Date: Sat, 2 Mar 2019 12:56:52 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Ian Lepore <ian@freebsd.org> Cc: Poul-Henning Kamp <phk@phk.freebsd.dk>, Mark Millard <marklmi@yahoo.com>, Mark Millard via freebsd-hackers <freebsd-hackers@freebsd.org>, Konstantin Belousov <kib@freebsd.org>, bde@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: <20190302105652.GD68879@kib.kiev.ua> In-Reply-To: <210dfd0f50ee6b1149c914ee503502654eb5f328.camel@freebsd.org> References: <D3D7E9F4-9A5E-4320-B3C8-EC5CEF4A2764@yahoo.com> <20190228145542.GT2420@kib.kiev.ua> <20190228150811.GU2420@kib.kiev.ua> <962D78C3-65BE-40C1-BB50-A0088223C17B@yahoo.com> <28C2BB0A-3DAA-4D18-A317-49A8DD52778F@yahoo.com> <20190301112717.GW2420@kib.kiev.ua> <679402FF-907C-43AF-B18C-8C9CC857D7A6@yahoo.com> <6669.1551473821@critter.freebsd.dk> <210dfd0f50ee6b1149c914ee503502654eb5f328.camel@freebsd.org>
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On Fri, Mar 01, 2019 at 02:11:33PM -0700, Ian Lepore wrote: > On Fri, 2019-03-01 at 20:57 +0000, Poul-Henning Kamp wrote: > > -------- > > In message <679402FF-907C-43AF-B18C-8C9CC857D7A6@yahoo.com>, Mark > > Millard via freebsd-hackers writes: > > > > > > I must admit that 2 seconds of interval where the timehands where > > > > not updated is too much. > > > > I have no idea how you got in that situation, but it is very far > > from how timecounters were designed to work. > > > > I wonder if it's fallout from reducing the number of timehands to 2, > which always struck me as a really bad idea. I know of at least one arm > configuration which fails because of it (it takes a combo of a single- > core system, and a pps capture driver that uses hardware latching of > the timer and the polling method for reading the latched value; given > all that, at least 4 sets of timehands are needed to avoid losing PPS > events due to generation changes). Using more than two timehands increases a chance of reader to try to use outdated timehands. In theory, on the hardware with very high inter-core propagation (e.g. with sufficiently large store buffers) it is possible for other CPU to use non-current timehands. More timehands you have, larger is the relative error. If you have very specific configuration which contradicts to the typical modern hardware configuration, I do not see a problem restoring some more timehands entries under a config option.
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