Date: Tue, 19 Jan 2010 18:27:43 +0100 From: Attilio Rao <attilio@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: FreeBSD Arch <arch@freebsd.org>, Ed Maste <emaste@freebsd.org> Subject: Re: [PATCH] Statclock aliasing by LAPIC Message-ID: <3bbf2fe11001190927m10f73775p7b68eb4d3ce0470a@mail.gmail.com> In-Reply-To: <201001191144.23299.jhb@freebsd.org> References: <3bbf2fe10911271542h2b179874qa0d9a4a7224dcb2f@mail.gmail.com> <20100116205752.J64514@delplex.bde.org> <3bbf2fe11001160409w1dfdbb9j36458c52d596c92a@mail.gmail.com> <201001191144.23299.jhb@freebsd.org>
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2010/1/19 John Baldwin <jhb@freebsd.org>: > On Saturday 16 January 2010 7:09:38 am Attilio Rao wrote: >> 2010/1/16 Bruce Evans <brde@optusnet.com.au>: >> > On Fri, 15 Jan 2010, Attilio Rao wrote: >> > >> >> I still see clock_lock in place (and no particular critical section >> >> code in that paths) or you meant to say that the clock_lock doesn't >> >> still provide enough protection alone? >> >> BTW, you were right about the lapic_timer_hz (I forgot to revert to >> >> hz). There is an updated patch: >> >> >> >> > http://www.freebsd.org/~attilio/Sandvine/STABLE_8/statclock_aliasing/statclock_aliasing4.diff >> > >> > It seems to have the same fundamental bugs as the previous version. >> > The atrtc interrupt is too slow to use for anything, so it should never >> > be used if there is something better like the lapic timer available >> > (even the i8254 is better), and using it here doesn't even fix the >> > problem (malicious applications can very easily hide from statclock >> > by default since the default hz is much larger than the default stathz, >> > and malicious applications can not so easily hide from statclock >> > irrespective >> > of the misconfiguration of hz, since statclock is not random). See my >> > previous reply and ftp://ftp.ee.lbl.gov/papers/statclk-usenix93.ps.Z for >> > more details. >> >> Well, the primary things I wanted to fix is not the hiding of >> malicious programs but the clock aliasing created when handling all >> the clocks by the same source. >> About the slowness -- I'm fine with whatever additional source to >> LAPIC we would eventually use thus would you feel better if i8254 is >> used replacing atrtc? >> Also note that atrtc is the default if LAPIC cannot be used. I don't >> understand why another source, even simpler (eg. i8254) would have >> been used in that specific case by the 'old' code. >> >> What I mean, then is: I see your points, I'm not arguing that at all, >> but the old code has other problems that gets fixed with this patch >> (having different sources make the whole system more flexible) while >> the new things it does introduce are secondarilly (but still: I'm fine >> with whatever second source is picked up for statclock, profclock) if >> you really see a concern wrt atrtc slowness. > > You can't use the i8254 reliable with APIC enabled. Some motherboards don't > actually hook up IRQ 0 to pin 2. We used to support this by enabling IRQ 0 in > the atpic and enabling the ExtINT pin to use both sets of PICs in tandem. > However, this was very gross and had its own set of issues, so we removed the > support for "mixed mode" a while ago. Also, the ACPI specification > specifically forbids an OS from using "mixed mode". > > My feeling, btw, is that the real solution is to not use a sampling clock for > per-process stats, but to just use the cycle counter and keep separate user, > system, and interrupt cycle counts (like the rux_runtime we have now). This > makes calcru() trivial and eliminates many of the weird "going backwards", > etc. problems. The only issue with this approach is that not all platforms > have a cheap cycle counter (many embedded platforms lack one I think), so you > would almost need to support both modes of operation and maybe have an #define > in <machine/param.h> to choose between the two modes. Generally that would be a good idea, but the problem is not only for the architectures not supporting it, but also for architectures that do (eg. TSC de-synchronization in some SMP environment). Attilio > Even in that mode you still need a sampling clock I think for cp_time[] and > cp_times[], but individual threads can no longer "hide" as we would be keeping > precise timing stats. Yes, cp_times do require a sampling clock, I guess. Attilio -- Peace can only be achieved by understanding - A. Einstein
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