Messages in this thread | | | Date | Tue, 15 Jan 2008 22:17:30 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles |
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* Steven Rostedt (rostedt@goodmis.org) wrote: > > On Tue, 15 Jan 2008, Mathieu Desnoyers wrote: > > > > Ok, but what actually insures that the clock->cycle_* reads won't be > > reordered across the clocksource_read() ? > > <looks at code> > > Hmm, interesting.I didn't notice that clocksource_read() is a static > inline. I was thinking that since it was passing a pointer to a function, > gcc could not assume that it could move that code across it. But now > looking to see that clocksource_read is simply a static inline that does: > > cs->read(); > > But still, can gcc assume that it can push loads of unknown origin > variables across function calls? So something like: > > static int *glob; > > void foo(void) { > int x; > > x = *glob; > > bar(); > > if (x != *glob) > /* ... */ > } > > I can't see how any compiler could honestly move the loading of the first > x after the calling of bar(). With glob pointing to some unknown > variable, that may be perfectly fine for bar to modify. > > > > > > > > > > > + cycle_raw = clock->cycle_raw; > > > > > + cycle_last = clock->cycle_last; > > > > > + > > > > > + /* read clocksource: */ > > > > > + cycle_now = clocksource_read(clock); > > So the question here is,can cycle_raw and cycle_last be loaded from > the unknown source that clock points to after the call to > clocksource_read()? > > I'm thinking not. >
I agree with you that I don't see how the compiler could reorder this. So we forget about compiler barriers. Also, the clock source used is a synchronized clock source (get_cycles_sync on x86_64), so it should make sure the TSC is read at the right moment.
However, what happens if the clock source is, say, the jiffies ?
Is this case, we have :
static cycle_t jiffies_read(void) { return (cycle_t) jiffies; }
Which is nothing more than a memory read of
extern unsigned long volatile __jiffy_data jiffies;
I think it is wrong to assume that reads from clock->cycle_raw and from jiffies will be ordered correctly in SMP. I am tempted to think that ordering memory writes to clock->cycle_raw vs jiffies is also needed in this case (where clock->cycle_raw is updated, or where jiffies is updated).
We can fall in the same kind of issue if we read the HPET, which is memory I/O based. It does not seems correct to assume that MMIO vs normal memory reads are ordered. (pointing back to this article : http://lwn.net/Articles/198988/)
Mathieu
> > > > > + > > > > > + /* calculate the delta since the last update_wall_time: */ > > > > > + cycle_delta = (cycle_now - cycle_last) & clock->mask; > > > > > + > > > > > + } while (cycle_raw != clock->cycle_raw || > > > > > + cycle_last != clock->cycle_last); > > > > > + > > > > > + return cycle_raw + cycle_delta; > > > > > +} > > > -- Steve >
-- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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