Messages in this thread | | | From | Andrew Lutomirski <> | Date | Fri, 12 Nov 2010 18:48:44 -0500 | Subject | Re: [PATCH] Improve clocksource unstable warning |
| |
On Fri, Nov 12, 2010 at 6:40 PM, john stultz <johnstul@us.ibm.com> wrote: > On Fri, 2010-11-12 at 16:52 -0500, Andrew Lutomirski wrote: >> On Fri, Nov 12, 2010 at 4:31 PM, john stultz <johnstul@us.ibm.com> wrote: >> > Ideas: >> > 1) Maybe should we check that we get two sequential failures where the >> > cpu seems fast before we throw out the TSC? This will still fall over >> > in some stall cases (ie: a poor rt task hogging the cpu for 10 >> > minutes, pausing for a 10th of a second and then continuing to hog the >> > cpu). >> > >> > 2) We could look at the TSC delta, and if it appears outside the order >> > of 2-10x faster (i don't think any cpus scale up even close to 10x in >> > freq, but please correct me if so), then assume we just have been >> > blocked from running and don't throw out the TSC. >> > >> > 3) Similar to #2 we could look at the max interval that the watchdog >> > clocksource provides, and if the TSC delta is greater then that, avoid >> > throwing things out. This combined with #2 might narrow out the false >> > positives fairly well. >> > >> > Any additional thoughts here? >> >> Yes. As far as I know, the watchdog doesn't give arbitrary values >> when it wraps; it just wraps. Here's a possible heuristic, in >> pseudocode: >> >> wd_now_1 = (read watchdog) >> cs_now = (read clocksource) >> >> cs_elapsed = cs_now - cs_last; >> wd_elapsed = wd_now_1 - wd_last; >> >> if ( abs(wd_elapsed - cs_elapsed) < MAX_DELTA) >> return; // We're OK. >> >> wd_now_2 = (read watchdog again) >> if (abs(wd_now_1 - wd_now_2) > MAX_DELTA / 2) >> bail; // The clocksource might be unstable, but we either just >> lagged or the watchdog is unstable, and in either case we don't gain >> anything by marking the clocksource unstable. > > This is more easily done by just bounding the clocksource read: > wd_now_1 = watchdog->read() > cs_now = clocksource->read() > wd_now_2 = watchdog->read() > > if (((wd_now_2 - wd_now_1)&watchdog->mask) > SOMETHING_SMALL) > bail; // hit an SMI or some sort of long preemption > >> if ( wd_elapsed < cs_elapsed and ( (cs_elapsed - wd_elapsed) % >> wd_wrapping_time ) < (something fairly small) ) >> bail; // The watchdog most likely wrapped. > > Huh. The modulo bit may need tweaking as its not immediately clear its > right. Maybe the following is clearer?: > > if ((cs_elapsed > wd_wrapping_time) > && (abs((cs_elapsed % wd_wrapping_time)-wd_elapsed) < MAX_DELTA) > // should be ok.
I think this is wrong if wd_elapsed is large (which could happen if the real wd time is something like (2 * wd_wrapping_time - MAX_DELTA/4)).
--Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |