Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 22 Oct 2018 19:04:21 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Steve Sistare <steven.sistare@oracle.com>
Cc:     mingo@redhat.com, subhra.mazumdar@oracle.com,
        dhaval.giani@oracle.com, rohit.k.jain@oracle.com,
        daniel.m.jordan@oracle.com, pavel.tatashin@microsoft.com,
        matt@codeblueprint.co.uk, umgwanakikbuti@gmail.com,
        riel@redhat.com, jbacik@fb.com, juri.lelli@redhat.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 00/10] steal tasks to improve CPU utilization
Message-ID: <20181022170421.GF3117@worktop.programming.kicks-ass.net>
References: <1540220381-424433-1-git-send-email-steven.sistare@oracle.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1540220381-424433-1-git-send-email-steven.sistare@oracle.com>
User-Agent: Mutt/1.5.22.1 (2013-10-16)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Oct 22, 2018 at 07:59:31AM -0700, Steve Sistare wrote:
> When a CPU has no more CFS tasks to run, and idle_balance() fails to
> find a task, then attempt to steal a task from an overloaded CPU in the
> same LLC. Maintain and use a bitmap of overloaded CPUs to efficiently
> identify candidates.  To minimize search time, steal the first migratable
> task that is found when the bitmap is traversed.  For fairness, search
> for migratable tasks on an overloaded CPU in order of next to run.
> 
> This simple stealing yields a higher CPU utilization than idle_balance()
> alone, because the search is cheap, so it may be called every time the CPU
> is about to go idle.  idle_balance() does more work because it searches
> widely for the busiest queue, so to limit its CPU consumption, it declines
> to search if the system is too busy.  Simple stealing does not offload the
> globally busiest queue, but it is much better than running nothing at all.

Why I don't dislike the idea; I feel it is unfortunate to have two
different mechanisms to do effectively the same thing.

Can't we improve idle_balance() instead of building this parallel
functionality?