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

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Date:   Wed, 18 May 2022 14:38:51 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Alex Shi <seakeel@...il.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 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]

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.
Thanks,
Suren.

>
> Thanks
> Alex