Date: Fri, 5 Apr 2019 16:21:28 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Bruce Evans <brde@optusnet.com.au> Cc: Michael Tuexen <tuexen@fh-muenster.de>, freebsd-hackers Hackers <freebsd-hackers@freebsd.org>, FreeBSD PowerPC ML <freebsd-ppc@freebsd.org> Subject: Re: powerpc64 head -r344018 stuck sleeping problems: th->th_scale * tc_delta(th) overflows unsigned 64 bits sometimes [patched failed] Message-ID: <20190405132128.GD1923@kib.kiev.ua> In-Reply-To: <20190405230717.D3383@besplex.bde.org> References: <20190306172003.GD2492@kib.kiev.ua> <20190308001005.M2756@besplex.bde.org> <20190307222220.GK2492@kib.kiev.ua> <20190309144844.K1166@besplex.bde.org> <20190324110138.GR1923@kib.kiev.ua> <E0785613-2B6E-4BB3-95CD-03DD96902CD8@fh-muenster.de> <20190403070045.GW1923@kib.kiev.ua> <20190404011802.E2390@besplex.bde.org> <20190405113912.GB1923@kib.kiev.ua> <20190405230717.D3383@besplex.bde.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Fri, Apr 05, 2019 at 11:52:27PM +1100, Bruce Evans wrote: > On Fri, 5 Apr 2019, Konstantin Belousov wrote: > > > On Thu, Apr 04, 2019 at 02:47:34AM +1100, Bruce Evans wrote: > >> I noticed (or better realized) a general problem with multiple > >> timehands. ntpd can slew the clock at up to 500 ppm, and at least an > >> old version of it uses a rate of 50 ppm to fix up fairly small drifts > >> in the milliseconds range. 500 ppm is enormous in CPU cycles -- it is > >> 500 thousand nsec or 2 million cycles at 4GHz. Winding up the timecounter > >> every 1 msec reduces this to only 2000 cycles. > >> ... > >> The main point of having multiple timehands (after introducing the per- > >> timehands generation count) is to avoid blocking thread N during the > >> update, but this doesn't actually work, even for only 2 timehands and > >> a global generation count. > > > > You are describing the generic race between reader and writer. The same > > race would exist even with one timehand (and/or one global generation > > counter), where ntp adjustment might come earlier or later of some > > consumer accessing the timehands. If timehand instance was read before > > tc_windup() run but code consumed the result after the windup, it might > > appear as if time went backward, and this cannot be fixed without either > > re-reading the time after time-depended calculations were done and > > restarting, or some globabl lock ensuring serialization. > > With 1 timehand, its generation count would be global. I think its ordering > is strong enough to ensure serialization. Yes, single timehands result in global generation. But it would not solve the same race appearing in slightly different manner, as I described above. If reader finished while generation number in th was not yet reset, but caller uses the result after tc_windup(), the effect is same as if we have two th's and reader used the outdated one. > > I think the fix in the kernel to use a global generation count (with > 1 > timehands) is simply s/th->th_generation/tc_generation/g. Oops, that > makes multiple timehands useless and gives some blocking. The critical > case is when a new timehands is under construction. The old timehands > becomes unsafe to use when the writer (tc_windup()) updates the offset. > tc_windup() currently sets th_generation to 0 to indicate that the new > timehands is unsafe to use. Doing the same with a global tc_generation > would give serialization at the cost of busy-waiting for tc_generation > to become nonzero. It would indicate that all timehands are unsafe > to use. > > In the library, does it just work to put the global generation count in > the shared page? libc always reload tk_current in the loop, so it works with any number of vdso timehands greater or equal to one.
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20190405132128.GD1923>