netdev - Re: [PATCH v2 bpf-next 4/8] tracepoint: compute num

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Date:   Thu, 22 Mar 2018 08:51:16 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        David Miller <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Network Development <netdev@...r.kernel.org>,
        kernel-team <kernel-team@...com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2 bpf-next 4/8] tracepoint: compute num_args at build
 time

On 3/22/18 6:36 AM, Steven Rostedt wrote:
> On Wed, 21 Mar 2018 15:05:46 -0700
> Alexei Starovoitov <ast@...com> wrote:
>
>> Like the only reason my patch is counting till 17 is because of
>> trace_iwlwifi_dev_ucode_error().
>> The next offenders are using 12 arguments:
>> trace_mc_event()
>> trace_mm_vmscan_lru_shrink_inactive()
>>
>> Clearly not every efficient usage of it:
>>          trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
>>                          nr_scanned, nr_reclaimed,
>>                          stat.nr_dirty,  stat.nr_writeback,
>>                          stat.nr_congested, stat.nr_immediate,
>>                          stat.nr_activate, stat.nr_ref_keep,
>>                          stat.nr_unmap_fail,
>>                          sc->priority, file);
>> could have passed &stat instead.
>
> Yes they should have, and if I was on the Cc for that patch, I would
> have yelled at them and told them that's exactly what they needed to do.
>
> Perhaps I should add something to keep any tracepoint from having more
> than 6 arguments. That should force a clean up quickly.

I was hesitant to do anything about iwlwifi_dev_ucode_error's 17 args,
because when the code is in such shape there are likely more
skeletons in the closet.
Turned out 'struct iwl_error_event_table' is defined twice with subtle
different field names and layout. While the same
trace_iwlwifi_dev_ucode_error() is used in what looks like two
different cases. I think I managed to refactor it from 17 args to 4
while keeping all bugs in place, but it really should be a job of
the author of the code to deal with such oddities.
Will send the 'fix' in the next respin.
So I definitely support the idea of build time warn for large
number of args.