On 11/07/13 07:34, Vince Weaver wrote: > > It turns out that the perf_event mmap page rdpmc/time setting was > broken, dating back to the introduction of the feature. Due > to a mistake with a bitfield, two different values mapped to > the same feature bit. > > A new somewhat backwards compatible interface was introduced > in Linux 3.12. A much longer report on the issue can be found > here: > https://lwn.net/Articles/567894/ > > Signed-off-by: Vince Weaver <vincent.weaver@xxxxxxxxx> Thanks, Vince. Applied. Cheers, Michael > diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2 > index 4ff9690..a443b6e 100644 > --- a/man2/perf_event_open.2 > +++ b/man2/perf_event_open.2 > @@ -1142,8 +1196,13 @@ struct perf_event_mmap_page { > __u64 time_running; /* time event on CPU */ > union { > __u64 capabilities; > - __u64 cap_usr_time : 1, > - cap_usr_rdpmc : 1, > + struct { > + __u64 cap_usr_time / cap_usr_rdpmc / cap_bit0 : 1, > + cap_bit0_is_deprecated : 1, > + cap_user_rdpmc : 1, > + cap_user_time : 1, > + cap_user_time_zero : 1, > + }; > }; > __u16 pmc_width; > __u16 time_shift; > @@ -1173,8 +1232,9 @@ A seqlock for synchronization. > A unique hardware counter identifier. > .TP > .I offset > -.\" FIXME clarify > -Add this to hardware counter value?? > +When using rdpmc for reads this offset value > +must be added to the one returned by rdpmc to get > +the current total event count. > .TP > .I time_enabled > Time the event was active. > @@ -1182,10 +1242,45 @@ Time the event was active. > .I time_running > Time the event was running. > .TP > +.IR cap_usr_time " / " cap_usr_rdpmc " / " cap_bit0 " (Since Linux 3.4)" > +There was a bug in the definition of > .I cap_usr_time > -User time capability. > +and > +.I cap_usr_rdpmc > +from Linux 3.4 until Linux 3.11. > +Both bits were defined to point to the same location, so it was > +impossible to know if > +.I cap_usr_time > +or > +.I cap_usr_rdpmc > +were actually set. > + > +Starting with 3.12 these are renamed to > +.I cap_bit0 > +and you should use the new > +.I cap_user_time > +and > +.I cap_user_rdpmc > +fields instead. > + > .TP > +.IR cap_bit0_is_deprecated " (Since Linux 3.12)" > +If set this bit indicates that the kernel supports > +the properly separated > +.I cap_user_time > +and > +.I cap_user_rdpmc > +bits. > + > +If not-set, it indicates an older kernel where > +.I cap_usr_time > +and > .I cap_usr_rdpmc > +map to the same bit and thus both features should > +be used with caution. > + > +.TP > +.IR cap_user_rdpmc " (Since Linux 3.12)" > If the hardware supports user-space read of performance counters > without syscall (this is the "rdpmc" instruction on x86), then > the following code can be used to do a read: > @@ -1195,7 +1290,6 @@ the following code can be used to do a read: > u32 seq, time_mult, time_shift, idx, width; > u64 count, enabled, running; > u64 cyc, time_offset; > -s64 pmc = 0; > > do { > seq = pc\->lock; > @@ -1215,7 +1309,7 @@ do { > > if (pc\->cap_usr_rdpmc && idx) { > width = pc\->pmc_width; > - pmc = rdpmc(idx \- 1); > + count += rdpmc(idx \- 1); > } > > barrier(); > @@ -1223,6 +1317,16 @@ do { > .fi > .in > .TP > +.I cap_user_time " (Since Linux 3.12)" > +This bit indicates the hardware has a constant, non-stop > +timestamp counter (TSC on x86). > +.TP > +.IR cap_user_time_zero " (Since Linux 3.12)" > +Indicates the presence of > +.I time_zero > +which allows mapping timestamp values to > +the hardware clock. > +.TP > .I pmc_width > If > .IR cap_usr_rdpmc , > @@ -1274,6 +1378,27 @@ enabled and possible running (if idx), improving the scaling: > count = quot * enabled + (rem * enabled) / running; > .fi > .TP > +.IR time_zero " (Since Linux 3.12)" > + > +If > +.I cap_usr_time_zero > +is set then the hardware clock (the TSC timestamp counter on x86) > +can be calculated from the > +.IR time_zero ", " time_mult ", and " time_shift " values:" > +.nf > + time = timestamp - time_zero; > + quot = time / time_mult; > + rem = time % time_mult; > + cyc = (quot << time_shift) + (rem << time_shift) / time_mult; > +.fi > +And vice versa: > +.nf > + quot = cyc >> time_shift; > + rem = cyc & ((1 << time_shift) - 1); > + timestamp = time_zero + quot * time_mult + > + ((rem * time_mult) >> time_shift); > +.fi > +.TP > .I data_head > This points to the head of the data section. > The value continuously increases, it does not wrap. > @@ -2221,6 +2387,17 @@ ioctl argument was broken and would repeatedly operate > on the event specified rather than iterating across > all sibling events in a group. > > +From Linux 3.4 to Linux 3.11 the mmap > +.I cap_usr_rdpmc > +and > +.I cap_usr_time > +bits mapped to the same location. > +Code should migrate to the new > +.I cap_user_rdpmc > +and > +.I cap_user_time > +fields instead. > + > Always double-check your results! > Various generalized events have had wrong values. > For example, retired branches measured > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
