Messages in this thread | | | Date | Sat, 6 Apr 2013 13:05:07 +0200 | From | Richard Cochran <> | Subject | Re: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples |
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On Fri, Apr 05, 2013 at 07:16:53PM +0100, Pawel Moll wrote:
> Ok, so how about the code below? Disclaimer: this is just a proposal. > I'm not sure how welcomed would be an extra field in struct file, but > this makes the clocks ultimately flexible - one can "attach" the clock > to any arbitrary struct file. Alternatively we could mark a "clocked" > file with a special flag in f_mode and have some kind of lookup.
Only a tiny minority of file instances will want to be clocks. Therefore I think adding the extra field will be a hard sell.
The flag idea sounds harmless, but how do you perform the lookup?
> Also, I can't stop thinking that the posix-clock.c shouldn't actually do > anything about the character device... The PTP core (as the model of > using character device seems to me just one of possible choices) could > do this on its own and have simple open/release attaching/detaching the > clock. This would remove a lot of "generic dev" code in the > posix-clock.c and all the optional cdev methods in struct posix_clock. > It's just a thought, though...
Right, the chardev could be pushed into the PHC layer. The original idea of chardev clocks did have precedents, though, like hpet and rtc.
> And a couple of questions to Richard... Isn't the kref_put() in > posix_clock_unregister() a bug? I'm not 100% but it looks like a simple > register->unregister sequence was making the ref count == -1, so the > delete_clock() won't be called.
Well,
posix_clock_register() -> kref_init() -> atomic_set(&kref->refcount, 1);
So refcount is now 1 ...
posix_clock_unregister() -> kref_put() -> kref_sub(count=1) -> atomic_sub_and_test((int) count, &kref->refcount)
and refcount is now 0. Can't see how you would get -1 here.
> And was there any particular reason that the ops in struct > posix_clock are *not* a pointer?
One less run time indirection maybe? I don't really remember why or how we arrived at this. The whole PHC review took a year, with something like fifteen revisions.
Thanks, Richard
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