linux-kernel - Re: [Patch v3 4/6] sched/fair: Consider the idle state of the whole core for load balance

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Message-ID: <5dbc278117107bcc4047dc55f783dad397da1704.camel@linux.intel.com>
Date:   Fri, 14 Jul 2023 15:16:18 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
Cc:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ricardo Neri <ricardo.neri@...el.com>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Len Brown <len.brown@...el.com>, Mel Gorman <mgorman@...e.de>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Valentin Schneider <vschneid@...hat.com>,
        Ionela Voinescu <ionela.voinescu@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        naveen.n.rao@...ux.vnet.ibm.com,
        Yicong Yang <yangyicong@...ilicon.com>,
        Barry Song <v-songbaohua@...o.com>,
        Chen Yu <yu.c.chen@...el.com>, Hillf Danton <hdanton@...a.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [Patch v3 4/6] sched/fair: Consider the idle state of the whole
 core for load balance

On Fri, 2023-07-14 at 18:32 +0530, Shrikanth Hegde wrote:
> 
> 
> 
> Tried on a symmetric system with all cores having SMT=4 as well. There was reduction in migrations here as well.
> Didnt observe any major regressions when microbenchmarks run alone. Such as hackbench, stress-ng. 
> 
> So. Here is tested-by. 
> Tested-by: Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>

Thanks for testing.  

> 
> 
> > +
> >  	/* Are we the first CPU of this group ? */
> >  	return group_balance_cpu(sg) == env->dst_cpu;
> >  }
> 
> One doubt though, Here a fully idle core would be chosen instead of first idle cpu in the 
> group (if there is one). Since coming out of idle of SMT is faster compared to a fully idle core,
> would latency increase? Or that concerns mainly wakeup path?

Yeah, I think that concern is for the wakeup path and not for the balance path.

Tim