> +		  __entry->page ? page_to_pfn(__entry->page) : 0,
> 
> pfn 0 should probably be reserved for the true 0th pfn - those exist 
> in some machines. Returning -1ll could be the 'no such pfn' condition?
> 

I took this from trace_mm_page_alloc() and other trace events from
trace/events/kmem.h. If we return -1 here to indicate "no such pfn",
should we change do this in kmem.h too?

>>> +	TP_STRUCT__entry(
>>> +		__field(unsigned long, pfn)
> 
> Btw., does pfn always fit into 32 bits on 32-bit platforms?
> 

Well, I think it does. cma_release() uses 'unsigned long' on all platforms.

>>> +		__field(unsigned long, count)
> 
> Does this have to be 64-bit on 64-bit platforms?
> 

Oops! I'm terribly wrong.
+		__field(unsigned int, count)

I guess it shouldn't be 64-bit on 64-bit platforms. It's the number of
pages being freed, and in cma_release() 'unsigned int' is used for it.

>>> +	),
>>> +
>>> +	TP_fast_assign(
>>> +		__entry->pfn = pfn;
>>> +		__entry->count = count;
>>> +	),
>>> +
>>> +	TP_printk("pfn=%lu page=%p count=%lu",
>>> +		  __entry->pfn,
>>> +		  pfn_to_page(__entry->pfn),
>>> +		  __entry->count)
> 
> So here you print more in the TP_printk() line than in the fast-assign 
> side.
> 

See above, I think it's the same case as in trace_cma_alloc() TP_printk().

> Again I'd double check the various boundary conditions.
> 

Sorry, I don't quite understand. Boundary conditions are already [should
be] checked in cma_alloc()/cma_release, we should only pass to a trace
event the information we want to be known, isn't it so?

I again terribly sorry, I also completely forgot about struct cma *
being passed to trace event. I think either it should be used somehow
(e.g. to print the number of CMA region) or shouldn't be passed...

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