Daniel, On Sun, Jun 15, 2014 at 2:18 PM, Daniel Lezcano <daniel.lezcano@xxxxxxxxxx> wrote: > On 06/04/2014 07:30 PM, Doug Anderson wrote: >> >> In (93bfb76 clocksource: exynos_mct: register sched_clock callback) we >> supported using the MCT as a scheduler clock. We properly marked >> exynos4_read_sched_clock() as notrace. However, we then went and >> called another function that _wasn't_ notrace. That means if you do: >> >> cd /sys/kernel/debug/tracing/ >> echo function_graph > current_tracer >> >> You'll get a crash. >> >> Fix this (but still let other readers of the MCT be trace-enabled) by >> adding an extra function. It's important to keep other users of MCT >> traceable because the MCT is actually quite slow. > > > > Hi Doug, > > could you elaborate ? I don't get the 'because the MCT ... slow' Sorry, I was trying to avoid duplication in the series and it's more obvious when you look at parts 2 and 3 of the series. ;) Doing the math (please correct any miscalculations) using the numbers from the other patches: You can see that the existing code takes 1323852 us for 1000000 gettimeofday in userspace. The fastest implementation (just shaving to a 32-bit timer) gets us as fast as ~1000000 us for 1000000 gettimeofday in userspace. >From profiling, I believe that gettimeofday from userspace is about 50% overhead (system call, multiplication, copies, etc) and about 50% MCT read. That means that the fastest you can possibly do an MCT read is in .5us or 500ns. I believe an A15 has something like 1 or 2 cycles per instruction. If it were 2 cycles per instruction, it can execute a normal instruction on a 2GHz machine in .5ns. That means we can execute 1000 normal instructions in the time it takes to do a since MCT access. ...so I guess that's what I'd call slow. ;) What do you think? I know that the MCT read shows up in whole system profiles of gettimeofday. -Doug -- To unsubscribe from this list: send the line "unsubscribe linux-samsung-soc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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