Messages in this thread | | | Date | Mon, 26 Jan 2009 15:55:13 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 1/2 v2] tracing/function-graph-tracer: various fixes and features |
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* Frederic Weisbecker <fweisbec@gmail.com> wrote:
> On Fri, Jan 23, 2009 at 12:00:37PM +0100, Ingo Molnar wrote: > > > > * Frédéric Weisbecker <fweisbec@gmail.com> wrote: > > > > > > Still needs a solution - if we do cross-CPU traces we want to have a > > > > global trace clock with 'seemless' transition between CPUs. > > > > > > So it doesn't only need a monotonic clock. It needs a global consistent > > > clock like ktime for example? Unfortunately this one uses seq_locks and > > > would add some drawbacks like verifying if the traced function doesn't > > > hold the write seq_lock and it will bring some more ftrace recursion... > > > > using ktime_get() is indeed out of question - GTOD callpaths are too > > complex (and also too slow). > > > > I'd not change anything in the current logic, but i was thinking of a new > > trace_option, which can be set optionally. If that trace option is set > > then this bit of ring_buffer_time_stamp(): > > > > time = sched_clock() << DEBUG_SHIFT; > > > > gets turned into: > > > > time = cpu_clock(cpu) << DEBUG_SHIFT; > > > > This way we default to sched_clock(), but also gain some 'global' > > properties if the trace_option is set. > > > Ok, yeah that's a good idea. > > > > Furthermore, another trace_option could introduce a third 'strongly > > ordered' trace-clock variant, which would use cmpxchg and per cpu > > timestamps, something like this: > > > > atomic64_t curr_time; > > > > DEFINE_PER_CPU(u64, prev_cpu_time); > > ... > > > > retry: > > prev_cpu_time = per_cpu(prev_cpu_time, cpu); > > cpu_time = sched_clock(); > > old_time = atomic64_read(&curr_time); > > > > delta = cpu_time - prev_cpu_time; > > if (unlikely((s64)delta <= 0)) > > delta = 1; > > > > new_time = old_time + delta; > > > > if (atomic64_cmpxchg(&curr_time, old_time, new_time) != new_time) > > goto repeat; > > > > time = new_time << DEBUG_SHIFT; > > > > This would be a monotonic, global clock wrapped around sched_clock(). It > > uses a cmpxchg to achieve it, but we have to use global ordering anyway. > > > > It would still be _much_ faster than any GTOD clocksource we have. > > > > Hm? > > > > And that would be even more faster that cpu_clock(). > > But why implement both? Wouldn't the above be more faster while playing > the same thing than cpu_clock()
cpu_clock() can be faster than a cmpxchg on a global variable - as cpu_clock() does not serialize globally at all as long as each CPU observes its own time only.
in any case, these are nuances - we need _some_ trace clock before we worry about details like that ;-)
Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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