linux-kernel - Re: [PATCH 2/3] DEBUGFS: Add per cpu counters

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Date:	Tue, 13 Dec 2011 18:08:17 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <andi@...stfloor.org>, Greg KH <greg@...ah.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH 2/3] DEBUGFS: Add per cpu counters

On Tue, 2011-12-13 at 14:56 -0800, Andi Kleen wrote:
>  
> > > +struct debugfs_counter {
> > > +	unsigned __percpu *ptr;
> > > +	const char *fn;
> > > +	const char *name;
> > > +} __attribute__((aligned(sizeof(char *))));
> > > +

> > 
> > See commit 654986462 for details.
> 
> Doesn't give a lot of details actually. Which target?
> 
> Note that my structure only has pointers, so there is not a lot 
> of potential for "evil" alignment.

We hit this with trace events as well, and your structure does have an
evil alignment, it's 3 pointers, which is not a base 2 number. Thus,
there were some compilers that liked to add padding to make the
alignment a power of 2. That is, between two sections of files we ended
up with something like:


.section file1
.ptra1
.ptra2
.ptra3
.ptrb1
.ptrb2
.ptrb3
.ptrc1
.ptrc2
.ptrc3
.ptrd1
.ptrd2
.ptrd3
.ptre1
.ptre2
.ptre3
<space>
.section file2
.ptrf1
.ptrf2
.ptrf3
[...]



It didn't happen often, heck, trace events and tracepoints were like
this for sometime before this blew up in our faces.

>From what I've seen is that 1 or 2 longs will pack nicely, but anything
else (except for maybe 4, 8, 16, etc) will run a risk of crashing.

-- Steve


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