linux-kernel - Re: [PATCH] sched/fair: Rate limit calls to update_blocked

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Date:   Thu, 13 May 2021 11:45:26 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Paul McKenney <paulmck@...nel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Dietmar Eggeman <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        Neeraj upadhyay <neeraj.iitr10@...il.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>
Subject: Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages()
 for NOHZ



On 5/12/21 6:59 AM, Qais Yousef wrote:
> On 05/11/21 10:25, Tim Chen wrote:
>>> update_next_balance() is only used in newidle_balance() so we could
>>> modify it to  have:
>>>
>>> next = max(jiffies+1, next = sd->last_balance + interval)
>>
>> Is the extra assignment "next = sd->last_balance + interval" needed?
>> This seems more straight forward:
>>
>> next = max(jiffies+1, sd->last_balance + interval)
> 
> I haven't been following the whole conversation closely, but it's always
> interesting when manipulating time in non time_*() functions.
> 
> Is this max() safe against wrapping?

Looking at the definition, seems like max doesn't take care of wrapping.
#define max(a, b) \
        ({ \
                typeof(a) __a = (a); \
                typeof(b) __b = (b); \
                MINMAX_ASSERT_COMPATIBLE(typeof(__a), typeof(__b)); \
                __a > __b ? __a : __b; \
        })


Probably need to do
next = time_after(jiffies+1, sd->last_balance + interval) ? jiffies+1 : sd->last_balance + interval;

Tim