linux-kernel - Re: [PATCH V6 1/6] sched/fair: Add trivial fair server

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Date: Thu, 11 Apr 2024 09:28:04 +0200
From: Daniel Bristot de Oliveira <bristot@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Daniel Bristot de Oliveira
 <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>,
 linux-kernel@...r.kernel.org, Luca Abeni <luca.abeni@...tannapisa.it>,
 Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
 Thomas Gleixner <tglx@...utronix.de>, Joel Fernandes
 <joel@...lfernandes.org>, Vineeth Pillai <vineeth@...byteword.org>,
 Shuah Khan <skhan@...uxfoundation.org>, Phil Auld <pauld@...hat.com>,
 Suleiman Souhlal <suleiman@...gle.com>,
 Youssef Esmat <youssefesmat@...gle.com>
Subject: Re: [PATCH V6 1/6] sched/fair: Add trivial fair server

On 4/10/24 19:24, Peter Zijlstra wrote:
> On Fri, Apr 05, 2024 at 07:28:00PM +0200, Daniel Bristot de Oliveira wrote:
>> From: Peter Zijlstra <peterz@...radead.org>
>>
>> Use deadline servers to service fair tasks.
>>
>> This patch adds a fair_server deadline entity which acts as a container
>> for fair entities and can be used to fix starvation when higher priority
>> (wrt fair) tasks are monopolizing CPU(s).
>>
>> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>> Signed-off-by: Daniel Bristot de Oliveira <bristot@...nel.org>
>> ---
>>  kernel/sched/core.c     | 24 ++++++++++++++++--------
>>  kernel/sched/deadline.c | 23 +++++++++++++++++++++++
>>  kernel/sched/fair.c     | 25 +++++++++++++++++++++++++
>>  kernel/sched/sched.h    |  4 ++++
>>  4 files changed, 68 insertions(+), 8 deletions(-)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 7019a40457a6..04e2270487b7 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -6007,6 +6007,14 @@ static void put_prev_task_balance(struct rq *rq, struct task_struct *prev,
>>  #endif
>>  
>>  	put_prev_task(rq, prev);
>> +
>> +	/*
>> +	 * We've updated @prev and no longer need the server link, clear it.
>> +	 * Must be done before ->pick_next_task() because that can (re)set
>> +	 * ->dl_server.
>> +	 */
>> +	if (prev->dl_server)
>> +		prev->dl_server = NULL;
>>  }
>>  
>>  /*
>> @@ -6037,6 +6045,13 @@ __pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>>  			p = pick_next_task_idle(rq);
>>  		}
>>  
>> +		/*
>> +		 * This is a normal CFS pick, but the previous could be a DL pick.
>> +		 * Clear it as previous is no longer picked.
>> +		 */
>> +		if (prev->dl_server)
>> +			prev->dl_server = NULL;
>> +
>>  		/*
>>  		 * This is the fast path; it cannot be a DL server pick;
>>  		 * therefore even if @p == @prev, ->dl_server must be NULL.
>> @@ -6050,14 +6065,6 @@ __pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>>  restart:
>>  	put_prev_task_balance(rq, prev, rf);
>>  
>> -	/*
>> -	 * We've updated @prev and no longer need the server link, clear it.
>> -	 * Must be done before ->pick_next_task() because that can (re)set
>> -	 * ->dl_server.
>> -	 */
>> -	if (prev->dl_server)
>> -		prev->dl_server = NULL;
>> -
>>  	for_each_class(class) {
>>  		p = class->pick_next_task(rq);
>>  		if (p)
> 
> This bit seems like a fix for 63ba8422f876 ("sched/deadline: Introduce
> deadline servers"), should it be a separate patch?

It was actually reported in a separated patch as a pre-review of this series. So I
think you can pick them as fixes already from there, and add the fixes tag?

https://lore.kernel.org/lkml/20240313012451.1693807-2-joel@joelfernandes.org/
https://lore.kernel.org/lkml/20240313012451.1693807-3-joel@joelfernandes.org/

Also add the Reviewed-by me...

-- Daniel