Messages in this thread | | | Date | Wed, 16 Apr 2008 13:46:09 +0200 | From | Ingo Molnar <> | Subject | Re: [BUG/PATCH] x86 mmiotrace: dynamically disable non-boot CPUs |
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* Pekka Paalanen <pq@iki.fi> wrote:
> > we should fix this restriction ASAP. Forcibly dropping to UP will > > cause mmiotrace to be much less useful for diagnostic purposes of > > Linux > > Ok, how do you propose we solve this? > > I have asked the question before, and then I had two ideas. Well, the > first one was actually your idea (so I hear) to solve the same problem for > kmemcheck. > - per-cpu page tables > - instead of single-stepping, emulate the faulting instruction and never > disarm pages during tracing. (Use and modify code from KVM.) > > I don't believe either of these is easy or fast to implement. Given > some months, I might be able to achieve emulation. Page tables are > still magic to me.
yeah - it looks complex. Not a showstopper for now :-)
but given that Xorg is usually just a single task, do we _really_ need this?
> > drivers. We want to enable the mmiotrace-ing of specific devices via > > some /sys flag. For example via: > > > > cat /sys/devices/pci0000\:00/0000\:00\:1f.2/mmiotrace > > > > this should start mmiotracing of that specific device - or something > > like that. Hm? > > Ooh, that sounds like a neat interface. I like it. I've been > half-thinking of different ways of specifying the set of devices to > trace, but none of them was particularly nice. This feature is for > post-2.6.26, right?
yeah, most likely.
> Ok, so first select mmiotrace tracer, at which point those sysfs files > appear, but mmiotrace is not activated yet. Then, when any of the > device specific files, or the global file in debugfs, is opened, > mmiotrace activates. And if the file is closed, mmiotrace deactivates.
sounds good to me!
> Should we support several "cats" at the same time?
if it's possible ...
> > i suspect the bug is that you bring the CPU down from an atomic > > (spinlocked or irq disabled) context. > > Hmm, it should not be... I have to double-check, but all the other > code, too, from where enter_uniprocessor() is called, may sleep. The > first thing the caller does is to acquire a mutex, which I assume > would complain loudly if spinlocked or irq-disabled. > > Ingo, thank you for fixing this patch, though I'd like to suggest to > leave it out for now, since there clearly are worse problems with it > than without it. And if we can solve the SMP issue, this is not > needed. For the time being we can just instruct users to disable all > but one CPU when try want to trace.
i think we still need to make this as 'transparent' to users as possible. Disabling CPUs can be tedious.
are lost events really a problem in practice, given Xorg's single-threadedness?
i'm leaving out this patch from the series for now.
Ingo
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