linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
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: 4+ 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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=YTouqsXeAGV6c5oV@cmpxchg.org \
    --to=hannes@cmpxchg.org \
    --cc=huangzhaoyang@gmail.com \
    --cc=ke.wang@unisoc.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=xuewen.yan@unisoc.com \
    --cc=zhaoyang.huang@unisoc.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).