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

[Chen Wandun]

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@xxxxxxxxx> 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
> > > .
> .