linux-kernel - Re: [PATCH 1/4] sched/eevdf: Fix vruntime adjustment on reweight

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Message-ID: <c2ceff07-e1b4-4dbc-b945-f91a9076375e@linux.alibaba.com>
Date: Thu, 29 Feb 2024 17:24:58 +0800
From: Tianchen Ding <dtcccc@...ux.alibaba.com>
To: Abel Wu <wuyun.abel@...edance.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Valentin Schneider <valentin.schneider@....com>,
 Barry Song <21cnbao@...il.com>, Benjamin Segall <bsegall@...gle.com>,
 Chen Yu <yu.c.chen@...el.com>, Daniel Jordan <daniel.m.jordan@...cle.com>,
 "Gautham R . Shenoy" <gautham.shenoy@....com>,
 Joel Fernandes <joel@...lfernandes.org>,
 K Prateek Nayak <kprateek.nayak@....com>, Mike Galbraith <efault@....de>,
 Qais Yousef <qyousef@...alina.io>, Tim Chen <tim.c.chen@...ux.intel.com>,
 Yicong Yang <yangyicong@...wei.com>,
 Youssef Esmat <youssefesmat@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] sched/eevdf: Fix vruntime adjustment on reweight

Hi Abel:

I'm not so familiar with eevdf and still learning. Here I've some questions 
about this patch.

1. You did proof that V will not change during reweight (COROLLARY #2). However, 
according to the original paper, the new V will be:
V' = V + lag(j)/(W - w_j) - lag(j)/(W - w_j + w'_j)
So the V' in paper will change (when lag is not zero).
Is the V in Linux code slightly different from the paper?

2. I print some log around reweight_entity(), mainly want to print V by calling 
avg_vruntime(cfs_rq). I found your algorithm only keeps the V unchanged during 
reweight_eevdf(), but not reweight_entity().

In detail:
If curr is true (i.e., cfs_rq->curr == se), we will directly run 
reweight_eevdf(), and the V is not changed.
If curr is false, we will have __dequeue_entity() -> reweight_eevdf() -> 
__enqueue_entity(). The V is finally changed due to dequeue and enqueue. So the 
result of reweight_entity() will be impacted by "cfs_rq->curr == se", is this 
expected?

Thanks!