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

[Johannes Weiner]

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).

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.