Re: [RFC PATCH] psi : calc psi memstall time more precisely

[Zhaoyang Huang]

On Thu, Sep 9, 2021 at 11:54 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> On Thu, Sep 09, 2021 at 08:00:24PM +0800, Huangzhaoyang wrote:
> > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> >
> > psi's memstall time is counted as simple as exit - entry so far, which ignore
> > the task's off cpu time. Fix it by calc the percentage of off time via task and
> > rq's util and runq load.
> >
> > Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
>
> Can you please explain what practical problem you are trying to solve?
>
> If a reclaimer gets preempted and has to wait for CPU, should that
> stall be attributed to a lack of memory? Some of it should, since page
> reclaim consumed CPU budget that would've otherwise been available for
> doing real work. The application of course may still have experienced
> a CPU wait outside of reclaim, but potentially a shorter one. Memory
> pressure can definitely increase CPU pressure (as it can IO pressure).
The preempted time which is mentioned here can be separated into 2
categories. First one is cfs task preempted because running out of the
share of schedule_lantency. The second one is preempted by RT,DL and
IRQs. IMO, the previous is reasonable to be counted in stall time,
while the second one NOT.
>
> Proportional and transitive accounting - how much of total CPU load is
> page reclaim, and thus how much of each runq wait is due to memory
> pressure - would give more precise answers. But generally discounting
> off-CPU time in a stall is not any more correct than including it all.
>
> This is doable, but I think there needs to be better justification for
> providing this level of precision, since it comes with code complexity
> that has performance and maintenance overhead.
The rq's utilization of load tracking provides an easy way to compute
such proportion. A new commit has been given out which mainly deals
with the 2nd scenario described above. The statistics of the precision
are provided together.