----- Original Message ----- > From: "Thomas Gleixner" <[email protected]> > To: "Mathieu Desnoyers" <[email protected]> > Cc: "LKML" <[email protected]>, "John Stultz" > <[email protected]>, "Peter Zijlstra" > <[email protected]>, "Steven Rostedt" <[email protected]> > Sent: Saturday, July 12, 2014 4:11:17 AM > Subject: Re: [patch 54/55] timekeeping: Provide fast and NMI safe access to > CLOCK_MONOTONIC[_RAW] > > On Fri, 11 Jul 2014, Mathieu Desnoyers wrote: > > > ----- Original Message ----- > > > From: "Thomas Gleixner" <[email protected]> > > > To: "LKML" <[email protected]> > > > Cc: "John Stultz" <[email protected]>, "Peter Zijlstra" > > > <[email protected]>, "Steven Rostedt" > > > <[email protected]>, "Mathieu Desnoyers" > > > <[email protected]> > > > Sent: Friday, July 11, 2014 9:45:19 AM > > > Subject: [patch 54/55] timekeeping: Provide fast and NMI safe access to > > > CLOCK_MONOTONIC[_RAW] > > > On the update side: > > > > > > tkf->seq++; > > > smp_wmb(); > > > update(tkf->base[0], tk; > > > > missing ")" > > :) > > > Any reason why the updater wouldn't do: > > > > tkf->seq++; > > smp_wmb(); > > update(tkf->base[1 - (tkf->seq & 0x01)], tk); > > > > instead of updating both array entries each time ? > > base[0]; <- Current active > seq++; -> Makes base[1] the active one for readers > update(base[0]); > > So readers are always one update cycle behind. Probably not an issue > most of the time, but think about fast wrapping clocksources and > NOHZ....
Ah, yep, got it. So instead of calling update() twice per update, could we do the following instead ? tkf->seq++; smp_wmb(); update(tkf->base[0], tk); tkf->seq++; smp_wmb(); memcpy(tkf->base[1], tkf->base[0], sizeof(tkf->base[1]); just in case "update" would happen to be expensive for some clock implementation. Thanks, Mathieu > > Thanks, > > tglx > > > -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- 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/
