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Date:   Fri, 29 Oct 2021 00:19:48 +0800
From:   Tao Zhou <tao.zhou@linux.dev>
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Aubrey Li <aubrey.li@linux.intel.com>,
        Barry Song <song.bao.hua@hisilicon.com>,
        Mike Galbraith <efault@gmx.de>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Tao Zhou <tao.zhou@linux.dev>
Subject: Re: [PATCH 1/2] sched/fair: Couple wakee flips with heavy wakers
Message-ID: <YXrNfHcfhp2LutiL@geo.homenetwork>
References: <20211028094834.1312-1-mgorman@techsingularity.net>
 <20211028094834.1312-2-mgorman@techsingularity.net>
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In-Reply-To: <20211028094834.1312-2-mgorman@techsingularity.net>
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Hi Mel,

On Thu, Oct 28, 2021 at 10:48:33AM +0100, Mel Gorman wrote:

> @@ -5865,6 +5865,14 @@ static void record_wakee(struct task_struct *p)
>  	}
>  
>  	if (current->last_wakee != p) {
> +		int min = __this_cpu_read(sd_llc_size) << 1;
> +		/*
> +		 * Couple the wakee flips to the waker for the case where it
> +		 * doesn't accrue flips, taking care to not push the wakee
> +		 * high enough that the wake_wide() heuristic fails.
> +		 */
> +		if (current->wakee_flips > p->wakee_flips * min)
> +			p->wakee_flips++;
>  		current->last_wakee = p;
>  		current->wakee_flips++;
>  	}
> @@ -5895,7 +5903,7 @@ static int wake_wide(struct task_struct *p)
>  
>  	if (master < slave)
>  		swap(master, slave);
> -	if (slave < factor || master < slave * factor)
> +	if ((slave < factor && master < (factor>>1)*factor) || master < slave * factor)

So, the check like this include the above range:

  if ((slave < factor && master < slave * factor) ||
       master < slave * factor)

That "factor>>1" filter some.

If "slave < factor" is true and "master < (factor>>1)*factor" is false,
then we check "master < slave * factor".(This is one path added by the
check "&&  master < (factor>>1)*factor").
In the latter check "slave < factor" must be true, the result of this
check depend on slave in the range [factor, factor>>1] if there is possibility
that "master < slave * factor". If slave in [factor>>1, 0], the check of
"master < slave * factor" is absolutly false and this can be filtered if
we use a variable to load the result of master < (factor>>1)*factor.

My random random inputs and continue confusing to move on.


Thanks,
Tao