On Tue, Jan 29, 2013 at 2:00 AM, Andi Kleen <[email protected]> wrote: > > On Tue, Jan 29, 2013 at 01:30:19AM +0100, Stephane Eranian wrote: > > >> The counter is reinstated to its state before the critical section but > > >> the PMI cannot be > > >> cancelled and there is no state left behind to tell what to do with it. > > > > > > The PMI is effectively spurious, but we use it to set back. Don't know > > > what you mean with "cancel". It already happened of course. > > > > > But when you do this, it seems you making INT_CP events unusable > > for sampling, because you're resetting their value under the cover. > > So what happens when you sample, especially with a fixed period? > > Sampling is forbidden for checkpointed events, the setup code > enforces that. It's unlikely to be useful anyways. > > The main use case for checkpointing is perf stat -T and related > counting usages. > > > >> > > >> > + if (event->attr.sample_period > 0 && > > >> > + event->attr.sample_period < 0x7fffffff) > > >> > + return -EIO; > > >> > + } > > Explain the 0x7fffffff to me? Is that the max period set by default when you > > just count? > > Originally I had just > 0, but then I found that perf stat from the > guest doesn't work anymore because it sets an very high overflow > to accumulate counters. > > The 0x7fffffff is a somewhat arbitary threshold to detect this case. > When I come in with the default frequency mode, then sample_period is a frequency and not a period anymore. So I could do:
$ perf record -F 100 -e instructions:intx_cp .... sample_period = 100; freq=1; That's a very low sampling rate, yet I think it would be rejected by your code. But I guess in frequency mode, there is no guarantee on the period. The kernel may lower the period to 1 in an attempt to achieve the desired rate for the tick period. And that may cause the spurious interrupts. But if I come in with frequency 0x7fffffff+1, then that's a very high frequency, thus small period, I would pass the test. So I think you need to reinforce the test for freq=1. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
