Messages in this thread | | | Date | Thu, 25 Sep 2008 14:25:17 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH 1/2 v2] Unified trace buffer |
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* Steven Rostedt (rostedt@goodmis.org) wrote: > > On Thu, 25 Sep 2008, Mathieu Desnoyers wrote: > > > > Is there a reason to use delta between events rather than simply write > > the 27 LSBs that I would have missed ? > > One answer is that your counter wrap problem is extended. > > That is, you have 27 bits of time between each event to not worry about > wraps. But if you go against the page itself, the last event on that page > is more likely to suffer. >
You can do the exact same thing and manage to keep the absolute time. You just have to adapt the reader like this : (this would be for per-event cycle count in the 32 LSBs, slight bitmask adaptation needed for 27 bits only).
keep a 64 bits TSC value in a per-buffer variable. The previous value is always re-used for the next read.
let's call it : tf->buffer.tsc (u64)
read_event() would look like :
u32 timetamp = read_event_timetamp();
if(timestamp < (0xFFFFFFFFULL & tf->buffer.tsc)) { /* overflow */ tf->buffer.tsc = ((tf->buffer.tsc & 0xFFFFFFFF00000000ULL) + 0x100000000ULL) | (u64)timestamp; } else { /* no overflow */ tf->buffer.tsc = (tf->buffer.tsc & 0xFFFFFFFF00000000ULL) | (u64)timestamp; }
This will detect 32 bits overflow and keep the tf->buffer.tsc in sync with the TSC representation on the traced machine as long as events are less then 27 bits apart. A "full tsc" header can also be easily managed with this by updating the tf->buffer.tsc value completely when such event is met.
Mathieu
> -- Steve >
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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