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Date:   Thu, 9 Sep 2021 11:56:26 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Huangzhaoyang <huangzhaoyang@gmail.com>
Cc:     Zhaoyang Huang <zhaoyang.huang@unisoc.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, xuewen.yan@unisoc.com,
        ke.wang@unisoc.com
Subject: Re: [RFC PATCH] psi : calc psi memstall time more precisely
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On Thu, Sep 09, 2021 at 08:00:24PM +0800, Huangzhaoyang wrote:
> From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> 
> 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@unisoc.com>

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.