linux-kernel - Re: [PATCH 3/4] gcov: add gcov profiling infrastructure

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Date:	Fri, 5 Jun 2009 02:34:34 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Peter Oberparleiter <oberpar@...ux.vnet.ibm.com>
Cc:	Amerigo Wang <xiyou.wangcong@...il.com>,
	linux-kernel@...r.kernel.org, andi@...stfloor.org,
	ying.huang@...el.com, W.Li@....COM, michaele@....ibm.com,
	mingo@...e.hu, heicars2@...ux.vnet.ibm.com,
	mschwid2@...ux.vnet.ibm.com
Subject: Re: [PATCH 3/4] gcov: add gcov profiling infrastructure

On Fri, 05 Jun 2009 11:23:04 +0200 Peter Oberparleiter <oberpar@...ux.vnet.ibm.com> wrote:

> Amerigo Wang wrote:
> > On Wed, Jun 03, 2009 at 05:26:22PM +0200, Peter Oberparleiter wrote:
> >> Peter Oberparleiter wrote:
> >>> Andrew Morton wrote:
> >>>> On Tue, 02 Jun 2009 13:44:02 +0200
> >>>> Peter Oberparleiter <oberpar@...ux.vnet.ibm.com> wrote:
> >>>>> +	/* Duplicate gcov_info. */
> >>>>> +	active = num_counter_active(info);
> >>>>> +	dup = kzalloc(sizeof(struct gcov_info) +
> >>>>> +		      sizeof(struct gcov_ctr_info) * active, GFP_KERNEL);
> >>>> How large can this allocation be?
> >>> Hm, good question. Having a look at my test system, I see coverage data 
> >>> files of up to 60kb size. With counters making up the largest part of 
> >>> those, I'd guess the allocation size can be around ~55kb. I assume that 
> >>> makes it a candidate for vmalloc?
> >> A further run with debug output showed that the maximum size is
> >> actually around 4k, so in my opinion, there is no need to switch
> >> to vmalloc.
> > 
> > Unless you want virtually continious memory, you don't need to
> > bother vmalloc().
> > 
> > kmalloc() and get_free_pages() are all fine for this.
> 
> kmalloc() requires contiguous pages to serve an allocation request 
> larger than a single page. The longer a kernel runs, the more fragmented 
> the pool of free pages gets and the probability to find enough 
> contiguous free pages is significantly reduced.
> 
> In this case (having had a 3rd look), I found allocations of up to 
> ~50kb, so to be sure, I'll switch that particular allocation to vmalloc().

Well, vmalloc() isn't magic.  It can suffer internal fragmentation of
the fixed-sized virtual address arena.

Is it possible to redo the data structures so that the large array
isn't needed?  Use a list, or move the data elsewhere or such?

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