> > On Wed, 22 Feb 2017, Liang, Kan wrote: > > > > So from what I understand, the issue is if we have an architecture > > > with full- width counters and we trigger a x86_perf_event_update() > > > when bit > > > 47 is set? > > > > No. It related to the counter width. The number of bits we can use > > should be > > 1 bit less than the total width. Otherwise, there will be problem. > > For big cores such as haswell, broadwell, skylake, the counter width is 48 > bit. > > So we can only use 47 bits. > > For Silvermont and KNL, the counter width is only 32 bit I think. So > > we can only use 31 bits. > > So on a machine with 48-bit counters I should just have a counting event > that counts to somewhere above 0x8000 0000 0001 and it should show > problems? Yes
> Because I am unable to trigger this. > > But I guess if anywhere along the line x86_perf_event_update() is run then > you start over? > Probably. It depends on the left. > I noticed your original reproducer bound the event to a core, is that needed > to trigger this? I don't think it's needed. But I didn't try anything without bound. > > Can it happen on a fixed event or only a genearl purpose event? I think it can happens on both. Because fixed counter and GP counter have same counter width and code path. > > > > So if I have a test that runs in a loop for 2^48 retired > > > instructions (which takes ~12 hours on a recent machine) and then > > > reads the results, they might be wrong? > > > > It only needs several minutes to reproduce the issue on SLM/KNL. > > Yes, but I only have machines with 48-bit counters. So it's going to take > 256 times as long as on a machine with 40-bit counters. > > I have an assembly loop that can consistently generate 2 instructions/cycle > (I'd be glad to hear suggestions for events that count faster) and on a > broadwell-ep machine it still takes at least 7 hours or so to get up to > 0x800000000000. I think you may use MSR tool to write a big number into IA32_PMC0 during your test. The writable IA32_PMC0 alias is 0x4C1. Thanks, Kan
