Messages in this thread | | | Subject | Re: [PATCH 2/3] DEBUGFS: Add per cpu counters | From | Steven Rostedt <> | Date | Tue, 13 Dec 2011 18:08:17 -0500 |
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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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