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Date:   Wed, 15 Apr 2020 12:07:41 +0800
From:   Aaron Lu <aaron.lwe@gmail.com>
To:     Peter Zijlstra <peterz@infradead.org>,
        Vineeth Remanan Pillai <vpillai@digitalocean.com>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Tim Chen <tim.c.chen@linux.intel.com>, mingo@kernel.org,
        tglx@linutronix.de, pjt@google.com, torvalds@linux-foundation.org,
        Aaron Lu <aaron.lu@linux.alibaba.com>,
        linux-kernel@vger.kernel.org, fweisbec@gmail.com,
        keescook@chromium.org, kerrnel@google.com,
        Phil Auld <pauld@redhat.com>,
        Aubrey Li <aubrey.intel@gmail.com>, aubrey.li@linux.intel.com,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Joel Fernandes <joelaf@google.com>, joel@joelfernandes.org
Subject: Re: [RFC PATCH 09/13] sched/fair: core wide vruntime comparison
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On Wed, Apr 15, 2020 at 11:34:08AM +0800, Aaron Lu wrote:
> On Tue, Apr 14, 2020 at 03:56:24PM +0200, Peter Zijlstra wrote:
> > On Wed, Mar 04, 2020 at 04:59:59PM +0000, vpillai wrote:
> > > From: Aaron Lu <aaron.lu@linux.alibaba.com>
> > > 
> > > This patch provides a vruntime based way to compare two cfs task's
> > > priority, be it on the same cpu or different threads of the same core.
> > > 
> > > When the two tasks are on the same CPU, we just need to find a common
> > > cfs_rq both sched_entities are on and then do the comparison.
> > > 
> > > When the two tasks are on differen threads of the same core, the root
> > > level sched_entities to which the two tasks belong will be used to do
> > > the comparison.
> > > 
> > > An ugly illustration for the cross CPU case:
> > > 
> > >    cpu0         cpu1
> > >  /   |  \     /   |  \
> > > se1 se2 se3  se4 se5 se6
> > >     /  \            /   \
> > >   se21 se22       se61  se62
> > > 
> > > Assume CPU0 and CPU1 are smt siblings and task A's se is se21 while
> > > task B's se is se61. To compare priority of task A and B, we compare
> > > priority of se2 and se6. Whose vruntime is smaller, who wins.
> > > 
> > > To make this work, the root level se should have a common cfs_rq min
> > > vuntime, which I call it the core cfs_rq min vruntime.
> > > 
> > > When we adjust the min_vruntime of rq->core, we need to propgate
> > > that down the tree so as to not cause starvation of existing tasks
> > > based on previous vruntime.
> > 
> > You forgot the time complexity analysis.
> 
> This is a mistake and the adjust should be needed only once when core
> scheduling is initially enabled. It is an initialization thing and there
> is no reason to do it in every invocation of coresched_adjust_vruntime().

Correction...
I meant there is no need to call coresched_adjust_vruntime() in every
invocation of update_core_cfs_rq_min_vruntime().