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Date:   Mon, 25 Nov 2019 16:36:50 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Qais Yousef <qais.yousef@arm.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] sched: rt: Make RT capacity aware
Message-ID: <20191125163650.6a624fee@gandalf.local.home>
In-Reply-To: <20191009104611.15363-1-qais.yousef@arm.com>
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Sorry for the very late response...

On Wed,  9 Oct 2019 11:46:11 +0100
Qais Yousef <qais.yousef@arm.com> wrote:

> ATM the uclamp value are only used for frequency selection; but on
> heterogeneous systems this is not enough and we need to ensure that the
> capacity of the CPU is >= uclamp_min. Which is what implemented here.

Is it possible that the capacity can be fixed, where the process can
just have a cpu mask of CPUs that has the capacity for it?

Not that this will affect this patch now, but just something for the
future.

> 
> 	capacity_orig_of(cpu) >= rt_task.uclamp_min
> 
> Note that by default uclamp.min is 1024, which means that RT tasks will
> always be biased towards the big CPUs, which make for a better more
> predictable behavior for the default case.
> 
> Must stress that the bias acts as a hint rather than a definite
> placement strategy. For example, if all big cores are busy executing
> other RT tasks we can't guarantee that a new RT task will be placed
> there.
> 
> On non-heterogeneous systems the original behavior of RT should be
> retained. Similarly if uclamp is not selected in the config.
> 
> Signed-off-by: Qais Yousef <qais.yousef@arm.com>

Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

-- Steve