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Date:   Sat, 21 May 2022 18:13:09 +0800
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Subject: Re: [PATCH 1/2] psi: add support for multi level pressure stall
 trigger
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To:     Chen Wandun <chenwandun@huawei.com>,
        Suren Baghdasaryan <surenb@google.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Alex Shi <alexs@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
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From:   Alex Shi <seakeel@gmail.com>
In-Reply-To: <9fc88a71-b484-c471-66cd-a4d87d8c02c4@huawei.com>
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On 5/21/22 15:23, Chen Wandun wrote:
> 
> 
> 在 2022/5/19 14:15, Alex Shi 写道:
>>
>> On 5/19/22 05:38, Suren Baghdasaryan wrote:
>>> On Wed, May 18, 2022 at 3:29 AM Alex Shi <seakeel@gmail.com> wrote:
>>>>
>>>>
>>>> On 5/17/22 20:46, Chen Wandun wrote:
>>>>>>>> This breaks the old ABI. And why you need this new function?
>>>>>>> Both great points.
>>>>>> BTW, I think the additional max_threshold parameter could be
>>>>>> implemented in a backward compatible way so that the old API is not
>>>>>> broken:
>>>>>>
>>>>>> arg_count = sscanf(buf, "some %u %u %u", &min_threshold_us,  &arg2, &arg3);
>>>>>> if (arg_count < 2) return ERR_PTR(-EINVAL);
>>>>>> if (arg_count < 3) {
>>>>>>       max_threshold_us = INT_MAX;
>>>>>>       window_us = arg2;
>>>>>> } else {
>>>>>>       max_threshold_us = arg2;
>>>>>>       window_us = arg3;
>>>>>> }
>>>>> OK
>>>>>
>>>>> Thanks.
>>>>>> But again, the motivation still needs to be explained.
>>>>> we want do different operation for different stall level,
>>>>> just as prev email explain, multi trigger is also OK in old
>>>>> ways, but it is a litter complex.
>>>> In fact, I am not keen for this solution, the older and newer
>>>> interface is easy to be confused by users, for some resolvable
>>>> unclear issues. It's not a good idea.
>>> Maybe adding the max_threshold as an optional last argument will be
>>> less confusing? Smth like this:
>>>
>>> some/full min_threshold window_size [max_threshold]
>> It's already confused enough. :)
> which point make you confused？
> Interface suggest by Suren is compatible with current version,
> I think it is more reasonable and there is no difficuty to understand it.

Your 3rd parameter has different meaning depends on the exists or non-exist
4th one. It's not a good design. 

>> BTW, I still don't see the strong reason for the pressure range.
> Considering this case:
> I divide pressure into multi levels, and each level corresponds to a
> hander,  I have to register multi triggers and wait for fire events,
> nowadays, these trigger is something like:
> echo “some 150000 1000000” > /proc/pressure/memory
> echo “some 350000 1000000” > /proc/pressure/memory
> echo “some 550000 1000000” > /proc/pressure/memory
> echo “some 750000 1000000” > /proc/pressure/memory
> 
> In the best case, stall pressure between 150000 and 350000,
> only one trigger fire, and only one wakeup.
> 
> In any other case,  multi triggers fire and multi wakeup, but it
> indeed is no need.
> 

Could you give more details info to show what detailed problem
which your propose could address, but current code cannot?


Thanks
Alex

> New implement make the fire and wakeup more precise,
> userspace code will be more simple, no confusing fire event,
> no need to filter fire event anymore, maybe minor performance
> improved.
> 
> Thanks.
>>
>>>> Also, if we do decide to add it, there should be a warning in the
>>> documentation that max_threshold usage might lead to a stall being
>>> missed completely. In your example:
>>>
>>> echo "some 150000 350000 1000000" > /proc/pressure/memory
>>>
>>> If there is a stall of more than 350ms within a given window, that
>>> trigger will not fire at all.
>> Right.
>> And what if others propose more pressure combinations?
>> Maybe leave them to user space is more likely workable?
>>
>> Thanks
>> Alex
>> .
>