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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
Date: Thu, 9 Sep 2021 11:56:26 -0400	[thread overview]
Message-ID: <YTouqsXeAGV6c5oV@cmpxchg.org> (raw)
In-Reply-To: <1631188824-25623-1-git-send-email-huangzhaoyang@gmail.com>

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.

  parent reply	other threads:[~2021-09-09 15:54 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-09 12:00 [RFC PATCH] psi : calc psi memstall time more precisely Huangzhaoyang
2021-09-09 13:07 ` Vlastimil Babka
2021-09-09 15:56 ` Johannes Weiner [this message]
2021-09-14  1:24   ` Zhaoyang Huang
2021-09-14  1:24     ` Zhaoyang Huang

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