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Subject: Re: [PATCH 04/10] sched/fair: Dynamically update cfs_overload_cpus
To:     Peter Zijlstra <peterz@infradead.org>
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, Steven Rostedt <rostedt@goodmis.org>
References: <1540220381-424433-1-git-send-email-steven.sistare@oracle.com>
 <1540220381-424433-5-git-send-email-steven.sistare@oracle.com>
 <20181022165631.GE3117@worktop.programming.kicks-ass.net>
From:   Steven Sistare <steven.sistare@oracle.com>
Organization: Oracle Corporation
Message-ID: <fbb2478e-41f0-37c7-7c22-f8748fa08389@oracle.com>
Date:   Mon, 22 Oct 2018 14:43:44 -0400
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On 10/22/2018 12:56 PM, Peter Zijlstra wrote:
> On Mon, Oct 22, 2018 at 07:59:35AM -0700, Steve Sistare wrote:
>> An overloaded CPU has more than 1 runnable task.  When a CFS task wakes
>> on a CPU, if h_nr_running transitions from 1 to more, then set the CPU in
>> the cfs_overload_cpus bitmap.  When a CFS task sleeps, if h_nr_running
>> transitions from 2 to less, then clear the CPU in cfs_overload_cpus.
> 
>> @@ -5111,8 +5147,12 @@ static inline void hrtick_update(struct rq *rq)
>>  		update_cfs_group(se);
>>  	}
>>  
>> -	if (!se)
>> +	if (!se) {
>>  		add_nr_running(rq, 1);
>> +		if (prev_nr == 1)
>> +			overload_set(rq);
>> +
>> +	}
> 
> This is very similar to what RT already has, except they also track if
> the tasks in question is in fact migratable (p->nr_cpus_allowed > 1).
> 
> And similarly, it might make sense to use the sparse mask thing over
> there.

Yes.  I forgot to mention RT as likely follow on work.

- Steve