Message-ID: <172c0580-279f-aa3e-817a-4216067bea10@akamai.com>
Date:   Thu, 18 Jun 2020 15:40:33 -0400
From:   Jason Baron <jbaron@...mai.com>
To:     jim.cromie@...il.com
Cc:     Petr Mladek <pmladek@...e.com>,
        LKML <linux-kernel@...r.kernel.org>, akpm@...uxfoundation.org,
        Greg KH <gregkh@...uxfoundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        Orson Zhai <orson.zhai@...soc.com>,
        Linux Documentation List <linux-doc@...r.kernel.org>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>
Subject: Re: [PATCH v3 20/21] dyndbg: add user-flag, negating-flags, and
 filtering on flags



On 6/18/20 3:11 PM, jim.cromie@...il.com wrote:
> On Thu, Jun 18, 2020 at 12:17 PM Jason Baron <jbaron@...mai.com> wrote:
>>
>>
>>
>> On 6/18/20 1:40 PM, Petr Mladek wrote:
>>> On Thu 2020-06-18 18:19:12, Petr Mladek wrote:
>>>> On Wed 2020-06-17 10:25:35, Jim Cromie wrote:
>>>>> 1. Add a user-flag [u] which works like the [pfmlt] flags, but has no
>>>>> effect on callsite behavior; it allows incremental marking of
>>>>> arbitrary sets of callsites.
>>>>>
>>>>> 2. Add [PFMLTU] flags, which negate their counterparts; P===!p etc.
>>>>> And in ddebug_read_flags():
>>>>>    current code does:       [pfmltu_] -> flags
>>>>>    copy it to:              [PFMLTU_] -> mask
>>>>>
>>>>> also disallow both of a pair: ie no 'pP', no true & false.
>>>>>
>>>>> 3. Add filtering ops into ddebug_change(), right after all the
>>>>> callsite-property selections are complete.  These filter on the
>>>>> callsite's current flagstate before applying modflags.
>>>>>
>>>>> Why ?
>>>>>
>>>>> The u-flag & filter flags
>>>>>
>>>>> The 'u' flag lets the user assemble an arbitary set of callsites.
>>>>> Then using filter flags, user can activate the 'u' callsite set.
>>>>>
>>>>>   #> echo 'file foo.c +u; file bar.c +u' > control   # and repeat
>>>>>   #> echo 'u+p' > control
>>>>>
>>>>> Of course, you can continue to just activate your set without ever
>>>>> marking it 1st, but you could trivially add the markup as you go, then
>>>>> be able to use it as a constraint later, to undo or modify your set.
>>>>>
>>>>>   #> echo 'file foo.c +up' >control
>>>>>   .. monitor, debug, finish ..
>>>>>   #> echo 'u-p' >control
>>>>>
>>>>>   # then later resume
>>>>>   #> echo 'u+p' >control
>>>>>
>>>>>   # disable some cluttering messages, and remove from u-set
>>>>>   #> echo 'file noisy.c function:jabber_* u-pu' >control
>>>>>
>>>>>   # for doc, recollection
>>>>>   grep =pu control > my-favorite-callsites
>>>>>
>>>>> Note:
>>>>>
>>>>> Your flagstate after boot is generally not all =_. -DDEBUG will arm
>>>>> compiled callsites by default, $builtinmod.dyndbg=+p bootargs can
>>>>> enable them early, and $module.dyndbg=+p bootargs will arm them when
>>>>> the module is loaded.  But you could manage them with u-flags:
>>>>>
>>>>>   #> echo '-t' >control             # clear t-flag to use it as 2ndary markup
>>>>>   #> echo 'p+ut' >control   # mark the boot-enabled set of callsites
>>>>>   #> echo '-p' >control             # clean your dmesg -w stream
>>>>>
>>>>>   ... monitor, debug ..
>>>>>   #> echo 'module of_interest $qterms +pu' >control # build your set of useful debugs
>>>>>   #> echo 'module of_interest $qterms UT+pu' >control       # same, but dont alter ut marked set
>>>>
>>>> Does anyone requested this feature, please?
>>>>
>>>> For me, it is really hard to imagine people using these complex and hacky
>>>> steps.
>>>
>>> I think that all this is motivated by adding support for module
>>> specific groups.
>>>
>>> What about storing the group as yet another information for each
>>> message? I mean the same way as we store module name, file, line,
>>> function name.
>>>
>>> Then we could add API to define group for a given message:
>>>
>>>    pr_debug_group(group_id, fmt, ...);
>>>
>>> the interface for the control file might be via new keyword "group".
>>> You could then do something like:
>>>
>>>    echo module=drm group=0x3 +p >control
>>>
>>> But more importantly you should add functions that might be called
>>> when the drm.debug parameter is changes. I have already mentioned
>>> it is another reply:
>>>
>>>     dd_enable_module_group(module_name, group_id);
>>>     dd_disable_module_group(module_name, group_id);
>>>
>>>
>>> It will _not_ need any new flag or flag filtering.
>>>
>>> Best Regards,
>>> Petr
>>>
>>
>> Yes, I'm wondering as well if people are really going to use the
>> new flags and filter flags - I mentioned that here:
>> https://urldefense.proofpoint.com/v2/url?u=https-3A__lkml.org_lkml_2020_6_12_732&d=DwIBaQ&c=96ZbZZcaMF4w0F4jpN6LZg&r=1fLh1mlLqbfetnnGsbwXfpwmGlG4m83mXgtV4vZ1B1A&m=vltk6sSzPDQIqO4gGkJeDY6jcEarG4xTztab2EHtPFY&s=6x1EHNoRxebA99Tu-C2i0s5dmdzyEF9bXVcv_cYoM_I&e= 
>>
> 
> yes, I saw, and replied there.
> but since that was v1, and we're on v3, we should refresh.
> 
> the central use-case is above, 1-liner version summarized here:
> 
> 1- enable sites as you chase a problem with +up
> 2- examine them with grep =pu
> 3- change the set to suit, either by adding or subtracting callsites.
> 4- continue debugging, and changing callsites to suit
> 5- grep =pu control > ~/debugging-session-task1-callsites
> 6- echo up-p >control   # disable for now, leave u-set for later
> 7- do other stuff
> 8 echo uP+p >control # reactivate useful debug-state and resume
> 
> 
>> The grouping stuff is already being used by lots of modules so
>> that seems useful.
> 
> I now dont see the need.
> 
> given N debug callsites, any group can be defined by <N queries,
> probably a lot less
> if module authors can use ddebug_exec_queries(), cuz its exported, (15/21)
> then they can act (+p or -p) on those sets defined by <N queries.
> 
> and now any callsite can be in any number of groups, not just one.
> It would be prudent to evaluate such groupings case by case,
> because the intersecting callsites are subject to "last manipulator wins"
> but its unnecessary to insist that all sets are disjoint.
> Unlike pr_debug_n, however its spelled.
> 

hmm - so I think you are saying there is then no need to change the
calling functions themselves - its still 'pr_debug()'. You could even
use the 'format' qualifier for example to implement your groups that
way.

For example:

pr_debug("failure type1: blah");
pr_debug("failure type2: blah blah");

and then do: ddebug_exec_queries("format type1 +p", module);

I would be curious to see what Stanimir thinks of this proposal
and whether it would work for his venus driver, which is what
prompted this module group discussion.

Thanks,

-Jason

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