Re: [PATCH v3] mm: make faultaround produce old ptes

[Vinayak Menon <vinmenon@codeaurora.org>]

On 1/24/2018 5:51 PM, Michal Hocko wrote:
> On Wed 24-01-18 17:39:44, Vinayak Menon wrote:
>> On 1/24/2018 4:41 PM, Michal Hocko wrote:
>>> On Wed 24-01-18 16:13:06, Vinayak Menon wrote:
>>>> On 1/24/2018 3:08 PM, Michal Hocko wrote:
>>> [...]
>>>>> Try to be more realistic. We have way too many sysctls. Some of them are
>>>>> really implementation specific and then it is not really trivial to get
>>>>> rid of them because people tend to (think they) depend on them. This is
>>>>> a user interface like any others and we do not add them without a due
>>>>> scrutiny. Moreover we do have an interface to suppress the effect of the
>>>>> faultaround. Instead you are trying to add another tunable for something
>>>>> that we can live without altogether. See my point?
>>>> I agree on the sysctl part. But why should we disable faultaround and
>>>> not find a way to make it useful ?
>>> I didn't say that. Please read what I've written. I really hate your new
>>> sysctl, because that is not a solution. If you can find a different one
>>> than disabling it then go ahead. But do not try to put burden to users
>>> because they know what to set. Because they won't.
>> What about an expert level config option which is by default disabled ?
> so we have way too many sysctls and it is hard for users to decide what
> to do and now you are suggesting a config option instead? How come this
> makes any sense?

Because by making it a expert level config we are reducing the users exposed to the configuration.

>> Whether to consider faultaround ptes as old or young is dependent on
>> architectural details that can't be gathered at runtime by reading
>> some system registers. This needs to be figured out by experiments,
>> just like how a value for watermark_scale_factor is arrived at. So the
>> user, in this case an engineer expert in this area decides whether the
>> option can be enabled or not in the build.
>> I agree that it need not be a sysctl, but what is the problem that
>> you see in making it a expert level config ? How is it a burden to a
>> non-expert user ?
> Our config space is immense. Adding more on top will not put a relief.
> Just imagine that you get a bug report about a strange reclaim behavior.
> Now you have a one more aspect to consider.
>
> Seriously, if a heuristic fails on somebody then just make it more
> conservative. Maybe it is time to sit down and rethink how the fault
> around should be implemented. No shortcuts and fancy tunables to paper
> over those problems.

Not sure if this is a fault around problem, because without the arch workaround to make the ptes young,
faultaround works well. But anyway let me see if I can do something to avoid tunables. Thanks.

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