Re: [PATCH] Revert "mm/page-writeback.c: print a warning if the vm dirtiness settings are illogical" (was: Re: [PATCH] mm: print a warning once the vm dirtiness settings is illogical)

[Yafang Shao <laoar.shao@gmail.com>]

2017-11-28 18:48 GMT+08:00 Michal Hocko <mhocko@suse.com>:
> On Tue 28-11-17 11:37:23, Jan Kara wrote:
>> On Mon 27-11-17 10:19:39, Michal Hocko wrote:
>> > Andrew,
>> > could you simply send this to Linus. If we _really_ need something to
>> > prevent misconfiguration, which I doubt to be honest, then it should be
>> > thought through much better.
>>
>> What's so bad about the warning? I think warning about such
>> misconfiguration is not a bad thing per se. Maybe it should be ratelimited
>> and certainly the condition is too loose as your example shows but in
>> principle I'm not against it and e.g. making the inequality in the condition
>> strict like:
>>
>>       if (unlikely(bg_thresh > thresh))
>>
>> or at least
>>
>>       if (unlikely(bg_thresh >= thresh && thresh > 0))
>>
>> would warn about cases where domain_dirty_limits() had to fixup bg_thresh
>> manually to make writeback throttling work and avoid reclaim stalls which
>> is IMHO a sane thing...
>
> If it generates false positives then it is more harmful than useful. And
> even if it doesn't, what is the point? Do we check that other related
> knobs are configured properly? I do not think so, we simply rely on
> admins doing sane things.

Not all admins are good at tuning this.
I don't think every SE knows how to tune vm.dirty_background_bytes and
vm.dirty_background_bytes. Only kernel experts could do that.

At least this warning could help them to learn what happend instead of
knowing nothing.



> Otherwise we would have a lot of warnings like
> that. They would be pain to maintain and I believe the additional value
> is quite dubious.
> --

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