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Date:   Mon, 14 Oct 2019 17:57:25 +0800
From:   Aaron Lu <aaron.lu@linux.alibaba.com>
To:     Vineeth Remanan Pillai <vpillai@digitalocean.com>
Cc:     Tim Chen <tim.c.chen@linux.intel.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
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        "Li, Aubrey" <aubrey.li@linux.intel.com>,
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        Nishanth Aravamudan <naravamudan@digitalocean.com>,
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        Ingo Molnar <mingo@kernel.org>,
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        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
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Subject: Re: [RFC PATCH v3 00/16] Core scheduling v3
Message-ID: <20191014095725.GA78693@aaronlu>
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On Sun, Oct 13, 2019 at 08:44:32AM -0400, Vineeth Remanan Pillai wrote:
> On Fri, Oct 11, 2019 at 11:55 PM Aaron Lu <aaron.lu@linux.alibaba.com> wrote:
> 
> >
> > I don't think we need do the normalization afterwrads and it appears
> > we are on the same page regarding core wide vruntime.

Should be "we are not on the same page..."

[...]
> > The weird thing about my patch is, the min_vruntime is often increased,
> > it doesn't point to the smallest value as in a traditional cfs_rq. This
> > probabaly can be changed to follow the tradition, I don't quite remember
> > why I did this, will need to check this some time later.
> 
> Yeah, I noticed this. In my patch, I had already accounted for this and changed
> to min() instead of max() which is more logical that min_vruntime should be the
> minimum of both the run queue.

I now remembered why I used max().

Assume rq1 and rq2's min_vruntime are both at 2000 and the core wide
min_vruntime is also 2000. Also assume both runqueues are empty at the
moment. Then task t1 is queued to rq1 and runs for a long time while rq2
keeps empty. rq1's min_vruntime will be incremented all the time while
the core wide min_vruntime stays at 2000 if min() is used. Then when
another task gets queued to rq2, it will get really large unfair boost
by using a much smaller min_vruntime as its base.

To fix this, either max() is used as is done in my patch, or adjust
rq2's min_vruntime to be the same as rq1's on each
update_core_cfs_min_vruntime() when rq2 is found empty and then use
min() to get the core wide min_vruntime. Looks not worth the trouble to
use min().