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

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Message-ID: <29d66a46-d141-2d02-45dd-a8931786588e@gmail.com>
Date:   Thu, 19 May 2022 14:15:21 +0800
From:   Alex Shi <seakeel@...il.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Chen Wandun <chenwandun@...wei.com>,
        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



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. :)
BTW, I still don't see the strong reason for the pressure range.

> > 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