linux-kernel - Re: [PATCH 1/2] psi: add support for multi level pressure stall trigger

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Date:   Sat, 21 May 2022 18:34:14 +0800
From:   Chen Wandun <chenwandun@...wei.com>
To:     Alex Shi <seakeel@...il.com>,
        Suren Baghdasaryan <surenb@...gle.com>
CC:     LKML <linux-kernel@...r.kernel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Alex Shi <alexs@...nel.org>, Jonathan Corbet <corbet@....net>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH 1/2] psi: add support for multi level pressure stall
 trigger



在 2022/5/21 18:13, Alex Shi 写道:
>
> 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@...il.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.
>
some/full min_threshold window_size [max_threshold]

In this format, the meaning of 3rd parameter keep unchanged.
This format is compatible with current version.

>>> 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?
Current code also can handle this, but thing become complex，jsut
as explained above.

Thanks
Wandun
>
> 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
>>> .
> .