Re: [PATCH v1] dynamic_debug: add support for logs destination

[Łukasz Bartosik]

śr., 18 paź 2023 o 05:08 <jim.cromie@xxxxxxxxx> napisał(a):
>
> adding in Jason, not sure how he got missed.
>
> On Mon, Oct 16, 2023 at 9:13 AM Łukasz Bartosik <lb@xxxxxxxxxxxx> wrote:
> >
> > czw., 12 paź 2023 o 20:48 <jim.cromie@xxxxxxxxx> napisał(a):
> > >
> > > > If you want the kernel to keep separate flight recorders I guess we could
> > > > add that, but I don't think it currently exists for the dyndbg stuff at
> > > > least. Maybe a flight recorder v2 feature, once the basics are in.
> > > >
> > >
> > > dyndbg has   +p    writes to syslog
> > > +T  would separately independently write the same to global trace
> > >
> > > This would allow  graceful switchover to tracefs,
> > > without removing logging from dmesg, where most folks
> > > (and any monitor tools) would expect it.
> > >
> > > Lukas (iiuc) wants to steer each site to just 1 destination.
> > > Or maybe (in addition to +p > syslog) one trace destination,
> > > either global via events, or a separate tracebuf
> > >
> > > Im ambivalent, but thinking the smooth rollover from syslog to trace
> > > might be worth having to ease migration / weaning off syslog.
> > >
> > > And we have a 4 byte hole in struct _ddebug we could just use.
> >
> > I'm glad you brought that up. This means we can leave class_id field
> > untouched, have separate +T in flags (for consistency)
> > and dst_id can be easily 8 bits wide.
> >
> > Also can you point me to the latest version of writing debug logs to
> > trace events (+T option).
> > I would like to base trace instances work on that because both are
> > closely related.
> >
>
> Ive got 3 branches, all stacked up on rc6
> (last I checked, they were clean on drm-tip)
>
> https://github.com/jimc/linux/tree/dd-fix-7c
> the regression fix, reposting this version next.
> it also grabs another flag bit: _DPRINTK_FLAGS_INCL_LOOKUP
>
>
> https://github.com/jimc/linux/tree/dd-shrink-3b
> split modname,filename,function to new __dyndbg_sites section
> loads the 3 column values and their intervals into 3 maple trees
> and implements 3 accessor functions to retrieve the vals
> from the descriptor addresses.
> it "works" but doesnt erase intervals properly on rmmod
> it also claims another flag - _DPRINTK_FLAGS_PREFIX_CACHED
> and uses it to mark cached prefix fragment that get created for enabled calls.
>
> https://github.com/jimc/linux/tree/dd-kitchen-sink
> this adds the +T flag stuff.
> its still a little fuzzy, esp around the descriptor arg -
> using it to render the prefix str at buffer-render time
> got UNSAFE warnings - likely due to loadable module
> descriptors which could or did go away between
> capture and render (after rmmod)
>

Great, I will use commits related to adding +T from this tree as a
baseline for adding trace instances.

>
>
>
> > > Unless the align 8 is optional on 32-bits,
> >
> > I verified with "gcc -g -m32 ..." that the align(8) is honored on 32 bits.
> >
>
> this is sorta the opposite of what I was probing, but I think result
> is the same.
> istm  align(8) is only for JUMP_LABEL, nothing else in the struct
> appears to need it
> so moving it to the top trades the hole for padding.
>

Ahh I see what you meant now.  Although I have to admit that looking
at the jump label code and static key usage cases
around the kernel code I don't see where align(8) requirement for jump
label comes from.

>
>
> > > I think we're never gonna close the hole anywhere.
> > >
> >
> > :)
> >
> > > is align 8 a generic expression of an architectural simplifying constraint ?
> > > or a need for 1-7 ptr offsets ?
> > >
> > >
> > >
> > >
> > > > > That's my idea of it. It is interesting to see how far the requirements
> > > > > can be reasonably realised.
> > > >
> > > > I think aside from the "make it available directly to unpriviledged
> > > > userspace" everything sounds reasonable and doable.
> > > >
> > > > More on the process side of things, I think Jim is very much looking for
> > > > acks and tested-by by people who are interested in better drm logging
> > > > infra. That should help that things are moving in a direction that's
> > > > actually useful, even when it's not yet entirely complete.
> > > >
> > >
> > > yes, please.  Now posted at
> > >
> > > https://lore.kernel.org/lkml/20231012172137.3286566-1-jim.cromie@xxxxxxxxx/T/#t
> > >
> > > Lukas, I managed to miss your email in the send phase.
> > > please consider yourself a direct recipient :-)
> > >
> > > thanks everyone
> > >
> > > > Cheers, Sima
> > > > --
> > > > Daniel Vetter
> > > > Software Engineer, Intel Corporation
> > > > http://blog.ffwll.ch